Diet and prey selection of cormorants (Phalacrocorax carbo) at Loch Leven, a major stocked trout fishery

Abstract

Numbers of great cormorants Phalacrocorax carbo feeding in inland waters during winter have increased throughout Europe resulting in concerns over their impacts on fisheries. Loch Leven is a world-renowned sport fishery for trout and supports a nationally important population of wintering cormorants. The relationships between brown trout stocking, regional cormorant numbers and cormorants counted during winter on Loch Leven were examined. Stocked brown trout were fin-clipped before release and gut contents from a sample of 96 cormorants shot under scientific licence were analysed. The data suggest that the number of cormorants at this site is influenced by both the stocking policy and the regional population level. Cormorant diet varied with the sex and age of birds. Overall, trout made up c. 85% of the total content by weight. Brown trout predominated in the diet, making up c. 70%, while rainbow trout occurred less frequently, making up c. 6%. Trout which could not be identified to species, made up the remainder (c. 9%). There was no significant difference in the ratios of wild and stocked brown trout sampled from the loch and from cormorant stomachs in winter. A model was constructed to investigate the likely loss of trout to cormorants roosting on the loch. It was estimated that over a 7-month period cormorants consumed 80 803 (41 617–128 248) brown and 5213 (830–12 454) rainbow trout. These estimates compared to average annual fishery catches of 5828 brown and 12 815 rainbow trout (1996–2000). Comparison with published estimates of trout populations in Loch Leven indicated a high potential for competition between the birds and the fisheries for available fish. It is suggested that stocking has led to an increase in cormorant numbers and the subsequent increase in predation may have prevented any increase in the trout population or in fishery catches.