© Baker, J.L. (2009) Marine Species of Conservation Concern in South Australia Full citation Baker, J.L. (2009) Marine Species of Conservation Concern in South Australia: Volume 1 - Bony and Cartilaginous Fishes. Report for the South Australian Working Group for Marine Species of Conservation Concern. J. Baker (consultant); Science and Conservation Division, and Coast and Marine Conservation branches of S.A. Department for Environment and Heritage (DEH); Marine and Coastal Community Network of S.A. (MCCN), and Threatened Species Network (TSN). Web version published by Reef Watch, South Australia.

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Silver Trevally / White Trevally / Skippy

Family Name:	Carangidae
Scientific Name:	Pseudocaranx georgianus (formerly P. dentex) (Bloch and Schneider, 1801)
Recommended Status:	South Australia: Data Deficient, possibly Near Threatened WA, Tasmania, Victoria: Data Deficient, possibly Near Threatened NSW & Commonwealth waters of SE Australia: Near Threatened
Rationale:  Although Silver Trevally has a broad geographic distribution across southern Australia, and is an abundant species, it is included here because (i) it is a relatively long-lived, relatively slow-growing, aggregative and highly schooling species, and all of these features increase the vulnerability of populations to over-exploitation; (ii) Silver Trevally has been fished in large quantities both commercially and recreationally, and until recent decades, little was known of the biology or population dynamics. There is still inadequate information about population sizes over space and time, and relationships between fished stocks across the range. The previous lack of information to ensure the sustainability of fished populations, coupled with heavy fishing pressure, has resulted in documented declines in some areas; (iii) catch rate may not provide an accurate index of stock abundance for this species, because it may remain stable, even if the stock is significantly depleted, due to the aggregative nature of the fish. Therefore, serious declines may be masked (whilst catch rates are maintained) until the stock reaches a critically low level of abundance; (iv) although the status of the species in the Commonwealth-managed South East fishery was listed as uncertain throughout the 1990s, the unstandardised catch rate (kg/hr) during the early 2000s was less than half that recorded during the mid to late 1980s. By the late 1990s, the Commonwealth status was “probably over-fished”, and from 2003 to 2007, the status was “over-fished, with over-fishing occurring”. In 2008, the status was described as “over-fished status and overfishing status uncertain” despite no evidence of improvements in the population size since the previous assessment (of Silver Trevally being over-fished). Catches have been considerably reduced in the South East fishery in recent years. However, it is not known whether the reduced catches will enable the stock to rebuild to target levels within an acceptable timeframe. This species is also classified as “over-fished” in both the New South Wales Ocean Hauling Fishery, and in the Ocean Fish Trawl fishery (from which 50% of the catches are taken); (v) size-composition data for the early years of the South East fishery are inadequate (e.g. prior to mid 1980s), but strongly suggest the mean size of Silver Trevally in both commercial and recreational catches has dropped significantly since the late 1980s (particularly off New South Wales), and there has been a significant effect on the size structure of the stock. Yield modelling also indicates that Silver Trevally are being caught well below the size for optimum biological yield. The average size of fish in commercial catches during 1997–99 was 28.4cm and 500g. In more recent years, the modal size of trevally in commercial catches has been about 20-25cm, which is very small for a fish that can grow to around 60–70 cm long. The size reduction issue is further complicated by the higher market price received for smaller trevally (25 – 35cm), compared with larger fish, for which the flesh is considered to be of lower quality (Shelf Resource Assessment Group, 2008), which lessens the incentive for fisher’s to reduce pressure on stocks of small trevally, and wait until they reach a bigger size; (vi) the fact that the modal length of P. georgianus in commercial catches has declined in recent years is consistent with the conclusion that this species is being growth-overfished in south-eastern Australia, with too many fish being caught well below their optimum size; (vii) in south-eastern and southern Australia, this species is legally caught before it is reproductively mature; (viii) there is a possibility that size at first maturity has decreased over time in this species south-eastern Australia, as a population response to heavy commercial fishing pressure; (ix) there are reports from recreational fishers that in waters near some high population areas, Silver Trevally are not as prolific as they once were; (x) in addition to over-fishing, loss of habitat may be a threatening process. Silver Trevally are also prevalent in shallow inshore waters, and any reduction in the quality of inshore waters may have adverse affects on Trevally populations, but there are no specific data; and (xi) there are inadequate data on spawning sites and spawning behaviour (e.g. aggregation patterns); size and age composition in both commercial and recreational sectors); species interactions, and the role of environmental drivers / oceanographic processes in determining recruitment strength. In some southern Australia States, such as WA, SA, Tasmania and Victoria, there are insufficient data on population sizes, distribution, and recreational and commercial fishing trends, to determine population status.

Page Contents

Current Conservation Status

There are no formal listings, but management authorities in Australia consider the species to be over-fished in south-eastern Australian fisheries (AFFA, 2004a; Tilzey, 2004). By the late 1990s, the status was probably overfished, and from 2003 to 2007, the status was overfished, with overfishing occurring (Caton and McLoughlin, 2005; McLoughlin, 2006). In 2008, the status was described as overfished status and overfishing status uncertain (Larcombe and Begg, 2008).

In New South Wales, this species is described by the Department of Primary Industries as being growth overfished.

In Australia, a Sustainable Seafood Guide (AMCS, 2004) listed Silver Trevally a species that should not be consumed, due to its over-fished status.

In New Zealand, Silver Trevally is ranked as a category “D” fish (i.e. “Caution”, in terms of a guide to sustainable seafood for consumers), due to its low to moderate growth rate; relatively long life span (40+, in New Zealand); bycatch in snapper trawls; uncertainty of the stock assessment; decline in the catch per unit effort (CPUE) and estimated biomass in some parts of the fished area; absence of quantitative stock assessment for most areas; impact of bottom trawling, and the lack of a management plan (Weeber and Szabo, 2005).

Distribution

Global

A number of closely related species, previously all referred to Silver Trevally P. dentex (a close relative of P. georgianus) are widely distributed in sub-tropical and warm temperate waters. These species are found through the Western Atlantic (e.g. Bermuda; and from North Carolina, U.S.A. south to southern Brazil); Eastern Atlantic (e.g. Mediterranean, Azores, Madeira, the Canary Is., Cape Verde, Ascension Is. and St. Helena); the Indo-Pacific region (e.g. South Africa, Japan, Hawaii, Australia, Lord Howe I., New Zealand, Kermadec Is., Chatham Is., Rapa, Pitcairn and possibly New Caledonia). Also known from Hawaiian Archipelago and Easter Island (Paxton et al., 1989; Roberts, 1991; Francis, 1993; Wantiez, 1993; Randall, 1996; Smith-Vaniz, 1999).

In New Zealand, Silver Trevally (as P. dentex) are most abundant around the North Island (e.g. Kendrick and Francis, 2002; Weeber and Szabo, 2005) and the top part of the South Island, but also occur in other areas, such as the west coast of the South Island (Roberts et al., 2005), occasionally as far south as Banks Peninsula in the summer (SMCEC, 2004).

Australia

Australia In Australia, closely related species of Silver Trevally (Smith-Vaniz and Jelks, 2006) occur around much of the coastline, particularly the southern half, ranging from North Reef Light, Queensland (23o11'S) to Northwest Cape, Western Australia (21o47'S) (Australian Museum, 2003x), and northern Tasmania (e.g. May and Blaber, 1989; Australian Museum records; Museum of Victoria records, OZCAM, 2009).

P. georgianus is generally considered to range from northern New South Wales through to south-eastern Western Australia, including Tasmania (Gomon et al., 2008). There are few records from south-eastern Tasmania. Edgar et al. (1999) recorded this species in the Huon and Esperance estuaries.

Silver Trevally P. georgianus is considered to be one of the most common trevally species in W.A., and was previously assigned to P. dentex prior to a revision of the group by Smith-Vaniz and Jelks (2006). In Western Australia, P. georgianus is common along the south and south-west coasts, as far north as approximately the Perth / Rottnest I. Area (Smith-Vaniz and Jelks, 2006). A similar species, P. dinjerra, extends from Geraldton / Houtman Abrolhos to North-West Cape (Smith-Vaniz and Jelks, 2006).

South Australia

The Silver Trevally P. georgianus is widely distributed in South Australia, and is known from areas such as the eastern Great Australian Bight; southern Eyre Peninsula (and around offshore islands); Spencer Gulf and Gulf St Vincent; northern and north-western Kangaroo Island; Encounter Bay region; and the south-east of S.A. (e.g. Branden et al., 1994; MLSSA, 1999 and undated; Tanner et al., 2003; Shepherd and Baker, 2008, and Shepherd and Baker, in prep.; Australian Museum record, Museum of Victoria records, S.A. Museum records, cited in OZCAM database, 2009).

It is noted that during a survey of more than 220 shallow coastal reef locations in South Australia, highest densities of Silver Trevally were recorded at a shallow rocky bay known as Smooth Pools, in the eastern Great Australian Bight (e.g. 200 per 2000 square m, recorded in December 2003: S. Shepherd, unpubl. data).

Habitat

In general, Silver Trevally occur in schools, in coastal waters of the continental shelf. Adults are found either at the surface (particularly in deeper shelf waters), in mid-water, or near the sea floor (Annala, 1994). Adults often form schools near the sea bed on the continental shelf, as they are attracted to bait fish grounds that offer some cover (Kailola et al., 1993; Fishnet web site, 2003; Sea-Ex Australia, 2004).

