Long-term drivers of catch variability in south-eastern Australia’s largest portunid fishery

Portunid crab fisheries are socially and economically valuable yet are characterised by high inter-annual variability. Competing hypotheses about factors contributing to this variability concern the environment, climate, and fisher behaviour. Here, we investigate variability in commercial catch, through a case study of the most heavily exploited estuary in southeastern Australia (Wallis Lake). Four main hypotheses were identified based on the broader portunid published literature: 1) Freshwater flow may cause poor recruitment due to increased offshore spawning and unfavourable dispersal of larvae, 2) Winter and spring harvesting may impact the following summer harvest by removing the spawning stock biomass (recruit overfishing); 3) Environmental factors including sea surface temperature and onshore winds may influence supply-side processes and recruitment success, contributing to catch fluctuations; and 4) Climatic indices such as the Inter Pacific Oscillation and Pacific Decadal Oscillation (PDO) may correlate with longer-term fluctuations in the fishery. We find evidence in support of hypotheses 2 and 4, with winter harvest negatively impacting the following January - April harvest and the PDO being positively correlated with January - April catch rates. Harvesting in June – November disproportionally impacts large females, and it may be possible to incorporate controls on winter fishing effort through co-management arrangements to achieve the best economic, environmental and social outcomes from the fishery. Such controls may lead to improved summer catch rates by protecting unspawned eggs during the winter months. The hypotheses explored here may provide insight in the variability observed in portunid fisheries around the world.