Year-class strength of plaice Pleuronectes platessa in the Southern Eight of the North Sea:: a validation and analysis of the inverse relationship with winter seawater temperature

The previously (1974-1982) observed inverse relationship between the February seawater temperature at the spawning grounds in the Southern Eight of the North Sea and subsequent year-class strength of 0-group plaice on the Balgzand tidal flats (western Dutch Wadden Sea) was revalidated for the period 1991-1998. However, seawater temperatures determined from NOAA satellite images showed that water temperatures during egg and larval drift in the open sea were more constant than the February temperatures at the spawning grounds suggested. In all years, and under all conditions, there was a persistent tongue of English Channel water entering through the Dover Strait into the Southern Eight of the North Sea, in which plaice eggs and larvae developed and were transported. The intensity of this plume varied between years, but even during the coldest period it penetrated as far as offshore Texel and the Marsdiep inlet. The absence of interruptions in the pattern of larval immigrations at the nursery implies a link between variations in water temperatures and in the period of drift. Between and within years, the number of settling larvae at Balgzand was inversely related to the median temperature during drift. Overall, also year-class strength estimates were still inversely related to water temperature during drift. However, the relationship broke down for the very strong 1996 year-class, suggesting that other factors were acting too. Cold winters (reflected in low temperatures in the coastal zone) are associated with persistent winds from the east which induce an atypical water circulation pattern that affects the strength and direction of the residual current. Easterly winds might also generate upwelling near the coast stimulating larval immigration.