Publication Abstract

Impacts of migratory behaviour on population structure in North Sea plaice

E. Hunter, J.D. Metcalfe, G.P. Arnold and J.D. Reynolds

1. Migration is widespread among marine fishes, yet little is known about variation in the migration of individuals within localities. We tested the hypothesis that variation in the migratory behaviour among plaice (Pleuronectes platessa) in the North Sea could be explained by large-scale differences in the speed and directions of the tidal streams, which the fish use as a transport mechanism.

2. Towards this end, 752 mature female plaice tagged with electronic data storage tags were released at eight locations with contrasting tidal flow properties, between December 1993 and September 1999.

3. The experiment yielded 20 403 days of data from 145 plaice. Individual tags recorded depth and temperature for up to 512 days. The position of each fish was determined at intervals throughout the liberty period using the tidal location method.

4. The results show 3 geographically discrete feeding aggregations during the summer, located in warm, thermally mixed water in the eastern and western North Sea, and in deeper, cold, thermally stratified water to the north. These dispersed over the southern North Sea and Eastern English Channel to spawn during winter.

5. During spawning (December-April), eastern and northern sub-unit plaice occurred simultaneously on the German Bight spawning grounds, and individuals from all three sub-units visited spawning grounds in the Southern Bight. The only fish that left the North Sea were western sub-unit plaice that visited spawning grounds in the eastern English Channel.

6. Our results re-affirmed the major role of the tidal streams in the southern North Sea in structuring plaice dispersion, both by providing transport and guidance, and by delimiting the extent of distribution due to thermal stratification during the summer. Northern North Sea plaice also migrated south, but did not use the tidal stream currents to migrate.

7. These results confirm the prediction that large-scale variation in migration behaviour can be explained in part by the tidal guidance and transport mechanisms available. They have revealed features of spatial dynamics not previously observed from a century of conventional tagging experiments and illustrate how the study of individual fish can successfully define the migratory characteristics of populations.

Reference:

E. Hunter, J.D. Metcalfe, G.P. Arnold and J.D. Reynolds (2004) Impacts of migratory behaviour on population structure in North Sea plaice. Journal of Animal Ecology, 73: 377-385