Assess causes and implications of anomalous low lobster catch rates in the shallow water areas near the centre of the Western Rock Lobster fishery
University of Western Australia (UWA)
Tim J. Langlois
A reduction in fishing effort in 2008 and change to quota management in 2010 resulted in record low harvest rates and high biomass levels across the Western Rock lobster fishery. Counter to this trend however has been the increasingly low catch rates that have been observed in the shallow water areas (<20 m) near the centre (and possibly other areas) of the fishery over a much longer time period. The adjacent deep water areas show good catch rates comparable with the rest of the fishery. The low catch rates in the shallow water (<20 m) of the fishery are particularly surprising given the relatively high levels of puerulus recruitment immediately to the north and south of this region, which would suggest there should be high levels of sub-legal and legal biomass, and thus good catch rates in the shallows. Unlike areas that have recently been impacted by the 2011 marine heat wave (e.g. Kalbarri), the processes behind the atypical catch rates in this central shallow water region are unknown and appear to form a worsening long term trend. In the short term, this trend could impact stock assessments. In the longer term, an expansion of a low catch rate region could result in significant reduction in the overall productivity of the fishery. Understanding the processes behind the unexpected low catch rates will allow prediction of future trends, management adaptation and the potential for mitigation.
1. Determine the spatial extent and temporal trends in regions exhibiting abnormally low legal catch rates throughout the lobster fishery.
2. Identify the lobster life stage(s) resulting in abnormally low legal catch rates in the main area of low catch rates.
3. Examine factors that may be causative of the abnormally low legal catch rates.
4. Identify the implications of the low-catch regions to the stock assessment and management of the fishery.