Changes in the behaviour of the sole,Solea vulgaris, during cold winters, and the relation between the winter catch and sea temperatures

Kurzfassung

Während kalter Winter werden ungewöhnlich große Seezungenfänge eingebracht. Diese Tatsache hängt offenbar zusammen mit der durch das kalte Wasser verursachten Ansammlung großer Bestände in relativ kleinen Gebieten, mit der erhöhten Verwundbarkeit durch die Schleppnetzfischerei bei niedrigen Temperaturen sowie mit der längeren Anwesenheit der Seezungen in küstennahen Schleppnetzgründen.Solea vulgaris ist offenbar besonders empfindlich gegenüber niedrigen Temperaturen. Es konnte eine deutliche Beziehung zwischen Winterfangergebnissen und der Wassertemperatur nachgewiesen werden.

Summary

1. Exceptionally large catches of soles are made in the North Sea during very cold winters. It was considered that these catches were due to increased vulnerability of the fish to trawling at low sea temperatures, and to the concentration of fish into smaller areas, of high density, by the cold water masses. A third contributory factor was that the soles remained on the offshore trawling grounds for a longer time in cold winters.

2. Data from fishing reports, sole catches in different regions of the southern North Sea, and the positions of recapture of tagged soles in the winter of 1962/63 were used to illustrate the changes in the behaviour of this fish.

3. Although soles suffered high mortalities from low temperatures, predation, and from fishing during winter 1962/63, reducing the numbers of potential spawning fish, it was suggested that the failure of the coastal spawning fisheries was also partly due to a large part of the stock failing to migrate inshore in this year.

4. It was considered that since soles appear to be particularly sensitive to low temperatures there might be a direct relationship between the winter catches of soles and the sea temperatures. This relationship was investigated and seems to hold good, at least for the post-war years since 1948.