Brown shrimp abundance in northwest European coastal waters from 1970 to 2010 and potential causes for contrasting trends

Abstract

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.

Predation and habitat modification synergistically interact to control bivalve recruitment on intertidal mudflats

“…In the Dutch Wadden Sea, shrimp numbers are over twice as high compared to other European coastal waters (Tulp et al, 2012), peaking in summer at over 100 individuals/m 2 on intertidal flats (van der Veer et al, 1998). Furthermore, shore crab densities increased over 10-fold since 1995, with numbers still rising (Tulp et al, 2012). At present, the underlying causes are unknown, but our results suggest that high predator numbers now hamper bivalve recruitment.…”

“…Apart from deteriorated substrate conditions, predation pressure by crustaceans is increasing in many coastal areas, often due to overfishing of top-predators resulting in mesopredator release (Worm and Myers, 2003). In the Dutch Wadden Sea, shrimp numbers are over twice as high compared to other European coastal waters (Tulp et al, 2012), peaking in summer at over 100 individuals/m 2 on intertidal flats (van der Veer et al, 1998). Furthermore, shore crab densities increased over 10-fold since 1995, with numbers still rising (Tulp et al, 2012).…”

Quantifying Tidal Movements of the Shore Crab Carcinus maenas on to Complex Epibenthic Bivalve Habitats

“…In the Dutch Wadden Sea, annual sampling in the tidal channels revealed that the shore crab population increased considerably in the last 20 years (Tulp et al 2012) and is therefore expected to have noticeable impacts on the different bivalve populations. However, little is known of the potential impact of C. maenas on epibenthic bivalve populations.…”

“…Since both species also differ in their attachment mechanisms, aggregations of multiple specimens differ considerably in structural complexity. Mussels are adhered to the substratum via temporary byssus threads (Bell and Gosline 1996), and the continuous process of generating new threads leads to flexible and dynamic meshworks of individual mussels (van de Koppel et al 2005). In contrast, Pacific oysters remain permanently attached to each other via an organic-inorganic adhesive (Burkett et al 2010) and continuous larval settlement onto conspecifics leads to the creation of rigid and persisting structures (Walles et al 2015).…”

Unexpected dietary preferences of Eurasian Spoonbills in the Dutch Wadden Sea: spoonbills mainly feed on small fish not shrimp

“…The fact that the food of spoonbills might be limiting, and that several colonies reached plateau levels more than 10 years ago (e.g. on the islands of Terschelling and Schiermonnikoog), can possibly be explained either by (1) shrimp becoming more abundant (Tulp et al 2012), but shrimp not actually being the staple food of these birds; or (2) by shrimp availability actually being lower than thought, e.g. as a result of high fishing pressure, as reported by Tulp et al (2016).…”