Daily records of crab and lobster catch and effort data from fishermen's log-books in the pot fishery off Devon, England were examined to determine the effect of pot immersion time on catch per unit effort (cpue) here defined as the weight of crabs or lobsters caught per 100 traps lifted. Seasonal changes in cpue occur; female crabs were most catchable in the autumn, large male crabs in the winter, and lobsters in the spring and for a short period in the autumn. When male crabs were most abundant there were some increases in cpue with pot immersion time, indicating that the traps continued to fish. At other times, stock density appeared to be too low to enable increased catches of male crabs to be taken with increased immersion time, on the same fishing ground. In the autumn, when female crabs were most abundant, female cpue decreased with immersion time, probably due to gear saturation. Lobster cpue decreased after the first day's immersion and this can possibly be related to the greater manoeuvrability of lobsters compared with crabs, which enabled them to escape. The various factors which could cause the observed variation in the cpue and pot immersion time relationship are discussed, and attention is drawn to the necessity to consider these variations when studying the population dynamics of such stocks. The need for more information on the behavioural reactions of crabs and lobsters to baited traps is emphasized.
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David Bennett (1974) studied this question.