Developing a comprehensive understanding of the movement patterns and stock structure of a fish species is crucial for identifying the appropriate scale and strategy for management.
In south eastern Australia, there are several adjacent Snapper fisheries operating in South Australian (SA), Victorian and Commonwealth waters that are managed using different strategies. The SA fishery, currently Australia’s largest Snapper fishery, is divided into contiguous regions whose relative contributions to total catch have changed dramatically in recent years. The extent to which these different regional or jurisdictional populations represent a single or multiple stocks is poorly understood because of the limited understanding of the patterns of fish movement. For example, have the recent high catches of Snapper from Northern Gulf St. Vincent and South East resulted from high levels of biomass built up through local demographic processes or have fish moved in from adjacent regions or possibly even from Victorian or Commonwealth waters? If large-scale movement is involved, it must be temporally complex as it appears to not conform to a regular, annual pattern. The need here is to elucidate the regions of origin and movement patterns of fish that currently contribute to high regional catches in SA to inform about the demographic processes that drive the spatial and temporal variation in fishery productivity. This will point to the appropriate spatial scale for management. It would also provide insight for resource allocation amongst the different fisheries, which is currently being considered by AFMF to improve resource sharing arrangements for Snapper.
The study included the largest and most comprehensive application of an otolith chemistry study for snapper, as well as the first completed acoustic telemetry study on snapper in Australia. From the various findings, hypotheses were developed to account for the recent regional trends in population dynamics. Fundamentally, these relate to inter-annual variation in recruitment to the three primary nursery areas of south eastern Australia, i.e. Northern Spencer Gulf (NSG), Southern Spencer Gulf (SSG), and Port Phillip Bay, Victoria (PPB). The other regional populations of South Australia depend on supplementation through emigration from these source populations. The latter regions included: the South East (SE) which depends on recruitment into and emigration from PPB; Southern Spencer Gulf (SSG) and probably also the west coast of Eyre Peninsula (WC) that depend on emigration from Northern Spencer Gulf (NSG); and Southern Gulf St. Vincent (SGSV) that depends on movement from Northern Gulf St. Vincent. The relationship between the three nursery areas and the regions to which fish emigrate determines the stock structure. As such, the three stocks that influence fishery catches in South Australia are the SG/WC Stock, GSV Stock and Western Victorian Stocks.