A 25-year study of climatic and density-dependent population regulation of common shrimp Crangon crangon (Crustacea: Caridea) in the Bristol Channel

Abstract

The results of a 25-year study of the population dynamics of the common shrimp, Crangon crangon, in the Bristol Channel are presented. The population size varied seasonally, with maximum abundance occurring in early autumn at the completion of annual recruitment. The number of recruits changed greatly between years, and was positively correlated with both average water temperature from January to August, and river flow rate, and negatively correlated with the Winter North Atlantic Oscillation Index. A wide range of other physical and biotic variables was found to have no significant impact on C. crangon abundance. The positive relationship between temperature and C. crangon abundance observed for the Atlantic coast during this study is the opposite of that found for southern North Sea populations. Similar contradictory responses have been noted previously for flatfish such as sole, Solea solea. This suggests that global variables may act to produce different outcomes for Atlantic and North Sea populations of the same species. Over-winter mortality was found to vary with population size so that the adult C. crangon population in spring was found to be remarkably stable, and little influenced by temperature or other variables. The mortality rate increased with population size producing clear evidence of density-dependent control. It is suggested that this stability is linked to the constant limited availability of suitable habitat, with individuals unable to find shelter vulnerable to a range of predatory fish. Given the pivotal role of C. crangon within the northern European estuarine ecosystems, this stability may be a critical component for the overall stability of the system. A particular feature of this study was the exceptional recruitment observed in October 2002. This did not result in any subsequent increase in adult C. crangon numbers, possibly because there was a synchronous increase in a wide range of predators. While the adult population has remained stable and showed no temporal trend, there has been an increase in both the average magnitude and between year variability in recruitment, which can be related predominately to the recent increase in water temperature. The difficulty of predicting the response of this population to continued climate warming is discussed. If temperature continues to rise, the present power law describing the increase in recruitment with temperature must inevitably break down. If this were to occur, the future trajectory of the C. crangon population could not be predicted, and the continued stability of this ecosystem would no longer be assured.

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

“…This corroborates previous observations (Siegel et al 2005, Henderson et al 2006 reflecting shrimp production in the coastal zone (Kuipers & Dapper 1984). Also, commercial landings are highest in autumn and lowest in spring (ICES 2008).…”

“…Nevertheless, none of these factors was found to influence brown shrimp abundance in spring (Siegel et al 2005). In contrast, in the Bristol Channel, the number of recruits was found to be negatively correlated with the WNAOI in autumn and positively related to river run off and water temperature from January to August (Henderson et al 2006). The severity of the winter and hence, winter temperature, has also been identified as a determinant for interannual oscillations in brown shrimp abundance: after severe winters, C. crangon abundance is much lower than beforehand.…”

“…Future monitoring will reveal whether brown shrimp abundance will recover from the recent downward trend. In the Bristol Channel, a general increasing trend was also found until 2005 (Henderson et al 2006), while in the German Wadden Sea, no trend was found in brown shrimp densities between 1974and 2002(Siegel et al 2005. Brown shrimp landings in the North Sea have also been increasing since the 1970s, despite a considerable decline from the mid to late 1980s of about one third of previous landings, and reached a maximum of 16 000 t in 2006 (ICES 2008).…”

“…As hypothesised, predator abundance was observed to consistently influence brown shrimp autumn abundance, explaining up to 85% of the variance in autumn models when considered altogether, reflecting their high importance. Predation and even cannibalism, which represents about 20% of the species annual food consumption (Pihl & Rosenberg 1982), are known to be important causes of brown shrimp natural mortality (Henderson & Holmes 1989), though fluctuations in predator abundance might be difficult to relate to Crangon crangon stock fluctuations (Henderson et al 2006). Nevertheless, brown shrimp densities have been identified as one of the explanatory variables for the long-term trends in abundance of several fish species observed in Dutch waters (Tulp et al 2008).…”

Seasonal variations in larval biomass and biochemical composition of brown shrimp, Crangon crangon (Decapoda, Caridea), at hatching

“…Predation is a major cause of larval mortality in C. crangon (e.g. Henderson et al 2006), with various fish species being well-known predators (Tiews 1978). As large ''winter larvae'' show a prolonged planktonic development time, they are also exposed to a higher risk of predation.…”