Brown shrimp abundance in northwest European coastal waters in the period 1970 - 2010 and potential causes for contrasting trends

Samenvatting

We investigated long-term trends in abundance of the NE Atlantic population of brown shrimp Crangon crangon based on data collected in annual autumn surveys carried out along the coasts of the North Sea in The Netherlands, Germany and Denmark. Surveys covered some estuaries and intertidal areas, as well as shrimp fishery grounds. The 40 yr period showed distinct regional trend differences, but only in estuarine areas: the Western Dutch Wadden Sea (increase), the Eastern Dutch Wadden Sea (decrease) and the Oosterschelde (decrease). The decrease in the Oosterschelde coincided with the closure of this sea arm from fresh water inflow. Increases in the Western and Eastern Dutch Wadden Sea were driven primarily by increased abundance of small shrimp, suggesting that the causative mechanism likely affected younger shrimp. Trends in potential causes were examined in the 2 regions. Of all abiotic factors investigated, only water clarity decreased significantly in the west but not in the east. Mean abundance of potential predators (cod, whiting, shore crab and swimming crab) was used as an indicator of predation pressure. Shore crab showed similar increases in both areas. In the other 3 species, predator abundance in the east was generally initially higher than in the west, but declined towards similar densities in recent years. Shrimp fishing pressure in NW Europe has increased strongly since the 1990s. Preliminary, spatially resolved logbook data show that shrimp landings from the Western Wadden Sea followed this increase, but those from the Eastern Wadden Sea remained stable. Apart from mechanisms related to fishing pressure and water clarity, a candidate cause of differences in abundance may be differences in food condition or competition for food. Generally biomass of benthos, a primary food for shrimp, increased in the west and remained stable in the east. However, because brown shrimp are omnivorous, quantification of changing fractions of food available to them is difficult