The species occurs in a variety of habitats, including vegetated or rocky inshore reefs and other rough bottom habitats, including “broken bottom” (dissected reef); vegetated reef patches between sand; bays and inlets; shallow waters off beaches, over open grounds of sand or gravel; behind breaking waves at the edge of beaches; or areas of strong current around the edges of rocky islands. Silver Trevally (particularly adults) are also found aggregating around numerous structures, including offshore reefs; artificial reefs and ship wrecks; structures in or near the entrance to estuaries; as well as near rock groynes, jetties, channel markers, wharves, recalls, anchored boats, and bridge pylons (May and Maxwell, 1986; Branden et al., 1994; Ayvazian and Hyndes, 1995; MLSSA, 1999; Williams et al., 2001; Australian Museum, 2003x; Stagles, 2003b; FishNet web site, 2003; Taylor, 2003; Speare et al., 2004; Sea-Ex Australia, 2004; Hutchins, 2005; Gilmour et al., 2006; Correia, 2008).

In New Zealand, schools of adult Silver Trevally occur at the surface, in mid-water and on the bottom, and are often associated with reefs and rough substrate (New Zealand Ministry of Fisheries, 2006b). In New Zealand, Silver trevally are commonly found in mid shelf waters (Weeber and Szabo, 2005). During a survey of the fish fauna of Lord Howe Island, Silver Trevally characterised hard-bottom, macroalgae-dominated habitats (Speare et al., 2004).

Juveniles are found in mid-water groups, in estuaries (e.g. in Tasmania and New South Wales), harbours and coastal rivers and lakes (e.g. in W.A. and N.S.W.), bays and other shallow continental shelf waters (Lenanton and Potter, 1987; Kailola et al., 1993; Potter and Hyndes, 1994; Edgar et al., 1999; West and Jones, 2001; Sea-Ex Australia, 2004; SMCEC, 2004). For example, in the Corner Inlet area of Victoria, small schools of juvenile Silver Trevally are found in bare patches of sand, sparse seagrass, or within the channels (O’Hara et al., 2002, cited by Plummer et al., 2003). At Mallacoota, schools of Silver Trevally have been observed over soft subtidal sediment, but also congregated in proximity to the subtidal reef habitat (Bloink et al., 2005). In Westernport Bay, they occur in seagrass; over bare sand; and in channel habitats (Edgar and Shaw, 1995a). In various parts of Victoria, juveniles have been recorded in the vicinity of Zostera seagrass near adjacent saltmarsh (e.g. Billows et al., 2006); Heterozostera tasmanica (e.g. in Port Phillip Bay: Hindell et al., 2000), and mixed Heterozostera seagrass and Caulerpa macroalgae (e.g. Gilmour et al., 2006). In northern Tasmania, juveniles have been recorded in shallow Heterozostera seagrass beds and over mud habitats, in bays (Jordan et al., 1998; Edgar et al., 1999). In S.A., small numbers are occasionally recorded in the Murray Mouth estuary (e.g. Ye et al., 2002). In New Zealand, juveniles smaller than 4cm are often found sheltering among the tentacles of jellyfish (SMCEC, 2004).

During a study of this species in Western Australia, juveniles were reported to be typically found in inshore areas (< 20m) where there are structures, e.g. reefs, rocks, pillars, jetties, or patches of seagrass and/or detached macrophytes. Adults typically occupy waters associated with reefs, and generally in deeper water. In W.A., the largest, oldest Silver Trevally are found in deeper offshore waters (> 60m deep) (Farmer et al., 2005). Some fishers catch large, schooling Silver Trevally in deep waters (e.g. 100m) off the Perth coast (Correia, 2008). Examples of habitats in which the species was recorded in the W.A. study include intermittent reef / sand interspersed with seagrass beds; sand substrate near rock groynes and reef, bare sand with patches of seagrass, and artificial reef. Individuals less than 250mm were mainly caught over bare sand, and larger, older fish (> 250mm) were caught over reefs (Farmer et al., 2005). Another study in W.A., reported Silver Trevally to be found in variety of reef habitats, including high profile, low profile, and sand-inundated low profile reef (Harvey et al., 2004). Recorded abundances were similar in habitats with sparse and dense macroalgae (e.g. Ecklonia, and other brown macroalgae), and other habitats in which this species was recorded included bare sand, rhodolith beds, vegetated sand, and seagrass beds of sparse, medium and dense coverage (Harvey et al., 2004).

In south-eastern Australia, there are records of juveniles from mangroves (e.g. Smith and Hindell, 2005).

The species is found from the shallow subtidal, to the edge of the continental shelf. The reported maximum depth is 200m (Smith-Vaniz, 1999).

Notes on Biology and Behaviour

Age and Growth

In Australia, Silver Trevally reach a maximum length of about 94cm (Gomon, in Gomon et al., 1994; Daley et al., 1998), and a maximum weight of about 10kg (New South Wales fishing Clubs Association record, 1983; Gomon, in Gomon et al., 1994; Daley et al., 1998). Large specimens (6-8kg) have occasionally been taken in W.A. (Stagles, 2003b). The Western Australian State fishing record for this species is 8.4kg, being a specimen taken off Dongara, in 2006. Globally, the maximum recorded weight is about 10.7kg (Smith-Vaniz, 1999).

More commonly, Silver Trevally in southern Australia are known at a size and weight of about 50cm and 1.5kg, respectively (Daley et al., 1998).

In New Zealand, Silver Trevally normally average between 30 and 60cm in length although occasional specimens approaching 1m are seen in the far north of that country (SMCEC, 2004). Growth rates in south-eastern Australia are slower than those reported in New Zealand (Shelf Resource Assessment Group, 2008f).

Silver Trevally in New Zealand waters are about 37cm total length by 5 years (James, 1984, cited by Kailola et al., 1993). Growth slows significantly after the onset of sexual maturity (see below).

Silver Trevally is a relatively long lived and slow growing species. A study in New South Wales reported a maximum age in excess of 20 years (Rowling and Raines, 2000), and the species possibly lives to 25 to 30 years in southern Australian waters (Larcombe and Begg, 2008). The maximum reported age in New Zealand is about 46 - 49 years, according to ageing work in that country (James, 1984, cited by Kailola et al, 1993; Paul, 1992; Walsh et al., 1999). There is considerable variation in size at age, according to the results of age and growth studies in New South Wales and New Zealand. Work in New Zealand has also reported a high level of inter-annual variability in age composition of commercially caught Silver Trevally (Langley, 2003). One example of age composition data is for a commercial fishing block in New Zealand, where age composition of the 2001-02 purse-seine catch included a wide range of age classes, dominated by 5-16 year age classes, but also with a relatively high proportion of fish of 20 years and older (Langley, 2003).

According to Farmer et al. (2005), previous studies of the biology of P. georgianus in New South Wales were either biased towards faster-growing individuals of younger age classes (Rowling and Raines, 2000) or contained little data for individuals > 5 years of age (Kalish and Johnston, 1997). During a study of this species in W.A., the maximum recorded total lengths of females and males of Silver Trevally in inshore waters (< 60 m depth) were ~69 and 66cm, respectively, and the maximum age of both sexes was 13 years (Farmer et al., 2005). The maximum total length and age in offshore waters (> 60 m depth) were far greater, i.e. 88.5cm and 18 years of age, reflecting greater growth rates after fish have reached ~35cm and 5 years of age. In the W.A. study, the growth curves for Silver Trevally in inshore and offshore waters of the lower west coast were significantly different. The length derived at any given age was far greater for fish in offshore waters than for those in inshore waters at the same age. For example, at 5, 8, 10 and 15 years of age, this species was estimated to reach lengths of 36.79, 51.2, 59.5 and 77.1cm, respectively, in offshore waters compared with 34.9, 41.4, 43.6 and 46.1cm, respectively, in inshore waters (Farmer et al., 2005). In W.A., females and males of Silver Trevally in shallow inshore waters (< 60 m depth) grow at similar rates and have attained lengths of ~13.5, 27, 35 and 44cm by the completion of their first, third, fifth and tenth year of life, respectively. In contrast, females in deeper water have already attained a length of 59.5cm by the end of their tenth year of life. The fish caught in offshore waters were generally larger and older than those in shallower, inshore waters. It is not clear at present whether the inshore and offshore assemblages are part of the same stock (Farmer et al., 2005). In the W.A. study, geographical differences in growth rate were also noted. The maximum lengths of females and males were both greater in the upper than lower west coast waters, i.e. 69 and 65.8cm compared with 65 and 56.8cm, respectively (Farmer et al., 2005). The mean lengths of 4+, 5+, 6+ and 7+ Silver Trevally from the lower and upper west coast regions were also significantly different, for both males and females (Farmer et al., 2005).

Migration / Aggregation

In New Zealand, tagging of adult fish has shown that the large offshore schools do not normally move far, and tend to remain associated with one group of islands for some time (Seafriends Marine Conservation and Education Centre, 2004). Although there is no evidence for major seasonal migration (according to tagging studies), there may, however, be some movement of adult fish between demersal and pelagic habitats on a seasonal basis, as indicated in New Zealand studies (James, 1984, and Paul, 1986, cited by Kailola et al., 1993).

