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Title:

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

Project Number:

2009-047

Organisation:

Agriculture Victoria

Principal Investigator:

Adrian Linnane

Project Status:

Completed

FRDC Expenditure:

$651,086.00

Program(s):

Environment

Need

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.

Objectives

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.

Final Report - 2009-047-DLD - Sustainability of the rock lobster resource in south-eastern Australia in a changing environment: implications for assessment and management

Final Report
ISBN:978-1-921563-49-2
ISSN:
Author(s):Adrian Linnane
Date Published:September 2013
Principal Investigator: Adrian Linnane 

Keywords:  CPUE Standardisation; CPUE environmental correlation, recruitment, growth, catchability, MSE

Summary
:
Outcomes achieved to date:
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 and pre-recruitment are robustly monitored, but determining trends in catchability remains problematic.​