Seasonal changes in the level and diel pattern of activity in the European lobster Homarus gammarus

Abstract

European lobsters are large, mobile crustaceans of ecological and commercial significance. Until recently, the lack of a reliable technique for long-term, in situ monitoring of activity has hindered study of seasonal variation in activity and its determinants. Electromagnetic telemetry has been used to measure seasonal changes in locomotor activity and body movements of lobsters on an artificial reef in Poole Bay, southern England. Both measures of activity showed marked seasonal variation, related primarily to water temperature, with greatest activity in late summer and minimum values in winter. Subsidiary effects of water movement and seabed illumination were also indicated. Throughout the spring and summer, excursions outside shelter were almost exclusively nocturnal, with movement peaking in the early part of the night. Body movements followed a similar pattern, but with less abrupt changes, suggesting a degree of activity within shelters during the day. Diet timing of lobster activity appeared to be mainly governed exogenously by light level, since the length of the nocturnal active period and the timing of the activity peak varied seasonally with the times of sunset and sunrise, and there was a relative increase in diurnal activity during periods of low seabed light levels caused by increased turbidity. During low temperatures in lid to late winter, activity was low throughout the 24 h cycle. These findings suggest that there are likely to be significant seasonal changes in the relationship between fishing effort and fishing mortality that need to be taken into account in lobster stock assessment.