Selective tidal stream transport and the migration of plaice (Pleuronectes platessa L.) in the southern North Sea

Abstract

The migrations of plaice which spawn in the Southern Bight of the North Sea have been studied using paired midwater trawl hauls made on consecutive northerly and southerly tides mainly within the line of the Knoll Stream between latitudes 52°00'N and 52°45'N and longitudes 02°00'E and 02°30'E. Fifty paired tows made in November and December (“autumn” pairs) and 22 in February (“winter” pairs) gave 39 and 21 pairs respectively in which plaice were caught in one or both tows of a pair. In 33 of the “autumn” pairs more plaice were caught in the tow made during the south-going tide; in 20 of the “winter” pairs more plaice were caught in the tow made during the north-going tide. The ratio of catch rates for the two tides showed a similar reversal from “autumn” to “winter”. The results provide direct evidence that in November and December ripening plaice migrating into the Southern Bight by the western route use selective tidal stream transport from the Leman Ground southwards. Spent plaice returning northwards in February use the same transport system at least as far as Smiths Knoll. The results are discussed in relation to the role of water currents in the life history of the plaice and of tidal stream paths in stock separation.