There is a supposition of movement from inshore to offshore waters (e.g. Harrison, 2001). Although little tracking work has been done in southern Australia, it is noted that work on groups of the closely related species (P. dentex) from inshore and offshore reefs in the Azores islands, showed that inshore Trevally moved daily alongshore, using large activity spaces, while the short-term movements of offshore Trevally were restricted to reef summits. During the summer spawning season, both groups displayed frequent migrations of up to several kilometres, but inshore fish remained inshore, whereas offshore reef fish expanded their range to include visits to inshore sites. This behaviour eventually resulted in low long-term residence within the study area, especially that of inshore fish. During the spawning season, one inshore site was visited by most of the fish. However, instead of gathering in large aggregations at a single location, it appears that the trevally adopted a multiple-site visiting behaviour, which may increase mating opportunities, through mixing between inshore and offshore fish that are otherwise segregated for most of the year (Alfonso et al., 2009).

Silver Trevally (particularly adults) often aggregate around structures (see section on Habitat).

Diet and Feeding Behaviour

Silver Trevally are opportunistic feeders, and thus consume a variety of prey. The diet and feeding behaviour of Silver Trevally change with age. At 10-15cm, trevally feed on small planktonic animals, and also pick parasites off other fishes to supplement their diet. These small fishes are usually solitary and are frequently seen swimming in the company of larger fishes (SMCEC, 2004). Fishes from about 15–40cm often school in coastal waters and feed on small invertebrates grubbed from sand or mud bottoms, as well as on planktonic animals (SMCEC, 2004). Fishes larger than about 40cm feed in large groups, on euphausiid krill (see Behaviour, below) (SMCEC, 2004).

Dietary items differ according to the position of Silver Trevally in the water column, with major differences in the diet of schooling, pelagic trevally, compared with trevally near the sea floor.

Pelagic schooling Silver Trevally can feed on euphausiid plankton (James, 1978, and Winstanley, 1985, cited by Kailola et al., 1993) by “ram-filtering” and suction feeding (Sazima, 1998). A study in Victoria (Hindell et al., 2000) showed that 99% of the food items found in a sample of Silver Trevally were planktonic crustaceans (i.e. zooplankton).

Silver Trevally near the sea floor feed on benthic invertebrates found amongst sand and rocks, such as polychaete worms, bivalve molluscs, crustaceans (e.g. amphipods and crabs), and echinoderms (sea urchins, sea stars, brittle stars) (Thomson, 1957; James, 1978; Winstanley, 1985, cited by Kailola et al., 1993; Annala, 1994; Sazima, 1998, cited by Froese and Pauly, 2007; Sea-Ex Australia, 2004).

A study in north-eastern New Zealand (Russell, 1983, cited by Froese and Pauly, 2009) showed that a sample of Silver Trevally had consumed around 51% benthic amphipods, 43% planktonic euphausiids, 4% planktonic copepods, and 2% benthic gastropod molluscs.

In South Australia, Silver Trevally have been observed to feed around and within Southern Bluefin Tuna grow-out cages used by the aquaculture industry (ASIC, 1996; A. Smart, pers. comm., cited by Kemper and Gibbs, 2001).

Reproduction

In Australia, the N.S.W. State Pollution Control Commission (1981, cited by Kailola et al., 1993 and New South Wales DPI, 2008d) reported a size at maturity of 28cm. In south-eastern Australia, this species is reported to mature comparatively early, at about 2 years of age (AFMA, 2007e; Larcombe and Begg, 2008). In Western Australia, female P. georgianus become mature at about 31cm (4 years of age), compared with 28cm (3 years of age) for males, and all individuals of both sexes attain maturity by the end of their seventh year of life (Farmer et al., 2005). In W.A., the length at which 50% of female P. georgianus are mature is 6 cm greater than the current minimum legal size of 25cm, hence females are exposed to one year of fishing mortality before they are able to spawn (Farmer et al., 2005). In New Zealand, previous research showed that Silver Trevally mature at 32cm - 37cm fork length, equivalent to an age of 4 or 5 years (James, 1978, cited by Kailola et al., 1993; Annala, 1994). As with the maximum age, the length at maturity of P. georgianus varies markedly among populations, being attained at a far smaller length in eastern Australia than in New Zealand (James, 1978; Kalish and Johnston, 1997; Rowling and Raines, 2000, cited by Farmer et al, 2005).

Silver Trevally have been found spawning in both estuaries and shelf waters (Kailola et al., 1993; Sea-Ex Australia, 2004). This species does not appear to have defined spawning grounds, nor does it school to spawn.

Silver Trevally is a serial spawner with indeterminate fecundity, i.e. they spawn on more than one occasion during the spawning season and the number of eggs released by individual females is not determined prior to that season (Farmer et al., 2005). Silver Trevally are usually partial spawners, releasing pelagic eggs in small batches at intervals over a period of several weeks (James, 1978, cited by Kailola et al., 1993; Sea-Ex Australia, 2004). Fecundity is estimated at 220,000 eggs for a 37cm female and in larger females may be up to 1 million eggs. Smaller fish may have a fecundity of 50,000 – 200,000 eggs (Rowling and Raines, 2000; New South Wales DPI, 2008d).

In both New Zealand and Australia, protracted spawning has been recorded mainly during the spring and summer months (December to February) (James, 1984; Winstanley, 1985; Kailola et al., 1993; Sea-Ex Australia, 2004). Silver Trevally in spawning condition were reported off the north coast of New South Wales from October to December; however the distribution and occurrence of larvae suggest that spawning is widespread, protracted and regionally variable in its timing. Spawning may occur in N.S.W. from spring to autumn (Rowling and Raines, 2000). During a study off the New South Wales coast, Silver Trevally larvae (mostly at flexion stage: 4 to 6 mm in length) were two to three fold more abundant in November than in January; also more abundant in subsurface compared with surface nets; and were more abundant at the 50m station rather than 100m station (Syahailatua, 2005). In W.A., this species spawns mainly between September and November on the lower west coast and mainly between August and December on the upper west coast (Farmer et al., 2005).

Hatching occurs 28 hours after fertilisation and larvae develop into juvenile fish when they are about 12mm long (James, 1976, cited by Kailola et al., 1993), after 2 – 3 months in the plankton.

The pelagic larvae can disperse widely across the continental shelf prior to coastal recruitment, depending on currents (Smith, 2003). Larval fishes have also been recorded in more than one season of the year, but spring is common (e.g. Gray, 1993; Gray and Kingsford, 2003). During a study of larval distribution off the New South wale coast, Silver Trevally larvae were concentrated in thermoclines, including shallow, sharp thermoclines at 15m – 30m (Gray and Kingsford, 2003). Larvae may enter estuaries after their period in coastal waters, and then settle out over summer and autumn (New South Wales DPI, 2008d). Larvae have been caught entering Lake Macquarie in N.S.W. from December to February (Miskiewicz, 1987), and in coastal waters off Sydney and other parts of south-eastern Australia, from August or September to May (Gray et al., 1992; Gray, 1993). During a study in south-eastern Australia, highest concentrations of Silver Trevally larvae (e.g. 1250 to 4240 larvae per 1000 m3) were sampled off the N.S.W. coast. Lower concentrations (20 to 150 larvae per 1000 m3) were sampled in the Nelson and East Gippsland areas (Bruce and Brayford, 2002, cited by Hayes et al., 2007).

Work in New Zealand on length and age composition of catch data reveal the existence of strong and weak year classes, in terms of recruitment strength (Langley, 2003).

Behaviour

When over 40cm in size, trevally become deeper bodied and develop a pronounced hump on their foreheads, and these adult fishes form large schools, sometimes covering many hectares of sea surface. In New Zealand, around the offshore islands and pinnacles of eastern North Island they often feed intensively at the surface, gulping down euphausiid crustaceans (krill) with their humped foreheads out of the water (SMCEC, 2004).

In New Zealand, schools of Silver Trevally are sometimes mixed with Decapterus koheru and Arripis trutta (Annala, 1994, New Zealand Ministry of Fisheries, 2006b).

Tagging of adult Trevally in New Zealand has shown that large offshore schools do not normally move far and tend to remain associated with one group of islands for some time.

In New Zealand, young fish enter a demersal phase from about 1 year old until they reach sexual maturity. At this stage adult fish move between demersal and pelagic phases. Schools occur at the surface, in mid-water and on the bottom, and are often associated with reefs and rough substrate (New Zealand Ministry of Fisheries, 2006b).

Though there is no evidence of seasonal migration, studies in New Zealand indicate that adult fish may move seasonally between demersal and pelagic habitats (Stagles, 2003b; New Zealand Ministry of Fisheries, 2006b).

Other Information

In southern Australia, Southern Bluefin Tuna (AFMA, 2002e) and Yellowtail Kingfish (Caines, 2005) are two of the large predatory fish species that prey on Silver Trevally. During a study of the diet of juvenile Southern Bluefin Tuna in Western Australia, P. georgianus (as P. dentex) was recorded as one of 4 main prey species in the diet (Davis, 1999). During a study in South Australia, in which the stomach contents of 42 Yellow-tail Kingfish were examined, fishes made up 51% of the restructured biomass (NB cephalopods 46%), and of the fishes, Silver Trevally made the most important contribution (20.5%) to total biomass (Caines, 2005).

In New Zealand, the Striped Marlin Tetrapturus audax preys upon Silver Trevally (Nakamura, 1985, cited in Froese and Pauly, 2009).

