Declining catches in the Western Zone of Victoria, the Southern Zone of South Australia, and the NW region of Tasmania have been observed over the past 4–5 years. These areas have always been the most productive and most important part of the south-eastern rock lobster fishery, and the declines in observed CPUE do not align with median predictions from each State’s stock assessment models. The lobster fishery in each State is managed primarily with output controls using Total Allowable Catches determined by stock assessment modelling. CPUE is used by the model as a proxy for lobster abundance and the observed trend may be caused by below average recruitment. However, catch rates may also be driven by exogenous changes in catchability influenced for example by environmental effects, fleet dynamics, fisher behaviour, or rock lobster behaviour. Hence, there is a pressing need to determine whether the observed falling CPUE represents an apparent decline in relative abundance caused by reduced catchability or an actual decline caused by reduced recruitment, reduced growth, or increased natural mortality, or a combination of these factors. Importantly, what are the implications for future assessments and what monitoring and management strategies are most robust in the face of these uncertainties.
Project number: 2009-047
Budget expenditure: $651,086.00
Principal Investigator: Adrian Linnane
Organisation: Agriculture Victoria
Project start/end date: 31 Mar 2010 - 29 Sep 2012
1. Undertake initial evaluation of catch and effort data for a selection of vessels (or skippers) for CPUE standardisation and undertake spatial analysis of rock lobster to depict annual CPUE trends within discrete regions standardised for effects of vessel (or skipper), season, and spatial cell defined by grid-cell and depth range.
2. Extend CPUE analyses to test for and standardise for, where feasible, the effects of oceanographic variables such as bottom temperature, dissolved oxygen, currents, and wave strength using available data from the Bonney Coast and then test the applicability of these results to western Tasmania and determine additional data requirements for extending the analyses to this region.
3. Apply various analyses such as within-season depletion models and each State’s stock assessment models using available catch and effort data, other monitoring data, and tag release-recapture data to explore variation in annual estimates of catchability and recruitment through time.
4. Investigate evidence for temporal trends in lobster recruitment across the three States, examine evidence of a declining trend since 2003, and examine relationships between yearly environmental signals, and the yearly puerulus index to yearly environmental signals.
5. Undertake growth analyses of available tag release-recapture data to explore variation in annual estimates of growth through time.
6. Undertake stock assessment modelling to explore the sensitivity of biomass projections to altered values of catchability, recruitment, and growth, and, if necessary, make appropriate corrections to components of the stock assessment models.
7. Undertake management strategy evaluation, testing stock assessments and exploring implications of alternative assumptions for catchability, recruitment, and growth.
Author: Adrian Linnane