Differential catchability of male and female European spiny lobster Palinurus elephas (Fabricius, 1787) in traps and trammelnets

摘要

A study was carried out to assess the size and sex dependent catchability of Palinurus elephas in traps by comparing trap catches to trammelnet catches in the Marine Reserve of Columbretes Islands (western Mediterranean) during three different periods. The results indicate that catches of P. elephas taken with traps do not adequately represent the size structure of the population and that the sex ratios are biased towards females. Trap selectivity primarily affected the estimates of the males' size structure. The effects on the females' size structure were limited by their restricted size range. No significant seasonal changes in female catchability in traps relative to nets were noted, but catchability of mature males in traps appeared to be higher before reproduction. Additionally, the catchability of male P elephas in traps and trammelnets was highly reduced during moulting. As a result of the above, changes in population structure observed in P. elephas trap assessment surveys in the reserve appear to be primarily due to seasonal variations in male size structure (operating through trap selectivity) and behaviourally induced changes of male catchability in traps. We discuss the implications of these findings for population assessment. From the point of view of the exploitation and conservation strategy, traps are preferred to trammelnets because they allow for the escape of a certain fraction of immature P. elephas and they protect the largest males.