Silver Trevally has been recorded as an important component of the diet of Yellowtail Kingfish (Caines, 2005).

A pilot stock-structure study (unreferenced, in Caton and McLoughlin, 2005, and AFMA, 2008f) suggested that there is a common stock in south-eastern Australian waters, distinct from that around the North Island of New Zealand.

Fisheries Information

Southern Australia – Commercial (Commonwealth Fisheries)

Silver Trevally (recorded as P. dentex) is a major species in the Commonwealth-managed South East Trawl fishery (SETF) component of the Commonwealth-managed Southern and Eastern Scalefish and Shark fisheries (SESSF), and is also taken in lesser tonnages in the Gillnet Hook and Trap (GHAT) fishery, and the Great Australian Bight Trawl Fishery. The total landings of Silver Trevally from all sectors during 2007 were 334t (AFMA, 2008f). Most of the Silver Trevally trawl catch is taken from waters less than 100 m deep (Tilzey, 1994). Silver Trevally caught by trawl in Commonwealth waters are subject to an annual Total Allowable Catch (TAC), and Individual Transferable Quotas (ITQs) (AFMA, 2003j). For all Commonwealth fisheries combined, the Total Allowable Catch was about 298t in 2008 (Larcombe and Begg, 2008). ITQ management arrangements apply for this species in the SETF (South East Trawl fishery component of the SESSF), the ECDWZ (East Coast Deepwater Zone within the SETF, previously called the East Coast Deepwater Trawl Fishery), the CVIT (Commonwealth Victorian Inshore Trawl Fishery) and the GHAT (Gillnet, Hook and Trap Fishery) (AFMA, 2003j). Catches of this species have been highly variable in the SETF (e.g. Prince, 2001; Caton and McLoughlin, 2005).

In the South East fishery component of the SESSF (formerly known as SETF), analyses of commercial catch data showed a decline in catches of Silver Trevally from at least 1,500t in 1985 to less than 500t in 1999. Much of the catch in this fishery comes from New South Wales (see below), where landings peaked at about 1800–2000t in the mid-1980s, then started to decline. New South Wales State catches account for nearly 80% of the total silver trevally catch. Catches in the South East fishery remained stable at between 500 – 600t during the late 1990s – early 2000s, until a decline in 2004 and 2005 (AFMA, 2007e). The level of discarding is reportedly low (less than 1%) (Smith and Wayte, 2005). More recent annual TACs and actual catches of Silver Trevally in the South East fishery are shown in the table below (from AFFA, 2004a; Caton and McLoughlin, 2005; Larcombe and Begg, 2008). A significant reduction in quota occurred between 2006 and 2007. “Trawl” allocations apply to the South East Trawl fishery; “non-trawl” allocations apply for the Gillnet, Hook and Trap Fishery. Walker et al. (2003) reported that P. georgianus is one of the top 4 scalefish species caught as by-product in the shark fishery in Bass Strait.

Silver Trevally Catches in the South East Fishery
	Total Allowable Catch (TAC)	Actual Catch
2001	624t (= 601t trawl; 23t non-trawl)	224t trawl 2t non-trawl 9t trawl discards
2002	399t (= 359t trawl, 40t non-trawl)	253t trawl (including 195t of State landings) 2t non-trawl
2003	Recommended combined TAC 320t Allocated TAC 320t (=283 t trawl; 37t non-trawl)	341t trawl (including 218t of State landings) 2t non-trawl 2t discards
2004	Recommended combined TAC 320t Allocated TAC 319t (= 286 t trawl; 33t non-trawl)	463t trawl (including 324t of State landings) 2t non-trawl 41t discards
2005	Recommended combined TAC 320t Allocated TAC 320t	96t trawl 2t non-trawl
2006	Recommended combined TAC 270t Allocated TAC 298t	69t trawl 2t non-trawl 2t discards
2007	Recommended combined TAC 146t Allocated TAC 159t	64t trawl 3t non-trawl
2008	Recommended combined TAC 296t Allocated TAC 313t
(AFMA, 2003j; AFFA, 2004a; Caton & McLoughlin, 2005; Larcombe, & Begg, 2008; Shelf Resource Assessment Group, 2008f)

Silver Trevally is a minor by-product species in the Great Australian Bight Trawl Fishery. Reported catches in recent years are shown in the table below.

Silver Trevally: Reported Commercial Catch in the GABTF
Year	Catch (t)
1995	0
1996	1
1997	10
1998	6
1999	27
2000	22
2001	3
2002	0
(from Lynch and Garvey, 2003)

Bycatch sampling in the GABTF reported by Brown and Knuckey (2002) showed that Silver Trevally were caught during the sampling periods in 2000 and 2001, and all of those caught were retained. During that period, Pseudocaranx dentex was observed in 11 of the 209 trawl shots; the average quantity retained was 3.5kg per trawl shot (Brown and Knuckey, 2002).

Silver Trevally is a minor bycatch species in the Commonwealth-managed Small Pelagics Fishery (Milton et al., 2004).

Southern Australia – Commercial (State Fisheries)

In southern Australia, the main fishery for Silver Trevally has been in New South Wales waters. Trevally are targeted by fishers using beach seines in bays, or from ocean beaches along the coast from Port Macquarie to the Victorian border (Kailola et al., 1993). Some of N.S.W. catch of Silver Trevally is part of the Commonwealth-managed South East Trawl Fishery (see above). Historically, catches in New South Wales have been large, with more than 1,100t per annum recorded in 1983/84 and 1988/89, with hundreds of tonnes per annum in other years of the 1970s, 1980s and 1990s (BRR, 1991).

Silver Trevally are a significant component of commercial fish catches in both estuarine and ocean waters of New South Wales. Ocean catches off New South Wales also form part of the Commonwealth-managed South East Trawl component of the SESSF (see above). Commercial catches are taken mainly with fish trawls (e.g. Ocean Fish Trawl fishery), fish traps and lines (which also target snapper and leatherjackets, in the Ocean Trap and Line fishery), and estuary hauling nets (e.g. in the Estuary General Fishery, particularly before Botany Bay became a “recreational fishing haven” and commercial hauling nets were banned in the area) (Kailola et al., 1993; Rowling and Raines, 2000). Minor bycatches come from the N.S.W. rock lobster fishery. In New South Wales State waters, about half the catch of Silver Trevally is taken by trawling in the Ocean Fish Trawl fishery (AFFA, 2004a; New South Wales DPI, 2008d). In the Ocean Fish Trawl fishery, Silver Trevally, flathead (Neoplatycephalus and Platycephalus spp.), redfish (Centroberyx affinis), John Dory (Zeus faber), and Eastern School Whiting (Sillago flindersi) are the main scalefish species taken (Larcombe et al., 2002). Target fishing for Silver Trevally using trawlers on continental shelf grounds commenced during the 1970s, following the development of markets for this species (Rowling, 2008). The New South Wales catch of Silver Trevally increased from less than 300t a year until 1980, to over 900t in 1989-90. Prior to the 1980s, the low price obtained for Silver Trevally deterred fishers from targeting the species, but the higher prices since that time encouraged fishers to target Silver Trevally schools when the availability of other more valuable species was low (Kailola et al., 1993). The commercial catch in 1991-192 was about 374t from N.S.W. ocean waters, plus 100t from the estuaries (New South Wales EPA, citing N.S.W. Fisheries, 1995). In New South Wales, commercial catches declined from about 1,500t in 1985 to 340t in 1999 (Rowling and Raines, 2000). Landings reported by N.S.W. fishers were 251t in 2002; 211t in 2003; 236t in 2005, 256t or 300t in 2006 and 215t in 2007 (Smith and Wayte, 2005; AFMA, 2007e, 2008f). In inshore waters, one of the principle fisheries in N.S.W. that takes this species is the Sydney inshore trawl-whiting fishery. About 130 tonnes of trevally were taken in 2005-06 in the whiting fishery (Graham, 2008). Silver Trevally is also taken in New South Wales by beach seines (e.g. in Botany Bay) (Gray et al., 2001), and is a major discard species (but sometimes retained seasonally) in the estuarine prawn and fish haul fishery, at locations such as the Manning and Shoalhaven rivers, Botany Bay, Lake Macquarie, and St Georges Basin (where a discard rate of ~ 20 - 80 fish per day was recorded, depending on season) (Gray and Kennelly, 2001). The retained and discarded catches of Silver Trevally in the haul fishery in Botany Bay from February 1998 to January 1999, were estimated to be 241,899 fish retained (SE = 101,224), and 18,361 discarded (SE = 14,678) (Gray and Kennelly, 2001). In St Georges Basin over the same period, the estimated numbers were 4866 retained (SE = 1,459) and 11,764 discarded (SE = 302). In N.S.W., Silver Trevally ranks as the second most important species in the Ocean Trap and Line (OTL) fishery by weight (Stewart and Miller, 2003). In some years, significant quantities (about 100t per annum) of Silver Trevally are taken by the OTL (New South Wales Department of Primary Industries, 2004).

