© 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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Derwent Flounder

Family Name:	Pleuronectidae
Scientific Name:	Taratretis derwentensis Last, 1978
Recommended Status in S.A:	Data Deficient
Rationale:  The species is included here because (i) South Australia is at the edge of geographic range of this mainly south-eastern species; (ii) there are few records of Derwent Flounder in S.A., and the distribution and relative abundance within S.A. are both poorly known; (iii) the species occurs in shallow sandy bays and estuaries, and may thus be vulnerable to numerous coastal impacts, but species-specific impact studies have not been undertaken; (iv) there is little indication of the total commercial and recreational catch of this species (which would be largely bycatch, because the species is not targeted), in relation to its relative abundance and distribution, including insufficient information about the extent of capture in prawn trawl fisheries (including all parts of S.A.); and (v) there is very little existing information about the relative abundance, biology and population dynamics of this species across its range.

Page Contents

Current Conservation Status

(No listings known)
 

Distribution

Southern Australia

The species occurs in southern Australia, from Bermagui (New South Wales) and Tasmania waters, through to S.A. (Gomon, in Gomon et al., 1994).
T. derwentensis is considered “rather common” in the sandy areas in which it is found (Gomon, in Gomon et al., 1994). During surveys of nearshore habitats in Tasmania, the species has been recorded from areas such as Stanley Bay (north-west coast), the D'Entrecasteaux Channel (DPIWE, 2002), and Derwent Estuary (Edgar et al., 1999), Prosser Bay, and Frederick Henry Bay (Jordan et al., 1998).
IMCRA Technical Group (1996) considered Taratretis derwentensis (Last, 1978) to be one of the indicator species for the Tasmanian Province, which extends from the north-eastern tip of Tasmania at Cape Naturaliste, encircling the west, south and east coasts, up to Cape Grim at its north-western extremity. The species was first described from the Derwent River estuary, near Hobart (Last, 1978, cited by Gomon, in Gomon et al., 1994, and Eschmeyer, 2001).
 

South Australia

South Australia is the edge of the species’ range, and examples of locations where the Derwent Flounder has been recorded in S.A., include eastern Investigator Strait / “heel” of Yorke Peninsula area; Spencer Gulf; Venus Bay and Anxious Bay area and the Ceduna area in the eastern Great Australian Bight (Gomon, in Gomon et al., 1994; Museum of Victoria records, South Australian Museum records, cited in OZCAM database, 2008).
 

Habitat

The species occurs in fine sandy habitat in protected coastal waters, at depths between 3m and 40m (Gomon, in Gomon et al., 1994) or 50m (Kuiter, 1996b; May and Maxwell, cited in Froese and Pauly, 2008).
In Tasmania, the species has been recorded in nearshore waters in various bays, in Heterozostera tasmanica seagrass, sand and mud habitats (Jordan et al., 1998).
 

Notes on the Biology

The species is small, growing to at least 9cm (Gomon, in Gomon et al., 1994) or 12cm (May and Maxwell, 1986, cited in Froese and Pauly, 2007; Kuiter, 1996b).
Flounders live on the sea floor, but are pelagic spawners (Nelson, 2006).
 
 
 
 

Fisheries Information

Commercial

Little species-specific commercial fishing data are available. However, it is noted that flounder species (unspecified) are taken in minor quantities by commercial fishers in southern Australian States (see section above, on Commercial Fishing for flounders as a group).
Although Taratretis is listed as a minor component of the commercial catch of flounder in Tasmania (which comprises mainly Rhombosolea tapirina, with a lesser component of A. rostratus) (Smith and Heran, 2001; Ziegler et al., 2006), this small species would be very unlikely to be speared or gillnetted, and any bycatch in trawls or dredges would be discarded (G. Edgar, TAFI, pers. comm., 2007).
In S.A., the Derwent Flounder has been recorded from areas in which prawn trawling operates (e.g. GSV / Investigator Strait, Spencer Gulf, and Anxious Bay on the west coast); however no data are available on bycatch of this species, if it occurs.
When caught in the bycatch of the S.A. Rock Lobster fishery, flounders are permitted to be retained for sale (Sloan, 20003a).
 

Recreational

Recreational survey catch statistics (e.g. Henry and Lyle, 2003) group recreational catches of flounders with soles and other flatfish, and summary statistics, as well as other State-level data, are provided in the section on Recreational Fishing, at the beginning of this chapter.
In southern States of Australia, legal minimum sizes for flounder are all larger than the maximum size of this species, which would preclude it from being taken as a recreational species. 
 

