DYNAMICS OF JUVENILE SHRIMP CRANGON-CRANGON IN A TIDAL-FLAT NURSERY OF THE WADDEN SEA AFTER MILD AND COLD WINTERS

Juvenile shrimp Crangon crangon (L.) were sampled frequently on a tidal flat in the westernmost part of the Wadden Sea during spring and summer from 1983 to 1991. High densities (several tens per m2) of small shrimp (length range 5 to 25 mm, mean generally <15 mm) were observed at low tide during late spring, summer and early autumn. The low numbers (ca 1 m-2) of overwintered shrimps observed on the tidal flats in March were positively correlated with the mean temperature of the foregoing months. Settlement of a new generation of postlarvae (ca 5 mm) started in April after mild winters and in May after cold winters. Maximal densities were observed in June after mild winters and in July after cold winters. Settlement continued with fluctuating intensity throughout summer and early autumn. Growth was rapid: as early as ca 1 mo after settlement the shrimps reached a length of 20 to 25 mm, at which point they leave (the higher parts of) the tidal flats. Thus, juvenile shrimp use tidal flats as a transit nursery, where several successive cohorts grow up during any one summer season. Earlier published estimates of time of recruitment, mean size and growth rate of shrimp living in the same area are refuted. In spring (up to mid-June), shrimp biomass on the tidal flat was much lower after cold than after mild winters. This difference could also help explain the generally highly successful recruitment ot the bivalve Macoma balthica after cold winters.