Stable isotopes indicate the extent of freshwater feeding by cormorants Phalacrocorax carbo shot at inland fisheries in England

1. The numbers of cormorants Phalacrocorax carbo feeding at English freshwa ter fisheries during winter have increased rapidly over the last 20 years, causing concern among fishery managers and anglers. 2, In order to assess the extent of freshwater feeding, stable isotope rati os of carbon and nitrogen (delta(13)C and delta(15)N) in feathers of wild c ormorants from inland freshwater fisheries were compared with those in the feathers of piscivorous birds with marine diets (captive 'marine-fed' cormo rants, free-ranging shags Phalacrocorax aristotelis) and freshwater diets ( juvenile goosanders Mergus merganser). 3. Isotope signatures of feathers represent the diet at the time of growth. Feathers grown at different times of the year were taken from wild cormora nts; each feather type therefore represented the diet over a different temp oral scale. 4, Isotopic analyses of feathers indicated that, when shot, nearly all of t he cormorants had been feeding entirely on freshwater prey. The mean delta( 13)C value of primary feathers growing when birds were shot was -22.2 parts per thousand, indicative of an entirely freshwater diet. 5. The move to freshwater habitats from coastal breeding grounds occurred o ver several months, but once established cormorants appear to have fed at f reshwater sites throughout the autumn and winter. 6. The suitability of using a two-source isotopic mixing model in order to quantify the extent of freshwater feeding in piscivorous birds is discussed . 7, Although the results indicate long-term residency and feeding in freshwa ter systems, they do not indicate whether birds were feeding regularly at t he sites at which they were shot, or the composition of the diet. It is rec ommended that further studies using telemetry and multiple isotope analyses be carried out in order to address these issues.