Silver Trevally are also taken in Victorian fisheries. BRR (1991) reported catches of 118t to 196t per annum, from 1978 to 1985, and catches of 40t – 60t during the 1960s and 1970s. In Victoria, the species is also a bycatch of gillnets used to target School Shark (Galeorhinus galeus) and Gummy Shark (Mustelus antarcticus), and it is noted that Kailola et al. (1993) reported that the level of catch from the shark fishery is uncertain due to misreporting of warehou (Seriolella species) as silver trevally. The reported State catch from Victoria was 211 tonnes in 2003 (Smith and Wayte, 2005) (which conflicts with Victorian State fisheries statistics – see below); 33t in 2006 (AFMA, 2007e) and 48t in 2007. According to NRE Victoria (2002, 2003), the commercial catch of Silver Trevally in Victorian waters in recent years is follows:

Commercial Catch of Silver Trevally in Victorian Waters 1997 – 2007
Year	Catch (t)
1997/98	40
1998/99	83
1999/00	78
2000/01	71
2001/02	93
2002/03	59
2003/04	48
2004/05	73
2005/06	42
2006/07	42
(DNRE Victoria and DPI Victoria statistics, 2002 - 2008)

In Western Australia, BRR (1991) reported catches of around 15t to 40t per annum, between 1964 and 1990, with higher tonnages in some years (e.g. 47 – 48t per annum from 1964 to 1966; 113t in 1977/78, and 56t in 1989/90). The bulk of the trevally catch in commercial trawls is reported to be fish of about 45cm and 1.4kg (Stagles, 2003b). Fish caught commercially in W.A. are fully recruited to the fishery at about age 4 to 5 years (Farmer et al., 2005). This species is also taken by commercial seine and “wet-line fishery” fishers. Juvenile Silver Trevally are also caught by commercial prawn trawlers operating over predominantly sandy substrates, e.g. in Cockburn Sound and Comet Bay (Farmer et al., 2005). Minor catches of adult fish are taken in the Western Deepwater Trawl Fishery (AFMA, 2004f), and the temperate demersal shark fishery (which uses gillnets and long-lines) (McAuley and Simpfendorfer, 2003). Reported catches of Trevally in W.A. waters are shown in the table below.

Reported Catches of Trevally from W.A. waters, 1997 to 2007
Year	“Trevally Other (Skippy)" Live weight (kg)	“Skipjack” P. georgianus Live weight (kg)
1997/98	195,075	8,950
1998/99	286,108	17,274
1999/00	221,602	8,033
2000/01	214,157	6,141
2001/02	201,815	5,630
2002/03	207,923	3,559
2003/04	219,758	6,163
2004/05	223,895	4,938
2005/06	193,567	3,778
2006/07	190,449	8,386
(W.A. State of the Fisheries reports, 2000/01 – 2007/08)

In Tasmania, Silver Trevally are caught by both Commonwealth-licensed (see section above) and State-licensed fishers. BRR (1991) reported highly variable catches between the 1960s and 1990, and some of the catches may have been misidentified (e.g. Jack Mackerel Trachurus declivis instead of P. georgianus. T. declivis was caught by purse seine in large quantities as feed for the aquaculture industry, before the stock of Jack Mackerel was exhausted, and during the period of large yields, Silver Trevally may have been a significant bycatch – G. Edgar, TAFI, pers. comm., 2009). During the 1980s and 1990s, small quantities of Silver Trevally were caught by trolling in Tasmanian coastal waters (Kailola et al., 1993). In Tasmania, Silver Trevally catches recorded in General Fishing Returns and Commonwealth logbook returns, ranged from approximately 15 tonnes in 1990/91 and 1994/95, to less than 5t annually from 2000/01 to 2004/05 (Ziegler et al., 2006). The reported State catch from Tasmania was about 5t per annum in 2003, 2006 and 2007 (Smith and Wayte, 2005; AFMA, 2007e, 2008f). Silver Trevally are caught in small (incidental) numbers in the gillnet fishery for Banded Morwong (Murphy and Lyle, 1999). Smith and Heran (2001) reported a commercial gillnet catch of about 15t in 1990/91 and 1994/95, and lower catches in all other years between 1990 and 2001. The commercial gillnet catch in 2000/01 was reported to be less than 5t, with a declining trend (Smith and Heran, 2001).

In S.A., Trevally catches from the mid 1980s to 2008 are shown in the chart below. BRR (1991) reported catches in the order of 1t – 5t per annum, from 1970 to 1990. The species is considered a “tertiary” commercial fish (i.e. fish for which catch over the past three years was less than 5t) (PIRSA, 2008c). Examples of locations where trevally are caught in the Marine Scalefish Fishery include Investigator Strait / northern Kangaroo Island (during the past decade, the highest catches in some years have been from this area); northern Gulf St Vincent; south-eastern Spencer Gulf / southern Yorke Peninsula; southern Spencer Gulf and waters southwards (towards the west coast of Kangaroo Island); southern Kangaroo Island, and Ceduna area in the eastern Great Australian Bight (SARDI data).

Reported Catches of Trevally from South Australia, 1984 to 2008

(Knight et al., 2002 – 2007; Fowler et al., 2008)

New Zealand – Commercial and Recreational

Silver Trevally are caught , mainly from age 3-4 onwards, around the North Island and the north of the South Island, with the main catches from the northern coasts of the North Island. The species is taken all year round by trawls (mainly in waters less than 80m), with a main season from October to April. Trevally is taken in the northern coastal mixed trawl fishery, mostly in conjunction with Pink Snapper and Jackass Morwong. Another large proportion of the commercial landings of Silver Trevally in New Zealand are taken in the purse-seine fishery, which targets schools seasonally (Taylor, 2003), and has been operating for more than 4 decades. It is also one of several by-products of the Arripis trutta salmon fishery. There is a smaller catch by set-net. The species is sold on the domestic market, and also exported. Some of the catch is processed and sold as frozen fillets and canned, often as smoked fillets. It is also reprocessed in “fish fingers”, and some are used as bait (Seafriends Marine Conservation and Education Centre, 2004). Silver Trevally is considered to be a premium one for sashimi (raw fish), marinating, and salt-curing (like gravalax), and is highly prized in Japan. In New Zealand, selected fish for the sashimi market are instantly killed upon landing (Ikijime process) to enhance flesh quality.

During the 1970s in New Zealand, pressure intensified on the limited inshore species such as trevally (Annala, 1995, cited in New Zealand State of the Environment Report, 1997). Since the mid 1970s, Trevally has been taken by purse seine, mainly in the Bay of Plenty, in variable but often substantial quantities. Purse seine fishing vessel companies charter planes to collect aerial survey data, such as estimates of the size and weight of Silver Trevally schools (Froude, 2000; Langley, 2004). During the early 2000s, the inshore fishery for Trevally off north-eastern North Island had a total allowable commercial catch (TACC) of 1506t (Annala et al., 2002, cited by Langley, 2004).

Set-net fishermen take modest quantities (New Zealand Ministry of Fisheries, 2004g). Other means of capture in New Zealand include long-lining, beach seining, gill netting (e.g. Hickford et al., 1997), Danish seine, and trawling. The species has been quota-managed since 1986. During the 1990s and up to the late 2000s, the annual quota was about 3,932t, and catches did not exceed this (New Zealand State of the Environment report, 1997; New Zealand Ministry of Fisheries, 2009a). Reported landings from 2000 to 2007 were between 2,897 and 3,548 tonnes per annum (New Zealand Ministry of Fisheries, 2009a).

Southern Australia – Recreational

Silver Trevally is an important fish species for recreational fishers in southern Australia. Various angling clubs and associations around southern Australia promote the Silver Trevally as “good sport” / “excellent sport”, and a “powerful fighter” / “fighting fish” that is “fun to catch”. The capture of both small and large trevally is promoted, in the various habitats in which the life stages reside. Trevally are caught using hand lines and rod and line methods (Kailola et al., 1993; Sea-Ex Australia, 2004) Winter and spring are popular times for catching trevally, when small fish are abundant in southern estuaries, and larger fish on southern offshore reefs. Australian National Sportfishing Association Ltd (ANSA) and the Gamefishing Association of Australia consider this to be a gamefish species, and ANSA maintains a recommended “trophy length” of 50cm for specimens of Silver Trevally.

The species is fished recreationally in all coastal areas of Australia except NT, and is especially popular in Victoria and Western Australia. The National Recreational and Indigenous Fishing Survey (Henry and Lyle, 2003) reported a minimum of 523,897 Silver Trevally were caught and kept across Australia by recreational fishers during the survey period (May 2000 to April 2001), which included approximately 2,602 from Queensland; 1,993 from New South Wales; 95,046 from Victoria; 22,584 from Tasmania; 27,743 from South Australia; and 373,929 from Western Australia. It is noted that an additional 456,704 trevally fish (unspecified species) were caught and kept during the survey period, including 213,551 from New South Wales; 163,456 from Queensland; 54,593 from S.A., and 16,141 from W.A., and it is possible that a proportion of the unspecified trevally catch was Silver Trevally.

