Do large rock lobsters inhibit smaller ones from entering traps? A field experiment

Indices of recruitment are often derived from trap surveys. If legal-sized lobsters inhibit smaller ones from entering traps, the overall catch-rate may depend on population composition and not just on overall abundance, and recruitment strength can be overestimated as average length decreases in a population. A controlled field experiment was used to examine whether trapping inhibition of Jasus edwardsii occurred during spring (November) or summer (February) in south-eastern Tasmania. Four treatments were applied. Baited traps were seeded with either: one large female, one large male or two sublegal-sized female lobsters. Baited traps with no seed were used as a control. Seeded traps always caught fewer sublegal-sized lobsters than control traps. When catches in both seasons were examined by sex of entrants, seeded traps caught fewer small lobsters than control traps in 11 of 12 comparisons. However, a strong inhibitory effect was observed only for female-seeded traps during summer. The data obtained in the present study suggest that sublegal-sized indices of recruitment are likely to be influenced in summer by the number of large females present. Spring trials suggest that corrections to a sublegal-catch index may be unnecessary during this season, but more work is required in order to better understand the complex, sex-specific and seasonal patterns of interactions of this species. © CSIRO 2006.