Episodic mass invasions of juvenile gadoids into the Wadden Sea and their consequences for the population dynamics of brown shrimp (Crangon crangon)

Abstract

The brown shrimp Crangon crangon is considered to be a key species in the coastal waters of the North Sea and in particular in the Wadden Sea, since it occurs in masses and acts as both a highly efficient predator and important prey. The Wadden Sea is the nursery ground for brown shrimp. In 1990, a mass invasion of 0-group whiting (Merlangius merlangus) reduced Crangon numbers to almost zero. Similar events, which had previously been reported in the 19th century, were observed in 1959 (whiting), 1970 (cod, Gadus morhua), and 1983 (cod and whiting). The 1990 invasion and the decline of the shrimp stock are documented on the basis of surveys that are compared with the years preceding and following. Consequences for the food web in the Wadden Sea and the reasons for the complete recovery of the shrimp stock in each case within one year after its collapse are discussed with regard to the Crangon crangon behaviour and reproductive potential. Sampling error and the impact of fisheries are also considered.