Determining variation in catchability of western rock lobsters to standardise important abundance indices used for stock assessment
Department of Primary Industries and Regional Development (DPIRD) WA
An outcome of a recent review of the WRL fishery (FRDC 2015-236) was to conduct a sensitivity analysis on a range of parameters utilised by the Western Rock Lobster (WRL) stock assessment model. This analysis highlighted that stock projections and MEY analysis are both sensitive to lobster assumptions associated with the catchability of lobsters during IBSS surveys. Recent stock estimates consider the population of the western rock lobster to be at record high levels. Therefore, even relatively minor differences in lobster catchability can have profound effects due to the multiplicative effect of increased stock size. With the recent marine heatwave off the WA coast producing record-high, projected increases in water temperature, and the substantial increased population abundance and size composition, there are likely to have been substantial changes in the catchability of lobsters during the IBSS, which may be impacting on the stock projections being produced for the fishery. The continued reliance of the stock assessment on independent indices while commercial fishing patterns (fishery dependent indices) become more consistent, requires that the variation in lobster catchability during these surveys is accounted for. If industry seek to increase WRL quotas in future seasons to take full advantage of the recent free trade agreement with China, it will be beneficial if the stock assessment is based on reliable indices of abundance to determine the appropriate TAC. Setting the quotas too low would affect industry’s profitability, while setting them too high would reduce fishers’ catch rates, affect the spawning stock and risk the sustainability of the fishery. Accounting for the biases impacting on abundance indices use to drive the stock assessment model (thus improving the accuracy of the model) will allow the fishery to maintain fishing at an appropriate level that achieves its harvest strategy target of maximising its economic yield and maintains a healthy spawning stock.
1. Determine factors (environmental and biological) causing variation in trap catchability of western rock lobsters
2. Quantify the relationships between factors and western rock lobster catchability
3. Standardise the empirical indices used for western rock lobster stock assessment for effects of catchability