Seasonal variation in the diet of common seals (Phoca vitulina) in the Moray Firth, north‐east Scotland, was determined from analysis of faecal samples collected at haul‐out sites during each month of 1988. Data on diet of common seals in 1987 are also presented. Limitations of the methods available for quantification of diet are discussed. Although some of the observed variation in diet from month to month may reflect changes in the sampling regime, a clear seasonal pattern was apparent, with clupeids predominating in the winter and sandeels in the summer. The trends observed are consistent with opportunistic feeding on the most abundant Prey.
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SUMMARY(1) Parallel studies of fish distribution and the diet and activity of common seals were made to assess the relationship between the seals' winter feeding activity and the distribution and abundance of their prey. Echosounder and trawling surveys revealed that a large part of the fish biomass was sprat and small herring, while faecal analyses showed that >90% of common seal prey (by weight) were clupeoid fish.(2) During the day, clupeoids concentrated in trenches and holes more than 12 m deep. Radio-tagged seals were located regularly over these areas. At night, clupeoid shoals rose in the water column and became more dispersed. Diel changes in seal activity patterns suggest that seals fed more often during the day.(3) Prey sizes were estimated from the size of otoliths retrieved from seal faeces. Estimated sizes of clupeoids taken by seals were similar to the sizes of fish caught in trawls, even though estimates were not corrected to allow for partial digestion of otoliths. This suggests that the rapid otolith digestion rates previously reported from captive seals may have been artificially high, or that the Moray Firth seals selected fish larger than those caught in trawls.
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