Defining the Stock Structure of Northern Australia's Threadfin Salmon Species

The requirement for Queensland, Northern Territory and Western Australian jurisdictions to ensure sustainable harvest of fish resources relies on robust information on the resource status. In northern Australia management of inshore fisheries that target blue threadfin (Eleutheronema tetradactylum) and king threadfin (Polydactylus macrochir) is independent for each of these jurisdictions. However, the lack of information on the stock structure and biology of threadfins means that the appropriate spatial scale of management is not known and assessment of the resource status is not possible. Establishing the stock structure of blue and king threadfin would also immensely improve the relevance of future resource assessments for fishery management of threadfins across northern Australia. This highlighted the urgent need for stock structure information for this species. The impetus for this project came from unsuccessful attempts in Queensland to conduct stock assessments for the king and blue threadfin resource, research that indicated the potential for localised stock structure, and the assessment that blue and king threadfin in Western Australia were fully and over-exploited respectively. The project objectives were to determine the stock structure of blue and king threadfin across their northern Australian range, and use this information to define management units and their appropriate spatial scales. We used multiple techniques concurrently to determine the stock structure of each species, including: genetic analyses (mitochondrial DNA and microsatellite DNA), otolith (ear bones) stable isotope ratios, parasite abundances, and life history parameters (growth and size at sex change). This holistic approach to stock identification gave the advantage of using techniques that were informative about the fish's life history at different spatial and temporal scales, increasing the likelihood of detecting different stocks where they existed and providing greater certainty in the signals given by the data. Genetics can inform about the evolutionary patterns as well as rates of mixing of fish from adjacent areas, while parasites and otolith microchemistry are directly influenced by the environment and so will inform about the patterns of movement during the fishes lifetime. Life history characteristics are influenced by both genetic and environmental factors. We adopted a phased sampling approach whereby sampling was carried out at broad spatial scales in the first year at locations along the east coast, within the Gulf of Carpentaria (GoC), and the Western Australian coastline. Using each of the techniques to compare fish samples collected from each of these locations we tested the null hypothesis for each species that they were comprised of a single homogeneous population across northern Australia. The null hypothesis was rejected after the first year leading us to re-sample the first year locations to test for temporal stability in stock structure, and to assess stock structure at finer spatial scales by sampling at other locations as well. Blue threadfin showed strong site fidelity with localised stock structuring evident and adjacent stocks separated by only tens of kilometres. This was found even where continuous habitat was present along coastlines with no obvious barriers to mixing. This was shown by clear and consistent signals of differences between fish from different locations including genetic differences. Blue threadfins also show what is called 'isolation by distance' whereby the farther apart stocks are from one another the greater the genetic differences between them. There was also extreme variability found in the life history characteristics among the different stocks. Similarly, king threadfin also showed fine scale stock structure with limited mixing between adjacent stocks separated by tens to hundreds of kilometres. Where there was sufficient distances separating them, or bio-geographical barriers such as headlands separating adjacent stocks, king threadfin were also genetically distinct. King threadfin also exhibited 'isolation by distance' though the pattern was notas strong as in blue threadfin. King threadfins also show a high degree of variation in their life history characteristics among the different stocks identified. Further, in the eastern Gulf of Carpentaria evidence of overfishing of king threadfin was evident in the truncation of size and age structures compared with samples taken over a decade ago, and the presence of females much smaller than found elsewhere or reported from the same region previously. The management implications of these results indicate the need for management of threadfin fisheries in Australia to be carried out on regional scales much finer than are currently in place. Given the fine spatial scale stock structure evident for both threadfin species management at local scales may not be pragmatic. At the very least management should consider these spatial dynamics by implementing monitoring and assessment of threadfin fisheries guided by the stocks identified in this study, and by the likely spatial scale of stocks indicated by these results. We also encourage the assessment of the threadfin resource status for the major fishery region in northern Australia. We recommend that the signals of overfishing detected for king threadfin in the Gulf of Carpentaria need to be investigated to assess the status of the stocks present in that region.