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The vertical and horizontal movements of mature plaice (Pleuronectes platessa) on spawning grounds west of Iceland were studied, using conventional anchor tags and electronic data storage tags measuring depth and temperature at specified time intervals. Concurrent to the taggings, information on sex ratios, maturation and spawning was analyzed using data from commercial catches. Males were more active than females during the time of spawning, an increase in swimming activity mainly being observed during the hours around midnight. A higher recapture rate of tagged males than females, indicates a greater catchability of males during the spawning season. Furthermore, males predominated in the catch and it is suggested that this may be explained partly by the observed difference in the behaviour of the sexes. The skewed sex ratios could also be caused by earlier sexual maturation of males, together with males spending more time on the spawning grounds than females. We conclude that sexual differences in behaviour, catchability, age distribution and duration of spawning time of plaice, should be accounted for in catch and survey data sampled from spawning grounds. |
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