In New South Wales, Silver Trevally comprise a significant component of recreational fish catches in both estuarine and ocean waters. It is considered to be a “sports fish” (DAFF, 2007) or “game fish”, and is included in game fishing tournaments (e.g. Murphy et al., 2002). During a survey of recreational trailer boat fishing in the marine waters of New South Wales (Steffe and Murphy, 1994), P. georgianus comprised 11% of the recorded catch in the Kingscliff (northern) area, and 13% of the catch in the Sydney area. Recreational catches of approximately 120t per annum have been reported for New South Wales ocean waters (New South Wales Department of Primary Industries, 2004), and similar amounts are believed to have been taken by recreational fishers from New South Wales estuarine waters in the early 1990s. Recreational catches in N.S.W. are not accurately known, but are thought to have declined from about 250t, estimated to have been taken in the early 1990’s (Rowling and Raines, 2000). The recreational catch of Silver Trevally from oceanic water in the mid 1990s was estimated to be 150-210t per annum (New South Wales DPI, 2008d), and more recently, about 87 t per annum (New South Wales Department of Primary Industries, 2004). Previously, the species was ranked as the 5th most common species in fish catches from the Sydney Harbour area (Henry, 1984, cited by Kailola et al., 1993). Recent recreational catch statistics include those from the Hawkesbury River (Silver Trevally comprised 3% of the fish catch recorded during 4,807 interviews with anglers and spearfishers) and 5% of the fish catch in the oceanic waters outside Broken Bay (based on 412 interviews) (New South Wales DPI, 2008c). This specie sis also taken by recreational fishers in estuaries, such as Lake Macquarie (e.g. Steffe and Chapman, 2003). This species is also promoted for spearfishing in New South Wales waters, and records are held by spearfishing clubs (e.g. Underwater Skindivers and Fisherman's Association Inc. record is 5.775kg specimen, caught on Jibbon Reef in 1981; Newcastle Neptunes Spearfishing Club record is 5.450kg specimen, caught off Broughton Island in 1962).

In Tasmania, juvenile Silver Trevally are caught in shallow sandy areas, in seagrass beds, estuaries (DAFF, 2007), or off wharves and jetties. According to Lyle et al. (2000), this species is also caught by gillnets in both northern and southern Tasmania. Smith and Heran (2001) reported an estimated recreational gillnet catch of about 17 tonnes per annum. They are often caught whilst line fishing for blue warehou, because the two species tend to favour the same inshore habitats. Larger individuals are more common in offshore waters and are occasionally taken in gillnets (Lyle et al., 2000; DPIWE Tasmania, 2004o). The recreational catch of Silver Trevally in Tasmania reportedly accounts for half of the total catch of that species in that State (Lyle, 2005, cited by Ziegler et al., 2006).

In Western Australia, a 12-month survey of coastal recreational boat fishing between Augusta and Kalbarri on the west coast of Western Australia during 1996-97 (Sumner and Williamson, 1999), estimated that during the survey period, approximately 120,439 Silver Trevally (SE = 15,665) were caught and kept by fishers using trailered boats, and 28, 380 were released. A smaller number (2,217) were estimated to have been caught and kept by fishers using non-trailered boats. Silver Trevally is one of the major species caught by recreational fishes in some parts of that region, including southern Perth and Mandurah regions (which incorporate Warnbro Sound and Comet Bay) (Sumner and Williamson, 1999). The importance of the Pseudocaranx complex of species to anglers is demonstrated by the fact that it was ranked third in the total shore-based catch recorded during the 1994/95 recreational shore survey along the lower west and south coast of Western Australia (G. Nowara, Department of Fisheries WA, pers. comm., cited by Farmer et al., 2005) and again during the subsequent 1996/97 recreational boat survey conducted between Kalbarri and Augusta, which included the Perth and Mandurah regions (Sumner and Williamson, 1999). Along the South Coast of W.A., a similar 12-month survey during 2002-03 showed that Silver Trevally were amongst the top 4 species caught and kept, with 20,951 trevally retained by beach fishers and other fishers during the survey period, from locations such as Oyster Harbour, Princess Royal Harbour, Walpole/ Nornalup Inlet and Bandy Creek (Smallwood and Sumner, 2007). The species is also taken in waters along the central coast of W.A., with Sumner et al. (2002) estimating the 1998/99 catch, in the Ningaloo Marine Park area, to be 770 specimens (SE = 1,066), with 652 released, and 498 eaten by sharks. In W.A., juvenile Silver Trevally are taken in the coastal river systems (including coastal rivers in the Perth area) and estuaries (e.g. Malseed et al., 2000; Malseed and Sumner, 2001); also close to shore, for example off beaches along the South Coast region (Department of Fisheries, W.A., 2004d). In W.A., adult Silver Trevally are promoted as a species that can be taken from various continental shelf habitats, such as offshore reefs (seasonally) and sand patches near vegetated bottom. Some anglers target the larger Silver Trevally offshore from structured reef offshore from Rottnest, and off rocky headlands along the south coast, including Esperance (Stagles, 2003b). Silver Trevally (including large specimens) are also taken near groynes, coastal jetties and bridges, particularly in areas of strong current flow. This species is also taken by charter boat fishers in W.A., with an estimated catch along the South Coast Bioregion of about 1-2t per annum, in each of 2002/03, 2003/04, and 2004/05 (W.A. State of the Fisheries report, 2005/06).

In South Australia, Silver Trevally are taken by recreational line fishers and spear fishers. All species of trevally, with specimens over 24cm length and 0.4kg weight, were listed as target species in the 54th Australian Spearfishing Titles 2006, held on northern Kangaroo Island (Australian Underwater Federation Inc., 2006). In the eastern Great Australian Bight, charter vessels visit bays, islands (e.g. Flinders Reef and Nuyts Archipelago), and other areas to catch species such as P. georgianus (McLeay et al., 2003).

A sports fishing web site has included Silver Trevally in the list of “prolific” fish species taken by sports fishers around Lord Howe Island (Lord Howe Island Sports and Gamefishing web site, 2004). One of the largest recorded Silver Trevally caught by a recreational angler weighed about 10kg, and was taken off Lord Howe Island (Sea-Ex Australia, 2004).

Aquaculture

In Western Australia, Silver Trevally is a currently authorised species on aquaculture licences for fish farmers in Jurien Bay (Everall, 1998).

In South Australia, a small number of licences have been issued for Silver Trevally to be produced in aquaculture facilities (PIRSA Aquaculture Public Register, August 2003).

Larval and juvenile Silver Trevally reared in hatcheries are prone to a disease called Viral Encephalopathy and Retinopathy (VER), or Viral Nervous Necrosis (VNN). The disease is characterised by neurological abnormalities, such as erratic swimming behaviour (spiral, whirling or belly-up at rest) and vacuolation of the central nervous tissues and of the nuclear layers of the retina. In general, younger fish have more severe lesions than older fish, in which the retina is often the main site of disease. The disease is controlled by a number of methods, such as treating infected eggs with ozone (Mori et al., 1992, 1998, cited by OIE, 2003).

Vulnerable Characteristics of the Species and Threatening Processes

Silver Trevally is a relatively long-lived, relatively slow-growing, aggregative species (e.g. Paul, 1992; Langley, 2003; Weeber and Szabo, 2005; Smith and Wayte, 2005) of low biological productivity (AFMA, 2007e; Shelf Resource Assessment Group, 2008f), and all of these features increase the vulnerability of populations to over-exploitation.

Silver Trevally have been fished in large quantities both commercially and recreationally, and until recent decades (e.g. Rowling and Raines, 2000 and Farmer et al., 2005) little was known of the biology or population dynamics. The previous lack of information to ensure the sustainability of fished populations, coupled with heavy fishing pressure, have resulted in documented declines in some areas. For example, in New South Wales, commercial catches declined from about 1,500t in 1985 to 340t in 1999 and 2000 (Rowling and Raines, 2000; Smith and Wayte, 2005). In the N.S.W. Ocean Trawl Fishery, catches have consistently declined since the 1980s, from 800-1000t to about 100t in recent years. The decline in landings has been apparent in both the N.S.W. area, and all ocean waters (which includes some landings from the Commonwealth-managed South-East fishery area) (New South Wales DPI, 2004). In the Commonwealth-managed South East fishery (of which State catches from N.S.W. are also a significant part), overall commercial landings declined consistently during the mid to late 1990s to well below the Total Allowable Catch at that time (Caton, 2002; AFFA, 2004a).

Silver Trevally is a highly schooling species, and catch rate may not provide an accurate index of stock abundance for this species (Smith and Wayte, 2004). For example, catch rate may remain stable, even if the stock is significantly depleted, due to the aggregative nature of the fish. Therefore, serious declines may be masked (whilst catch rates are maintained) until the stock reaches a critically low level of abundance.

During the 2000s, the TAC in the Commonwealth fishery has been reduced substantially (see table above). The unstandardised catch-rate has also declined since 1997, and in some years is below AFMA’s ‘trigger’ catch-rate (the lowest annual average CPUE between 1986 and 1994), and therefore does not always meet the catch rate performance criterion (Caton, 2002). Since 2000, catch rates have been variable (Smith and Wayte, 2005).

Although the status of the species in the Commonwealth-managed South East fishery was listed as uncertain throughout the 1990s (AFFA, 2002b), the unstandardised catch rate (kg/hr) during the early 2000s was less than half that which was recorded during the mid to late 1980s (see AFFA, 2004a). By the late 1990s, the status was probably overfished, and from 2003 to 2007, the status was overfished, with overfishing occurring (Caton and McLoughlin, 2005; McLoughlin, 2006). In 2008, the status was described as overfished status and overfishing status uncertain (Larcombe and Begg, 2008), despite no evidence of change in the population since the previous assessment. This species is also classified as overfished in both the New South Wales Ocean Hauling Fishery, and in the Ocean Fish Trawl fishery (from which 50% of the catches are taken). In a draft ecological risk assessment for the trawl component of the South East fishery, this species was ranked as being at “medium risk” of population impacts (Wayte et al., 2004).

