Predation by seals on salmonids in two Scottish estuaries

Detailed observations of the behaviour of harbour seals, Phoca vitulina L., at sites within the estuaries of the Rivers Dee and Don, in north-eastern Scotland, were made over two full years between 1993 and 1996. Small number s of grey seals, Halichoerus grypus Fab., were also present. The presence o f seals within the estuaries was strongly related to season, with maximum n umbers observed in winter and early spring; seals were virtually absent in June and July. The River Don was used largely as a haul-out site, while the River Dec was used predominantly as a foraging site, although it was not p ossible to determine whether the same seals were using the two estuaries. M ore seals were hauled-out on the River Don during twilight and dark than in daylight. The seals were observed to cat mostly salmonids, Salmo salar L. and S. trutta L., unidentified roundfish and flounder, Pleuronectes flesus L. The otoliths identified in seats collected at the mouth of the River Don belonged to marine species indicating that the seals were also feeding out side the estuaries. A minimum estimate is given of the numbers of large sal monids eaten in each river during the course of the year. Although no infor mation was available on the numbers of salmonids using the rivers or the re productive status of the fish eaten by the seals, as a cause of mortality, seal predation on large salmonids in estuaries is apparently an order of ma gnitude less important than mortality caused by angling within the river.