Size selectivity is an important aspect of southern rock lobster stock assessment models and has been estimated for different fleets, pot types, and areas. Temporal changes in size selectivity can lead to biased model results and can bias parameter estimates, such as pre-recruit indices (PRIs). These biases can manifest as trends that are unrelated to changes in the population characteristic they are designed to monitor. Several mechanisms for changes to size selectivity have created particular concern due to their potential to influence management decisions.

Firstly, in some areas southern rock lobster stocks are increasing significantly and are expected to continue to do so. One way in which these elevated densities may affect selectivity is if large lobsters deter small lobsters from entering pots, thus lowering the estimated PRI (an index used in TACC setting in Victoria)

Secondly, increased price differential between lobster size classes coupled with high CPUE is creating substantial incentives for high grading, both through discards and through changing fishing practices targeting different sized lobsters.

Lastly, seasonal changes in size selectivity have been observed in South Australia and may occur elsewhere. These have not been quantified and may occur in other regions.

Understanding these changes in size selectivity and mitigating the impact on the ongoing stock assessment modelling and harvest strategies will ensure robust assessments and avoid future management bias.