Foraging ranges, diets and feeding locations of gannets Morus bassanus in the North Sea:evidence from satellite telemetry

We used satellite telemetry to examine the foraging ranges, feeding locations and travel speeds of 17 chick-rearing gannets Morus bassanus from the Bass Rock, SE Scotland. Regurgitates indicated that birds at the colony exploited a wide range of prey, frequently including 0-group sandeels (<10 cm in length) and mature mackerel and herring (up to 33 cm) in the diet. The maximum foraging range was 540 km, and the mean distance to the furthest point from the colony on any one trip was 232 km. Destinations of foraging trips covered a wide area of the North Sea, with a non-random distribution and a higher than expected proportion of trips to the NE (generally in the vicinity of Buchan Deep and Halibut Bank) and to the SE (mostly between Farne Deep, Dogger Bank and Outer Silver Pit) of the colony. Foraging trips lasted 13 to 84 h, and trip duration explained 94% of the variance in maximum distance from the colony, indicating that distance travelled could be predicted with a high degree of accuracy from time spent at sea. However, the average speed of travel during foraging trips (15 km h-1) was considerably lower than maximum ground speed (~55 km h-1 in most cases). The results of this study suggest that gannets breeding at the Bass Rock utilize a wide range of species and sizes of prey over a large area of the North Sea, and that they focus their activity on bathymetric features that are probably associated with high primary production. Further data are now required to examine the foraging ranges and feeding locations of gannets in different oceanographic regions in order to obtain a broader understanding of how gannets make use of different marine environments.

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