Assessing the effectiveness of size limits and escape gaps as management tools in a commercial rock lobster (Jasus edwardsii) fishery

Minimum legal sizes (MLSs) and escape gaps are two commonly used management tools within commercial lobster fisheries. This study examined their effectiveness within the spatially expansive (~207,000km²) Northern Zone rock lobster (Jasus edwardsii) fishery of South Australia. Firstly, spatially explicit estimates of size of maturity (SOM) and relative reproductive potential were estimated to examine if the current minimum legal size of 105mm carapace length (CL) was appropriate. Size of maturity ranged from 105 to 118mm CL with 90% of reproductive potential coming from size classes above 105mm CL. SOM generally tended to increase from east to west and indicated that while the current MLS of 105mm CL may be appropriate for Marine Fishing Areas (MFAs) in the eastern part of the fishery, a higher MLS (>110mm CL) should be considered for MFAs in the western region of the zone. As size of maturity is age, rather than size dependent, these results may reflect differences in growth rates and highlight the importance of considering spatial variation in reproductive characteristics when applying size limits to lobster resources. Escape gap effectiveness was examined by estimating catch rates of undersized lobsters and commonly caught bycatch species, before and after mandatory introduction in 2003. After 2003, catch rates of undersized lobsters decreased by >60% while blue-throat wrasse (Notolabrus tetricus) and leatherjacket species (Meuschenia hippocrepis and Meuschenia australis) decreased by >50%. The findings have implications for the Southern Zone of South Australia where fishing effort is substantially higher but where escape gaps are not mandatory.