Impacts of migratory behaviour on population structure in North Sea plaice

Abstract

Summary 1.Migration is widespread among marine fishes, yet little is known about variation in the migration of individuals within localities, and the consequences for spatial population structure. We tested the hypothesis that variation in the migratory behaviour among plaice ( Pleuronectes platessa L.) in the North Sea could be explained by largescale differences in the speed and directions of the tidal streams, which the fish use as a transport mechanism. 2. We released 752 mature female plaice tagged with electronic data storage tags 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 that the fish were segregated into three discrete feeding aggregations during the summer non-breeding season. Two clusters were in warm, thermally mixed water in the eastern and western North Sea, respectively, and one was in deeper, cold, thermally stratified water to the north. 5. In the winter spawning period, fish from all three aggregations mixed together in the southern North Sea, and fish from the eastern and northern subunits spawned in the south-eastern North Sea. The only fish that left the North Sea were western subunit 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. However, plaice from the northern North Sea did not use tidal stream transport. 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 also revealed features of spatial dynamics not observed previously from a century of conventional tagging experiments and illustrate how the study of individual fish can successfully define the migratory characteristics of populations.

Consistency in the behaviour types of the Atlantic cod: repeatability, timing of migration and geo-location

“…However, despite intensive research on these 2 behaviour types, no studies have dealt with year-toyear repeatability of individual migrations, or the timing and routes of migrations. This information can be retrieved from DSTs (Hunter et al 2004, Metcalfe 2006 and may shed new light on feeding migration processes (i.e. whether individuals move in groups/ shoals) and on the consistency of the migration habits of these behaviours.…”

Large-scale variation in seasonal swimming patterns of plaice in the North Sea

“…DST studies of plaice have already largely confirmed the observations from acoustic-tracking and mid-water trawling experiments made at specific locations at particular times of year (e.g. Greer Walker et al 1978, Arnold & Metcalfe 1996, and have also provided insights into migration (Metcalfe & Arnold 1997, Hunter et al 2004a), distribution (Hunter et al 2004b) and reproductive behaviour (Hunter et al 2003, Solmundsson et al 2003. This work now allows the characterisation of swimming times and patterns of activity across the North Sea for the first time.…”

“…The analyses draw on data from 2 major field programmes targeting mature female plaice in the North Sea; the first of these programmes released 302 fish tagged with DST1s (Cefas Mk-1 DSTs) between 1993 and 1997 (Hunter et al 2004a), while the second programme released 452 plaice tagged with DST3s (Lotek-1200 DSTs, Lotek) between 1997 and 2000 (Hunter et al 2004b).…”

“…1). However, for the purposes of the current study individual fish were grouped according to membership of the 3 population sub-units identified by Hunter et al (2004b): western, northern and eastern feeding aggregations ( Fig. 1).…”

“…Although continuous electronic data storage tag (DST) behaviour records of migrating adult females have clearly linked STST to the pre-spawning and post-spawning migrations of plaice in areas of fastflowing tidal currents (Hunter et al 2004c), plaice form at least 3 major feeding aggregations throughout the North Sea during the summer (Hunter et al 2004b): these are located in the western, eastern, and central northern North Sea. While the eastern aggregation is also located near a fast-flowing tidal stream, the northern aggregation is located in an area of slow-flowing tidal currents where it is predicted that most plaice would be unable to gain any energetic advantage by using the tidal streams to migrate (Metcalfe et al 1990).…”