Size-composition data for the early years of the South East fishery are inadequate (e.g. prior to mid 1980s), but strongly suggest the mean size of Trevally in both commercial and recreational catches has dropped significantly since the late 1980s (particularly off New South Wales), and there has been a significant effect on the size structure of the stock. Length-frequency data show a significant reduction in the mean size of fish in both commercial and recreational catches. Yield modelling also indicates Silver Trevally are being caught well below the size for optimum biological yield, and that growth-overfishing has occurred, and is still is occurring, because too many young Silver Trevally are being taken (AFFA, 2004a). The average size of fish in commercial catches during 1997–99 was 28.4cm and 500g. In more recent years, the modal size of trevally in commercial catches has been about 20-25cm, which is very small for a fish that can grow to around 60–70 cm long (Caton, 2002; Smith and Wayte, 2005). The size reduction issue is further complicated by the higher market price received for smaller trevally (25 – 35cm), compared with larger fish, for which the flesh is considered to be of lower quality (Shelf Resource Assessment Group, 2008), which lessens the incentive for fisher’s to reduce pressure on stocks of small trevally, and wait until they reach a bigger size. The fact that the modal length of P. georgianus in commercial catches has declined in recent years is consistent with the conclusion that this species is being growth-overfished in south-eastern Australia, with too many fish being caught well below their optimum size (Caton and McLoughlin, 2005; Farmer et al., 2005). There is agreement that there has been a fish-down of Silver Trevally stocks since the beginning of the fishery, but the current level of depletion is reported to be uncertain. Catches have been considerably reduced in the South East fishery in recent years. However, it is not known whether the reduced catches will enable the stock to rebuild to target levels within an acceptable timeframe (Larcombe and Begg, 2008). There is a possibility that size at first maturity has decreased over time in this species in south-eastern Australia, as a population response to heavy commercial fishing pressure (Farmer, 2005). The earlier attainment of maturity of Silver Trevally in eastern Australia may reflect a response to the severe growth over-fishing to which, on the basis of a recent marked decline in its mean lengths in commercial catches, it has been concluded that this species is being subjected (Tilzey, 2004, cited by Farmer, 2005).

A study in N.S.W. reported that a large proportion of the specimens in the Silver Trevally catch in N.S.W. are reproductively immature (Rowling and Raines, 2000), although there may be an issue with the accuracy of data. Similarly in W.A., the average length of females at first maturity is 60 mm greater than the current MLL of 250 mm and thus the females are exposed to one year of fishing mortality before they are able to spawn (Farmer et al., 2005).

In W.A., this species is considered to have “moderate vulnerability” to both fishing impacts and environmentally-drive impacts, and both adult and juvenile fish are under high fishing pressure (Harrison, 2001).

Declines have also been documented in the New Zealand fishery, and catches are considered to be above the level of sustainable yield (New Zealand State of the Environment report, 1997). Concerns with the capture of this species in New Zealand include the uncertainty of the stock assessment (e.g. in parameter estimates, and model assumptions, and uncertainty of biomass estimates in most areas: New Zealand Ministry of Fisheries, 2003); decline in the catch per unit effort (CPUE) and estimated biomass in one of the major fishing areas; the absence of quantitative stock assessment for most areas; the impact of bottom trawling on habitat and benthic species; potential bycatch of Hector’s Dolphin; and the lack of a management plan. There appears to be a slowly declining trend in stocks in one of the major fishing areas in New Zealand. The status of this species is reported to be uncertain, but it has been ranked as code “D” (cautious management required) (Weeber and Szabo, 2005). In New Zealand, there is concern that when schools of larger (older) trevally are removed by purse seine fishing, then localised depletions occur for considerable time (SMCEC, 2004).

There are reports from recreational fishers (e.g. Stagles, 2003b, for W.A.) that in waters near some high population areas, Silver Trevally are not as prolific as they once were.

In addition to over-fishing, loss of habitat may be a threatening process. Silver Trevally are also prevalent in shallow inshore waters as well as offshore, and any reduction in the quality of inshore waters may have adverse affects on Trevally populations, but there are no specific data.

Silver Trevally is one of the species for which research has been undertaken on the potential impacts of offshore seismic survey noise. During trials, captive fishes showed a common fish 'alarm' response of swimming faster, swimming to the bottom, tightening school structure, or all three, at an estimated 2-5 km from a seismic source (McAuley et al., 2000).

Research Notes

The Commonwealth’s Shelf Resource Assessment Group has developed an “agreed catch history” for the Commonwealth’s South East fishery, incorporating annual catches from each sector, to assist the stock assessment process. Correction of historic catch data is important, because historically, in the South East fishery, fishers often recorded warehou as ‘trevalla’ or ‘trevally’.

During the mid 2000s, students (D. French and B. Farmer) at the Centre for Fish and Fisheries Research at Murdoch University in Western Australia undertook a project to investigate the biology of Silver Trevally in marine waters of the lower west coast of Australia (Murdoch University, 2006). Data have been collected and analysed on habitats, spawning periods, size and age compositions, and growth and reproductive biology in the northern and southern parts of the distribution of silver Trevally on the west coast of Australia (e.g. Farmer et al., 2005).

In W.A., a study of the movement of Silver Trevally has been undertaken, using acoustic telemetry (Bancroft, 2008).

In New South Wales, Silver Trevally is one of the species for which recreational catch statistics from anglers and spear fishers are being recorded (e.g. New South Wales DPI, 2008c).

During the mid 2000s, a structured method for setting Total Allowable Catches was developed , utilising clear decision rules in a 4-tiered system, depending on the amount and type of information available to assess stock status (Day and Klaer, 2006). Silver Trevally is one of the species for which this system is used to determine appropriate catch levels.

Management Notes

There are limit reference points for the commercial catch of this species in South Australia, such as 3rd lowest or 3rd highest catch; greatest inter-annual change (plus or minus), and greatest 3-year trend (plus or minus) (Fowler et al., 2008).
< br> In south-eastern Australia, management is complicated by the multi-jurisdictional and multi-sectorial nature of the fishery. For example, about two-thirds of the 2003 catch was taken by dual-endorsed South East fishery vessels from waters under State jurisdiction. The Total Allowable Catches from 2001 to 2003 were exceeded (Caton and McLoughlin, 2005). AFMA’s reductions in the TAC for the South East fishery have not limited and cannot control catches. A revised OCS arrangement, or State trip limits, are needed to effectively reduce the fishing mortality of Silver Trevally (Caton and McLoughlin, 2005).

During the mid-late 2000s, a total of 29 fishing businesses which caught Silver Trevally between 2000 and 2005 were bought out by the N.S.W. Government, due to the implementation of the Batemans Bay Marine Park. The implementation of the Marine Park also closed significant areas of trawl ground to fishing by trawlers which remain active on the south coast, including productive grounds for Silver Trevally fishing in the area of Montague Island, and these areas are now habitat protection zones within the Marine Park where trawling is banned. It was expected that these actions would result in a very significant decrease in trawl landings of Silver Trevally from southern N.S.W. waters in 2008 (Shelf Resource Assessment Group, 2008f), but it is also noted that the Commonwealth portion of the TAC was doubled in 2008 to account for the corresponding reduction in N.S.W. State catch. Such action cannot be considered responsible to the need for an overall reduction in fishing pressure from both State and Commonwealth sectors (see below).

It is noted that in New Zealand, fish-spotting planes are used to identify schools of the required size and species composition, to assist with setting the gear. The pilots also record details of the surface schools sighted, and such data are used to derive estimates of abundance over space and time. Although the data are opportunistic and difficult to analyse (Taylor, 2003), they provide additional and potentially useful information to assist the management of fisheries for Silver Trevally.

In New South Wales, the Ocean Trawl Fishery is one of the major commercial fisheries in which Silver Trevally is taken, and a recovery program has been developed under the management strategy for that fishery. The recovery program includes the imposition of a minimum legal length of 30 cm (total length) for Silver Trevally in N.S.W., and changes to the selectivity of fish trawl cod-ends to allow trevally smaller than the minimum legal length to escape from the net (New South Wales DPI, 2004). One of the stated objectives of the fisheries management strategy for this fishery was to restrict fish trawl cod-ends to a maximum circumference of 100 meshes with a joining ratio of 1:1 to the next forward section of the net, and for cod-ends to be constructed of single twine (max. 6-mm diameter) 90-mm mesh netting. However, recognising that such regulations would eliminate the trawl-whiting fishery in its current form, an interim arrangement was proposed whereby fish-trawl endorsement holders could continue using cod-ends made from double-braided twine in depths less than 55m, subject to a commitment to implement the results of future research into the development of appropriately selective cod-ends (Graham, 2008). The results (of experiments designed to investigate the selectivity attributes of cod-ends with either different circumferences to the extension-section of the cod-end, or different twine diameters) demonstrated that the only benefit to fish trawlers from the use of the heavy 5-mm 200-mesh cod-ends was the retention of school whiting, as mean catches of almost all other commercial species were not significantly different to the other 90-mm treatment cod-ends. These findings have reinforced the argument that 100-mesh circumference cod-ends of lighter and/or larger mesh are required for greater selectivity of almost all other species in the fishery (including Silver Trevally), as canvassed in the Fishery Management Strategy for the Ocean Trawl Fishery (DPI 2007; Graham, 2008). Data from an onboard observer program is being used to assess the effectiveness of the recovery program in preventing the capture and marketing of large numbers of small trevally, including the recording of any discarding of trevally smaller than the new minimum legal length. Over time, information from associated research and the observer program will be used to determine if a more appropriate cod-end mesh size needs to be implemented when fishers are targeting silver trevally. (New South Wales DPI, 2004, 2007; Graham, 2008).

