Sustainability of the rock lobster resource in south-eastern Australia in a changing environment: implications for assessment and management

This project has improved assessment of the sustainability of the southern rock lobster resource throughout southeastern Australia. Investigating whether declining catch rates reflect an actual decline in biomass or are a result of changing catchability or recruitment has improved our understanding of the environmental and catchability impacts on the rock lobster stocks. Consequently, this will improve application of the SRL- assessment model and improve our confidence in the modelling predictions that underpin sustainable management. Victorian trends in CPUE were not biased by data screening and data selection criteria in preparation for standardisation of CPUE, which provides a more reliable indicator of relative abundance than nominal CPUE (as reported by fishers). Setting TACCs in Victoria involved application of the SRL-fishery stock-assessment model using nominal CPUE up to and including 2011, but this was changed to use standardised CPUE for the 2012 and 2013 assessments. Although CPUE standardisation adjusts for the effects of fishing-year, fishing-month, longitudinal-range, depth-range, and vessel-fisher, it is not feasible, nor appropriate, at this time to incorporate environmental variables into CPUE standardisation for two reasons. One reason is that for the environmental variables tested, the daily fluctuations in CPUE when averaged over month or year have small or negligible effect on the pooled CPUE. The other reason is the lack of ongoing data on key environmental variables such as bottom temperature and dissolved oxygen at spatial and temporal resolutions compatible with the CPUE data. Hence, in the foreseeable future, any detected or hypothesised effect of environmental variables on CPUE or SRL abundance will need to be handled through the SRL-assessment model rather than through CPUE-standardisation models. The present project confirms spatial trends in puerulus settlement indices are similar in most parts of south-eastern Australia suggesting large-scale oceanographic processes are driving settlement. Puerulus monitoring is a relatively robust indicator of future fishery performance in terms of stock size and CPUE, and should therefore be regarded as important data for providing management advice for SRL resources in south-eastern Australia. Examination of seasonal stock depletion in Victoria provides a basis for linking catchability values across selected months in the SRL-assessment model, which can now estimate monthly values separately or grouped. Intra-annual cycles of depletion and recovery indicate growth and recruitment are protracted where females precede males by two months. Inter-annual differences in catchability allow correction of CPUE for changing fishing efficiency. Growth-transition matrices for each sex separately, updated and used in the SRL-assessment model for annual setting of TACCs in Victoria, are now available for the late 1970s, late 1990s and 2000s in each of the main regions and indicate large differences in growth rates between particular sites, but only subtle differences over time. Bimonthly growth-transition matrices are applied monthly for each of six months to account for the SRL moultgrowth- recruitment during the second half of the fishing year (November–September) in Victoria. Management strategy evaluation demonstrates the importance of periodically updating growth transition matrices from on-going tagging programs and the sensitivity to trends in recruitment and catchability for success in accurately determining the TACCs required to meet prescribed fishery management goals. Trends in recruitment