Size matters: large spiny lobsters reduce the catchability of small conspecifics

Abstract

Indices of lobster abundance and population demography are often derived from pot catch rate data and rely upon constant catchability. However, there is evidence in clawed lobsters, and some spiny lobsters, that catchability is affected by conspecifics present in pots, and that this effect is sex- and size-dependent. For the first time, this study investigated this effect in Panulirus cygnus, an economically important spiny lobster species endemic to Western Australia. Three studies: (1) aquaria trials, (2) pot seeding experiments, and (3) field surveys, were used to investigate how the presence of large male and female conspecifics influence catchability in smaller, immature P. cygnus. Large P. cygnus generally reduced the catchability of small conspecifics; large males by 26-33% and large females by 14-27%. The effect of large females was complex and varied seasonally, dependent on the sex of the small lobster. Conspecific-related catchability should be a vital consideration when interpreting the results of pot-based surveys, especially if population demography changes. Analysis of the mean catch rate of large P. cygnus over the past 29 yr indicates that fishery management changes have created significant variations in the abundance of large lobsters. This is likely to have affected the catchability of smaller lobsters, which will have implications for the use of time series catch rate data in the stock assessment and management of this fishery.