Pass the batten! Alternative pot design increases catch efficiency in a Southern Rock Lobster fishery

The South Australian Rock Lobster Fishery uses baited pots, traditionally ‘beehive’ in shape, to capture Southern Rock Lobster (Jasus edwardsii). Fishery harvest is controlled through annual quota that is set relative to performance indicators of relative abundance (Catch Per Unit Effort) for legal-size and pre-recruit lobsters. Under quota-based controls, improvements in catch efficiency through alternative pot designs offer an opportunity to reduce input costs and improve net economic return with low risk to stock sustainability. However, for performance indicators such as CPUE to remain robust, any changes in fishing efficiency must be accounted for in stock assessment. This study collected data from 13 fishers over 768 sampling days resulting in five treatments and 14,006 individual potlifts from the South Australian Northern Zone Rock Lobster Fishery. Geo-statistical methods, developed to control for temporal and spatial covariates, and variable lobster abundance, indicated higher catch efficiency of legal-size and undersize lobsters in ‘batten’ pots compared to ‘beehive’ pots. Ratios of mean legal-size catch weight $\left({\hat{\rho }}_{C_W\mathrm{PUE}}\right)$and undersize lobster (number) (${\hat{\rho }}_{\mathrm{PRI}})$from beehive pots to batten pots were estimated to be $0.62$ and $0.68$, respectively. Applying the ratio $\left({\hat{\rho }}_{C_W\mathrm{PUE}}\right)$with respect to effort, fishers adopting batten pot designs may reduce future effort (potlifts) to take quota by up to 38 %. Potential increases in undersize catches of up to 32 % for fishers using batten pot designs would be offset by an overall reduction in effort. The taxonomic composition of bycatch was similar in batten pots and beehive pots. Generally lower catches of all bycatch were observed from batten pots and further reductions in bycatch discard rates would be likely where effort is reduced via their use to take quota. The number of depredated lobsters recorded during testing was similar between batten pots and beehive pots also indicating that reduced effort to attain quota with batten pots could lower the absolute number of dead lobsters landed each season. Methods to account for differences in pot-specific catch efficiency in future harvest strategy decision rules are discussed.