A quantitative study of nocturnal foraging distances of the western rock lobster Panulirus cygnus George

Abstract

An electromagnetic tracking device was used to study the distances over which juvenile western rock lobsters Panulirus cygnus George forage at night. Up to 14 tagged lobsters were tracked continuously for up to 3 wk with the aid of an automatic tracking system. A total of 460 “lobster nights” (data on one lobster for 1 night) was recorded during the study. On 48 of these “lobster nights”, the lobsters did not forage. Approximately half of the remaining data were incomplete or “censored” due to lobsters moving outside the tracking area for part of the night. A Weibull distribution showed an extremely close fit to the data after adjustment for the bias of censoring. Through analyses of deviance, comparisons were made of the effect on distance travelled of sampling time, sex, and time since tagging in a way that was analogous to an analysis of variance. The results of the study indicate that nocturnal foraging distances were variable: 95% of the population covered between 72.5 and 585 m · night −1. These distances, which are much greater than previously reported, show that the animals have an unexpectedly large foraging area. The effect on survival of P. cygnus of limited food supply due to small foraging distances may not be as important as previously believed.