The role of industrial fisheries and oceanographic change in the decline of North Sea black-legged kittiwakes

1. Marine predators can be adversely affected by human activities in several potentially interacting ways. Industrial fisheries can affect predator populations adversely through competition for shared prey, and marine environmental change has also been implicated in population declines. In the North Sea, black-legged kittiwakes Rissa tridactyla have declined by > 50% since 1990, a period during which a lesser sandeel Ammodytes marinus fishery was active and profound oceanographic changes occurred. We studied the role of fisheries and oceanography in kittiwake declines on the Isle of May, south-east Scotland, where sandeels are the main prey. 2. Demographic data were collected from 1986 to 2002. We examined changes over time and correlations between population parameters, the local sandeel fishery and environmental factors, and incorporated the results in a deterministic and a stochastic matrix population model. 3. Breeding success and adult survival were low when the sandeel fishery was active (1991-98) and were also negatively correlated with winter sea temperature, with a 1-year lag for breeding success. Modelling showed that the observed changes in population parameters could explain the change in population growth rate from +8% year(-1) in the late 1980s to -11% year(-1) in the late 1990s. An observed improvement in breeding success from 2000 onwards has been insufficient to arrest the population decline. To stabilize the population, breeding success must increase to unprecedented levels or survival needs to increase substantially. Stochastic modelling indicated that the population was unlikely to increase if the fishery was active or sea temperature increased, and that the population was almost certain to decrease if both occurred. 4. Sandeel recruitment is reduced in warm winters, and we propose that this explains the temperature effects on kittiwake survival and breeding success. The fishery also had a strong effect on demographic performance, although the exact mechanism is unclear as kittiwakes and fishermen target different sandeel age groups. 5. Synthesis and applications. Poor breeding success of kittiwakes was associated with warm winters and the presence of a local sandeel fishery. Reversing the trend towards warmer winters may be impossible and, at best, would be a very slow process. Therefore, to safeguard kittiwake populations we recommend that the current closure of the commercial sandeel fishery remain in place indefinitely. This study shows that adequate monitoring of the effect of a fishery or of environmental change on seabird populations needs to include survival as well as breeding success.