The diet and foraging ecology of harbour seals (Phoca vitulina) in the Moray Firth, Scotland
The diet and foraging behaviour of harbour seals in the Moray Firth, Northeast Scotland, was investigated between 1989-1996. The digestion of fish otoliths and cephalopod breaks from prey fed to captive harbour seals was examined. In captive feeding trials, otolith length reduction (mean=27.5%) and recovery rate (mean=42%) varied with species and size-class and were positively correlated to fish size. For five species, the degree of digestion was graded using external characteristics. The subsequent use of calculated correlation factors improved estimates of the original size of prey fed. Otoliths and beaks recovered from 1820 samples represented 36 prey species. Seven species (sandeels, octopus, whiting, squid, flounder, cod and herring) contributed >92% of the total reconstructed mass consumed (307kg). The abundance of prey in the diet varied seasonally and interannually. Sandeels, octopus and clupeids fluctuated by >60% by mass, between years. Diet reflected local changes in food availability, especially overwintering clupeids. Radio-telemetry data on 31 seals indicated the majority of foraging was <30km from haul-out sites, primarily over sandy sediments of 10-50m in water depth. Individual differences were apparent. Additional data from time-depth recorders indicated seals were generally diving close to the seabed. Between-site differences in foraging habitat were related to variations in diet. The comparison of seal diet with fish prey abundance assessed from local fishery surveys suggested seals adopt either a pelagic or demersal foraging strategy, depending on the relative abundance of clupeids. Most fish consumed were 10-16cm in length. Despite marked interannual differences, winter clupeid consumption by seals represented a similar proportion (7%) of the biomass estimated from research surveys in the area. However, when clupeids were relatively scarce, consumption of gadoids represented a larger proportion of the estimated biomass. Overall, the mass of prey consumed annually was small (1%) in proportion with commercial landings in the region.