Biology and management of a multi-sector blue swimmer crab fishery in a subtropical embayment - Shark Bay, Western Australia

Abstract

Stock assessments to support sustainable management in data-limited fisheries present a challenge to fisheries scientists and managers. This is the case with the Shark Bay Crab Fishery, which has expanded rapidly in the past 10 years, to become Australia’s highest-producing blue swimmer crab fishery. The resource is harvested commercially by two sectors, the Shark Bay crab trap and trawl fisheries (combined catch of ~800 t), as well as supporting a small but important recreational fishery. Commercial catch and effort data have been collected for the fishery since the early 1980s, commercial trap-monitoring data since 1999, and fishery-independent trawl-survey data since 2001. There is conflicting evidence on the impact that significant increases in catch and effort over the past decade has made on this fishery, such as legal catch rates remaining relatively constant, but declines occurring in the abundance of large crabs. There has also been concern over the level of latent effort in the fishery, with the trap sector currently operating at 70–80% of its potential effort and the capacity for further increases in crab landings by the trawl fleet. Since July 2011, the relative abundance of all size classes of crabs declined significantly. The reasons for this unexpected decline are yet to be understood, but are likely to be linked to adverse environmental extremes (flooding and very warm water temperatures) during the summer of 2010–2011, associated with a very strong La Niňa event. Preliminary assessment indicated that the spawning stock that led to the low recruitment was within historic ranges. The current challenge for the research and management of this fishery is to clarify the causes for this recent decline, and establish suitable biological indicators that will determine the appropriate level of catch and harvest strategy to ensure the future sustainability of the stock.