Gemfish (Rexea solandri) is a benthopelagic snake mackerel of the Family Gempylidae. Gemfish is found on the continental shelf and slope in southern, southwestern and southeastern Australia and New Zealand. It is found at depths ranging from 100 to 800 metres, but commonly at 300 to 450 metres. Historically Gemfish formed part of a large trawl fishery off the east coast of Australia in the 1970s and 1980s. Catches peaked between 1978 and 1980 at around 5,000 tonnes per year, declining substantially after 1987. The east coast fishery remains in an overfished state with an unavoidable bycatch limit of 100 T. The most recent stock assessment estimated spawning biomass at 15 per cent of the 1968 level.
Fisheries for Gemfish have operated off western Tasmania, south western Victoria and south eastern South Australia as part of the Commonwealth Trawl Sector (CTS) of the Southern and Eastern Scalefish and Shark Fishery (SESSF) and in Great Australian Bight as part of the Great Australian Bight Trawl Sector (GABTS) of the SESSF. These fisheries are managed as a separate stock to the east coast and have a substantially different fishing history and level of exploitation. The most recent stock assessment of the western stock suggested biomass is at 74 per cent of virgin biomass levels.
Great Australian Bight Resources Assessment Group (GABRAG) had sufficient concerns regarding the population structure of the western stock and its impact on the stock assessment (whether western Gemfish constituted a single population and where the boundary between east and west arises) to reject the assessment.
Previous research found genetic subdivision between the eastern stock of Gemfish (eastern Australia including eastern and western Tasmania) and a western stock (South Australia, GAB and Western Australia). Mitochondrial DNA (mtDNA) data showed differences following the same pattern as previous data, but suggested a much stronger division between the two stocks. Both data sets suggested there were no genetic differences between eastern Australia and New Zealand. However, it was unclear whether these findings were confounded due to scientific design (i.e sample size).
This project was initiated through GABRAG to address these stock structure issues and was a collaboration between the Australian Bureau of Agricultural and Resource Economics and Sciences (Andy Moore) and the Molecular Fisheries Laboratory at the University of Queensland (Jenny Ovenden, Carlos Bustamante and Andy Moore). The expectation was that this study would confirm the distinction between the eastern and western stocks, particularly as the classes of genetic markers used for this study mirrored those used previously.
The objectives of this project were to:
1) Improve understanding of stock structure for Western Gemfish west of Bass Strait;
2) Improve understanding of spawning locations for western Gemfish west of Bass Strait; and
3) Provide the Australian Fisheries Management Authority with recommendations on stock structure and boundaries on the basis of this evidence.
Fish were sampled from the mid-western Great Australian Bight (GAB), from two locations off the southwestern Victorian coastline at Kangaroo Island, (KI) and Portland (Por), from one location in western Bass Strait (WBS) and from one location off the coast of western Tasmania (WT).
To provide genetic comparisons among these target populations, fish were also sampled from the east coast of Tasmania (ET), the eastern coast of New South Wales (NSW) as well as New Zealand (NZ).
Previous research had found large genetic differences between eastern and western populations. However there was large sample size variation between both locations and it was unclear if the genetic differences were the result of biological processes or a sampling artefact. Archived historical samples from this study were obtained from the Australian Museum to confirm the previous results and to test for temporal stability of genetic patterns.
This study confirmed that there are two distinct stocks of Gemfish in Australia, with western Bass Strait the boundary between both stocks. The level of differentiation between the stocks for all genetic markers was high. This, along with the largely fixed mitochondrial haplotype differences between populations indicated minimal gene flow between stocks. This level of genetic structuring for a migratory marine finfish species with planktonic larval dispersal and contiguous distribution is very rare. For fisheries management purposes Gemfish to the east and west of Bass Strait can be managed as separate management units.
Immigrant Gemfish were found in both east and west stocks. These immigrants were classified as hybrids as they had the nuclear DNA from one stock and the mitochondrial from the other. However, if full hybridisation is occurring the introgression of genetic material between stocks would lead to the breakdown of stock boundaries. The levels of genetic subdivision between stocks detected in this study indicates that this is not occurring.
The study also found evidence for a smaller genetic effective population sizes in eastern Gemfish than western Gemfish. The results are preliminary but warrant further investigation as they may provide insight into why the eastern Gemfish population is not recovering from its overfished state.
This research has delineated two stocks of Gemfish in Australia and defined the boundary between both stocks. These data are very useful for assigning data for the stock assessment and for changing the management boundary between both stocks.
The report recommends that both eastern and western Gemfish stocks be treated as separate management units and the management boundary between both stocks be moved to a more appropriate location to better reflect both genetically distinct populations. The report also recommends further investigating the small effective population size found for eastern Gemfish.
