Influence of behavioural interactions on lobster distribution and abundance as inferred from pot-caught samples

Stock assessments of lobster (Homarus gammarus (L.)) fisheries in England generally assume that the catch of lobsters in pots (or traps) is representative of the population on the ground. This article considers how behavioural interaction of animals in and around pots might influence our perception of the true distribution and abundance of lobsters on the ground. It shows from an analysis of the variance/mean ratio of the number of lobsters per pot on the east coast of England that lobsters are randomly or uniformly distributed amongst pots. Historical catch data from other fishing areas exhibited similar distributions, except that at very low catch rates lobsters were aggregated amongst pots. Catch rates were significantly reduced by pre-stocking pots with lobsters, suggesting that the presence of a lobster may deter entry of further individuals and generate a more uniform distribution of lobsters amongst pots than the true distribution on the ground. Pre-stocking pots with lobsters also reduced the catch rate of crabs (Cancer pagurus), and catch rates of lobsters and crabs are shown to be negatively correlated. Baiting pots with a soft recently moulted crab inhibited entry of crabs, and thus pots baited in this way may provide a better estimate of lobster abundance. Crabs may also deter entry of lobsters but this needs to be confirmed by further experiments. The results suggest that catch per unit effort may not be linearly related to stock abundance, as customarily assumed.