Vertical activity patterns of free-swimming adult plaice in the southern North Sea

The analysis of continuous records of behaviour from adult female plaice equipped with electronic data storage tags (DST), released in the Southern Bight of the North Sea, has yielded new insights into seasonal migration patterns and linked patterns of vertical activity associated with the annual migration cycle. Here we describe the migration-linked changes in patterns of vertical activity observed from 31 individual fish released between December 1993 and February 1997. Ground-tracks were reconstructed using tidal location method. Periodicity of behaviour was examined using actogram and periodogram analysis. Our results illustrate a predominantly southward migration following release in December and January, into Southern Bight and the eastern English Channel. Actogram analysis revealed spatially-varying, migration-linked changes in patterns of vertical activity. Circadian periodicity was dominant in 23 of 31 individuals (74 %), 13 of which also demonstrated significant 12 h periodicity, but with a lower value of q(p). The other 8 plaice exhibited a dominant 25 h period of activity. Periodogram analysis of the averaged population data revealed significant periodicity of 336 h (14 d), in phase with the spring-neap cycle, with periods of reduced activity associated with times of expected spring tides. By linking the spatial and temporal aspects of horizontal and vertical movement, our results provide important information with regards the behavioural regulation of migration by plaice, and also illustrate how availability of plaice to capture by demersal trawling may vary in space and in time.