The recent review of N.S.W. fishing regulations introduced a 30cm (total length) minimum size for silver trevally (commercial and recreational) in N.S.W. waters from September 2007. The daily bag limit for recreational fishers in N.S.W. is 20, for all trevally species (New South Wales DPI, 2008d). In Victoria, there is a minimum legal size of 23cm and a bag limit / possession limit of 20 trevally (DPI Victoria, 2009). In Tasmania, the size limit is 20cm, and the previous daily bag limit and possession limit of 30 and 45 respectively (DPIWE Tasmania, 2004o) have been reduced to a possession limit of 15 (DPIWE Tasmania, 2009a).

In South Australia, the minimum legal size of Silver Trevally taken by recreational fishers is 24cm, and there is a daily bag limit of 20, and a boat limit of 60 trevally (PIRSA, 2009). In W.A., the previous minimum legal size of 20cm (Department of Fisheries, Western Australia, 2005a) has been increased to 25cm, and the previous State-wide bag limit of 20 Silver Trevally, has been reduced to 8 in the West Coast and Gascoyne bioregions, and 12 along the South Coast (Department of Fisheries, Western Australia, 2008a, 2008c, 2009a, 2009b).

In New South Wales, commercial catches of Silver Trevally are reported to have decline significantly following the declaration of a Recreational Fishing Haven in Botany Bay in 2002 (New South Wales DPI, 2008d).

The implementation of the Batemans Marine Park in New South Wales reduced commercial trawl catches of this species in the area, due to licence buy-back (New South Wales DPI, 2008d). Between 127t and 218t of P. georgianus was landed annually from 2003 to 2005 in that area. Consequently, the total Allowable Catch for Commonwealth catches was increased by 150t to 296t for 2008 as a result (Larcombe and Begg, 2008), to compensate for the reduced State catch.

Research Recommendations

Given the differences in population dynamics (including longevity and age at maturity) recorded for Silver Trevally in southern Australia compared with New Zealand, the relation between these populations needs to be better elucidated.

More research is required to determine the spawning sites and spawning behaviour (e.g. aggregation) of Silver Trevally. There is inadequate information about population sizes over space and time, and relationships between fished stocks across the range.

The Commonwealth’s Shelf Resource Assessment Group (AFMA, 2008f) has recommended that monitoring of size composition of Silver Trevally caught by the different fisheries and fishing methods be continued and improved.

More ageing data are required than the current amount (i.e. one year of data), to strengthen stock assessments (Shelf Resource Assessment Group, in AFMA, 2008f).

Inclusion of the recreational sector catches should be considered in stock assessments (Shelf Resource Assessment Group, in AFMA, 2008f).

The Shelf Resource Assessment Group recommended that gear selectivity work be undertaken on the range of gears used to target Silver Trevally, to allow assessment of the changes in gear that would be required to move the size at first capture towards the optimum. The implications for catches of other species of such a change need to be assessed (Smith and Wayte, 2005).

Water temperature has been suggested (Shelf Resource Assessment Group: AFMA 2008f) as an environmental driver for populations of Silver Trevally. The relations of population abundance over space and time to environmental cues should be investigated.

Management Recommendations

Spawning sites for Silver Trevally should be protected from fishing impacts.

Key data for adequate stock assessment of this species in south-eastern Australia do not exist, including reliable estimates of historic and current biomass and age structure(e.g. see Shelf Resource Assessment Group, 2008f).

A 2-year, FRDC-funded study in N.S.W. (Rowling and Raines, 2000) reported that a large proportion of the catch in N.S.W. is of reproductively immature fish. Management measures to ensure that fish taken in both State and Commonwealth fisheries have the opportunity to spawn before being caught, would assist the sustainability of Silver Trevally stocks in southern Australia.

The N.S.W. study showed that, according to preliminary yield models, a significant improvement in yield-per-recruit to the fishery could be achieved, despite the slow growth rate of Silver Trevally, by increasing the size at which the fish are first caught by commercial and recreational fishers (Rowling and Raines, 2000; Caton and McLoughlin, 2005). A minimum legal size was suggested for commercial fisheries, which would require that fish gear selectivity issues be addressed, for those fisheries that currently catch small trevally (Rowling and Raines, 2000). The size limit change was implemented in New South Wales waters in 2007. However, whilst the 30cm catch limit does not technically apply to catches by Commonwealth-licensed fishers in the South East fishery (SEF), it is likely to cause compliance issues regarding SEF catches landed in New South Wales. If this size limit was applied to SEF trawl catches it would result in dumping of commercially valuable fish if the current gear configuration is used (Shelf Resource Assessment Group, 2008f). Therefore measures to reduce the overall catch of small fish less than 30cm are needed across all jurisdictions. This may require significant adjustments to fishing gear and method, and/or permitted catch locations.

The fisheries for this species are multi-sectorial, and multi-jurisdictional. Commercial catches are significant at both State and Commonwealth level, and recreational catches are also large. These factors complicate management arrangements. For example, about two-thirds of the 2003 catch was taken by dual-endorsed South East fishery vessels from waters under State jurisdiction, and the 2001 to 2003 TACs were exceeded. It is reported that the majority (average 80% over the last 4 years to 2007) of the total estimated Silver Trevally catch in south-eastern Australia was taken by a number of commercial fisheries under State jurisdiction, and by recreational fisheries. Management arrangements for State fisheries during the past decade, have reportedly not been based on direct control of Silver Trevally catches, and average State catches of around 380t have been taken during the last four years to 2007 (Shelf Resource Assessment Group, 2008f). Landings from Commonwealth waters have been less than 100t in recent years. According to Caton and McLoughlin (2005), AFMA’s reductions in the Total Allowable Catch for the South East fishery have not limited catches, and cannot control catches. A revised Offshore Constitutional Settlement Arrangement, and/or State trip limits, have been recommended to more effectively reduce the fishing mortality of Silver Trevally (Caton and McLoughlin, 2005).

Stock assessment methods are not adequate, especially in southern Australia, where only catch per unit effort (CPUE) is used as a reference point (see AFFA, 2004a). In New Zealand, where Silver Trevally schools are targeted by purse seine, CPUE is considered to be a particularly poor estimator of abundance (Taylor, 2003). In southern Australian fisheries for this species, improved methods of estimating abundance over space and time are required. In the longer term, development of more detailed stock assessment methods, including a population model, should be undertaken. More robust fishery performance indicators are also required, to assess the sustainability of the fisheries catching Silver Trevally, and provide better quality advice for their management.

Quotas for commercial fishing of Silver Trevally in Commonwealth and State waters should be regularly reviewed and decreased, in light of the apparent depleted nature of stocks. Increasing the size at first capture, and reducing the overall fishing mortality (both State and Commonwealth waters) may be critical factors in recovery of growth over-fished Silver Trevally in south-eastern Australia (e.g. Smith and Wayte, 2005; Shelf Resource Assessment Group, 2008f).

Due to reduction in N.S.W. catches from the closure to commercial fishing of the Batemans Bay Marine Park, the Commonwealth allocation of the TAC was corresponding doubled in 2008, to almost 300t. This management decision was not in keeping with a conservative approach for an obviously over-exploited resource, and such State to Commonwealth trade-offs must stop, to bring about a total reduction in the number of fish permitted to be caught, not simply a jurisdictional reduction.

Monitoring of the size composition and quantities of Silver Trevally caught by different fisheries and fishing methods (both State and Commonwealth waters) needs to be continued and improved (Smith and Wayte, 2005; AFMA, 2007e).

Size and bag limits should regularly be reviewed in recreational fisheries. For example, concern was expressed in W.A. that a minimum size limit of 20cm was too small (e.g. Stagles, 2003b), and the size limit has now been raised to 25cm in the South Coast bioregion (Department of Fisheries W.A., 2008a).

Other Information

Recent research has indicated that Silver Trevally in southern Australia and northern New Zealand differs from the tropical P. dentex (Smith-Vaniz and Jelks, 2006), hence the previous species name, P. georgianus, has been restored (New South Wales DPI, 2008d).

In South Australia, Silver Trevally have been recorded in Aquatic Reserves such as Aldinga and Noarlunga (S.A. diving club records), and likely occur in a number of other reserves which contain suitable habitat.

The species has been recorded in or near a number of protected areas in Victoria, such as Yaringa, French Island, Churchill, Wilsons Promontory, Corner Inlet, Point Cooke and Beware Reef (Plummer et al., 2003).

In New Zealand, P. dentex has been recorded in reserves such as the Poor Knights Island Marine Reserve (full protected from fishing) (Denny et al., 2003), the partially protected (from fishing) Mimiwhangata Marine Park (Denny and Babcock, 2004), and the Wairoa Hard fish nursery area (fully protected) (Hawkes Bay Regional Council, 2004).

This species is found in various marine parks in New South Wales (such as Jervis Bay: Marine Parks Authority, 2008), and also in the Lord Howe Island Marine Park (Speare et al., 2004).