Vulnerable Characteristics of the Species, and Threatening Processes

South Australia is at the edge of the species’ range.
Benthic fishes in general have limited mobility, more localised reproduction than most pelagic species, and limited opportunity for population dispersal, all of which can increase the vulnerability of populations to decline.
Derwent Flounder occurs in coastal areas such as estuaries and shallow sandy bays, and populations may thus be vulnerable to decline in coastal areas where impacts degrade the habitat. There are few pristine or near pristine estuaries remaining in South Australia, and most are subject to numerous impacts (see Bucher and Saenger, 1989; Lewis et al., 1998; Barnett, 2001; GeoScience Australia, 2001; Baker, 2004; Gillanders et al., 2008). A summary of many of the impacts in shallow coastal bays in S.A. is provided in Baker (2004). Also in other States, this species occurs in polluted bays and estuarine areas (e.g. the Derwent estuary in Tasmania, in which point sources of pollution include 10 sewage treatment plants and two large industries (paper mill and smelter), coupled with diffuse sources from urban runoff; waste / rubbish tips and contaminated sites; catchment inputs carried by the Derwent and Jordan Rivers; atmospheric contributions and wastes associated with shipping operations, port facilities and marinas (Green and Coughanowr, 2003). Some pollutants are also derived from contaminated sediments within the estuary itself. Contaminants associated with these various sources include pathogens, nutrients, organic matter, silt and gross solids, wood extractives such as resin acids, and a range of toxicants including heavy metals and hydrocarbons (Green and Coughanowr, 2003).
Other than for R. tapiria (see synopsis), there are few specific studies on the impacts of pollutants on flounders in Australia, but it is noted that in the northern hemisphere, sub-lethal effects of sediment contamination (from municipal sources such as  sewage; also industrial pollutants, and other contaminants such as tri-butyl tin from ship and boat anti-foulants) have been recorded. Impacts upon flounders from contaminants include changes in body chemistry, hepatic and other lesions, and reproductive impairment (e.g. Johnson et al., 1993; Myers et al., 1998; CSTEE, 1999; WWF, 1999; Rice et al., 2000).
Flounders are vulnerable to capture in prawn trawls, due to their benthic nature, existence in the same habitats as prawns are found, and their poor swimming ability (Carrick, 1997; PIRSA, 2003). The species is found in the prawn trawling grounds in S.A. (e.g. lower central GSV / Investigator Strait, central Spencer Gulf, and off the Anxious Bay area on the west cost). Mortality due to trawling may be a threatening process for some of flounder species of lesser abundance in S.A., but no species-specific data are available. To date, the species apparently has not been recorded in the limited species-specific studies of flounder bycatch in Spencer Gulf (e.g. Carrick, 1997). More recent studies of prawn trawl bycatch in Spencer Gulf (e.g. Dixon et al., 2005) grouped flounders and soles together as “flatfish”. No information is available on the prawn trawling bycatch composition or abundance in GSV, or in the eastern GAB / western Eyre Peninsula. Therefore, it is not possible to comment on the extent to which trawling may be a threatening process for Derwent Flounder populations in South Australia.
Tanner (2003 and 2005) discussed the impacts of prawn trawling on habitats and benthos in Gulf St Vincent, and this species has been previously recorded in these trawl grounds
 

Research Requirements

More information is required on the current geographical distribution and relative abundance of Derwent Flounder in South Australia, which is at the edge of the geographic range. Derwent Flounder is little known in this State, with few published records.
There is very little information on the biology, and population dynamics of this species. 
Where possible, flounders in prawn trawling bycatch from Gulf St Vincent, Spencer Gulf and the West Coast of S.A. should be recorded, with data analysed and monitored to determine the presence and relative abundance of this species over time. 
 

Management Requirements

Documentation and quantification of flounder bycatch in fisheries across the geographic range is required (see above).
As indicated in Research Requirements above, in S.A. a system should be developed for the ongoing collection and monitoring of bycatch data from the Gulf St Vincent Prawn Fishery and the West Coast Prawn Fishery (as currently occurs in the Spencer Gulf Prawn Trawl Fishery), sufficient to enable identification of long-term trends in bycatch (Australian Government DEH, 2004b; Dixon et al., 2005).
Measures (such as improvements in net design) to reduce the bycatch of benthic fish species in all trawl fisheries are recommended (see Management Notes).
An assessment of the sustainability of commercially fishing this species (e.g. in Tasmania) is required.
Further recreational fishing controls (e.g. legal minimum sizes, and possession limits) are required in areas where these do not yet exist, and enforcement is also required.
Generally required is the ongoing management and mitigation of the multiple impacts that have reduced the quality of estuarine habitats in S.A. and other southern States. This includes protection of shallow coastal bays from land-based discharges, and, where possible, from coastal activities (such as dredging, and aquaculture developments) that can reduce benthic habitat quality in such bays.  
 

Management Notes

It is noted that prawn fisheries in southern Australia have made significant efforts during the past decades to reduce the bycatch of finfish species. Examples for Spencer Gulf and Gulf St Vincent include the spatial and temporal organisation and “real time” management of the fishing fleet in some areas (e.g. Spencer Gulf) to minimise capture of undersized prawns and bycatch species, and developments in gear design to reduce bycatch, such as square-mesh cod-ends, bycatch chutes, hopper/conveyor systems, and the fitting of exclusion devices (e.g. MacDonald, 1998; Carrick, 1997; Broadhurst et al., 1999; South Australian Prawn Industry Association web site, 2000; PIRSA, 2003). PIRSA (2003) reported that field studies have indicated that survival of flounders (e.g. the commonly caught species Pseudorhombus jenynsii) is higher with hopper/conveyors systems, compared with the previously used sorting equipment. However, Australian Government DEH (2004b) reported that further work is required to confirm this assumption. 
 

Other Information

The species was long overlooked due to its small maximum size (Gomon, in Gomon et al., 1994).