Black-legged kittiwakes as indicators of environmental change in the North Sea; evidence from long-term studies

Top predators, particularly seabirds, have repeatedly been suggested as indicators of marine ecosystem status. One region currently under pressure from human fisheries and climate change is the North Sea. Standardized seabird monitoring data have been collected on the Isle of May, an important seabird colony in the northwestern North Sea, over the last 10–20 years. Over this period oceanographic conditions have varied markedly, and between 1990 and 1999 a major industrial fishery for sandlance (Ammodytes marinus), the main prey of most seabird species, was prosecuted nearby. Sandlance fishing grounds close to seabird colonies down the east coast of the UK were closed in 2000 in an attempt to improve foraging opportunities for breeding seabirds, particularly black-legged kittiwakes (Rissa tridactyla). Initially this closure seemed to be beneficial for kittiwakes with breeding success recovering to pre-fishery levels. However, despite the ban continuing, kittiwakes and many other seabird species in the North Sea suffered severe breeding failures in 2004. In this paper, we test the predictive power of four previously established correlations between kittiwake breeding success and climatic/trophic variables to explain the observed breeding success at the Isle of May in 2004. During the breeding season, kittiwakes at this colony switch from feeding on 1+ group to 0 group sandlance, and results up until 2003 indicated that availability of both age classes had a positive effect on kittiwake breeding success. The low breeding success of kittiwakes in 2004 was consistent with the late appearance and small body size of 0 group sandlance, but at odds with the two variables likely to operate via 1 group availability (lagged winter sea surface temperature and larval sandlance cohort strength in 2003). The reason for the discrepancy is currently unknown, but analysis of 1 group sandlance body composition indicated that lipid content in 2004 was extremely low, and thus fish eaten by kittiwakes during pre-breeding and early incubation were likely to be of poor quality. Monitoring of reproductive success of kittiwakes, although useful, was clearly not sufficient to tease apart the complex causation underlying the 2004 event. Monitoring programs such as this, therefore, need to be complemented by detailed research to identify the mechanisms involved, and to attribute and predict the effects of natural and human-induced environmental change.

Combined Effects Of Body Size Food Concentration And Season On Physiology Of Mytilus-Edulis

Multivariate experiments are used to study the effects of body size, food concentration, and season on the oxygen consumption, ammonia excretion, food assimilation efficiency and filtration rate of Mytilus edulis adults. Food concentrations and season affect both the intercept and the slope of the allometric equation describing oxygen uptake as a function of body size. Multiple regression and response surface techniques are used to describe and illustrate the complex relationship between metabolic rate, ration, season and the body size of M. edulis. Filtration rate has a relatively low weight exponent Q> = 038) and the intercept for the allometric equation is not significantly affected by food concentration, season or acclimation temperatures between 5 and 20 °C. Food assimilation efficiency declines exponentially with increasing food concentration and is dependent on body size at high food levels. The rate of ammonia excretion shows a similar seasonal cycle to that of oxygen consumption. They are both minimal in the autumn/winter and reach a maximum in the spring /summer.