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.
Final report
These objectives of the research were achieved, in particular standardised meshed pot catch and release surveys were highly useful to establish the extent of the low catch zone and that sub-legal to early juvenile lobster were found to be indicative of the low catch zone. The iterative assessment process, presented to fishers over multiple workshops, indicated that loss of essential habitat, relating to early juvenile lobster survival or recruitment success, was the most likely causative factor of the low catch zone.
To inform the stock assessment and management of the fishery, the project has highlighted the importance of data on 1) the abundance of sub-legal lobster in near shore habitats and 2) monitoring change in/condition of near shore habitats as a potential indicator of early juvenile lobster survival or recruitment success for the stock assessment and management of the WRL fishery. The project has highlighted that limited historical data is available on these potential indicators, but new FRDC WRL IPA funded projects have subsequently been created to further synthesise available information and collect new data for both early juvenile / pre-recruit lobster abundance (FRDC 2019-159 Independent Shallow Survey) and condition and change in near shore habitats (FRDC 2019-099 Habitat as a limit to Western Rock Lobster recruitment) to further inform the stock assessment and management of the fishery.
