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

Analysis of continuous behaviour records of adult female plaice Pleuronectes platessa tagged with electronic data storage tags, and released in the southern North Sea, has yielded new insights into spatial and temporal variation in vertical activity (swimming). Here we describe migration-linked changes in vertical activity patterns observed from 31 plaice released between December 1993 and February 1997. Fish migrations of up to 220 d were reconstructed using the tidal location method. Analysis of the periodicity of activity revealed high levels of vertical activity during the southward spawning migration into Southern Bight and the eastern English Channel following release in December and January. We observed spatially varying, migration-linked changes in vertical activity, with a tidal pattern of vertical activity during the pre- and post-spawning migrations and a circadian period of rhythmicity in the spawning grounds. Circadian periodicity was dominant in 23 of 31 individuals (74%), 13 of which also demonstrated significant 12 h periodicity. The other 8 fish exhibited a dominant 25 h periodicity. A significant 336 h (14 d) rhythm was observed in the averaged population data, in phase with the spring-neap cycle, with periods of reduced activity associated with times of expected high tides. Our results provide important information about systematic changes in migration-linked behaviour in plaice, and also illustrate how the accessibility of plaice to capture by trawling may vary in space and in time.