Changes in fecundity and size at maturity of rock lobster Jasus edwardsii in southern New Zealand – A cause for concern in the face of environmental change

Understanding reproductive parameters such as size at maturity (SAM) and fecundity is critical for the management of fished species as they are direct indicators of reproductive potential and therefore stock health. Rock lobster, Jasus edwardsii, support one of New Zealand’s most valuable inshore fisheries with commercial, recreational and customary significance. Reproduction in J. edwardsii is characterised by size-dependent fecundity, external brooding, and extended larval development, with regional variability in the timing of the reproductive period. This study assessed SAM and fecundity in female J. edwardsii from southern New Zealand, using fisheries-independent field data collected via standardised potting methods between 2021 and 2024. We found a 20 % decline in SAM compared to estimates from 1969. Also, while fecundity was higher across most sizes in 2024, the relationship between size and fecundity has changed since last measured in 1984. Our fecundity estimates utilised a traditional gravimetric approach, however we also trialled a non-invasive method for estimating fecundity and found no significant difference between the methods, supporting its utility for future monitoring with reduced impact on individuals. Results presented suggest that there has been a shift in key reproductive indicators over a 50-year period, which may have been driven by fishing pressure in a region where the commercial minimum legal size has remained low. These findings highlight the importance of shifting management strategies toward rebuilding reproductive capacity and population structure in a valuable fishery to ensure long-term persistence and enhance stock resilience into the future. • Size at maturity and fecundity estimates for Jasus edwardsii were assessed after five decades in Otago, New Zealand. • Size at maturity was found to have decreased by 20 %. • The fecundity-size relationship has shifted, likely as a result of individuals maturing at smaller sizes. • The Otago J. edwardsii population may have experienced fisheries-induced evolution. • Management strategies should focus on rebuilding reproductive capacity.