Fluctuations of brown shrimp Crangon crangon abundance in the western Dutch Wadden Sea

Abstract

Long-term trends in brown shrimp Crangon crangon abundance were studied near the entrance of the Dutch Wadden Sea using a 34 yr time series of fyke net catches. A general trend of increasing abundance was observed. In autumn, brown shrimp abundance was 5 times greater than in spring, and interannual fluctuations were also more pronounced. Biotic and abiotic conditions showed effects with time lags of up to 1 yr, which is consistent with the species' life cycle. The popu- lation responded rapidly to changes in environmental conditions: abrupt declines after adverse con- ditions, as in spring 1999, were followed by steep recoveries in the following autumn. Two hypothe- ses were tested and discussed: (1) autumn recruitment is related to predator abundance and temperature during previous warmer seasons, and (2) overwintering adult abundance is determined by predation pressure and abiotic conditions in winter. In general, predator abundance was the main factor correlated to brown shrimp abundance, especially in autumn. Multiple linear regression mod- elling showed that predators altogether accounted for up to 55% in spring and up to 85% in autumn of the explained variance. Previous winter conditions such as sunshine duration, salinity and fresh water discharge, and the North Atlantic Oscillation Index (NAOI) were also correlated with autumn abundance. Yet, temperature conditions during the previous growing season did not determine the autumn stock size. Winter sunshine duration and the winter NAOI were correlated with spring stock size. Additionally, temperature and salinity were relevant factors affecting abundance during spring. A significant positive relationship between spring and autumn abundance and annual commercial landings was also found.