Publication Abstract

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

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

S. Bearhop, D.R.Thompson, S. Waldron, I.C. Russell, G. Alexander and R.W. Furness 1. The numbers of cormorants Phalacrocoraxcarbo feeding at English freshwater fisheries during winter have increased rapidlyover the last 20 years, causing concern among fishery managers and anglers.

2. In order to assess the extent offreshwater feeding, stable isotope ratios of carbon and nitrogen (δ ¹³C and δ ¹⁵N) in feathers ofwild cormorants from inland freshwater fisheries were compared with those in the feathersof piscivorous birds with marine diets (captive 'marine-fed' cormorants, free-rangingshags Phalacrocorax aristotelis) and freshwater diets (juvenile goosanders Mergusmerganser).

3. Isotope signatures of feathers representthe diet at the time of growth. Feathers grown at different times of the year were takenfrom wild cormorants; each feather type therefore represented the diet over a differenttemporal scale.

4. Isotopic analyses of feathers indicatedthat, when shot, nearly all of the cormorants had been feeding entirely on freshwaterprey. The mean δ ¹³Cvalue of primary feathers growing when birds were shot was -22-2 per thousand, indicativeof an entirely freshwater diet.

5. The move to freshwater habitats fromcoastal breeding grounds occurred over several months, but once established cormorantsappear to have fed at freshwater sites throughout the autumn and winter.

6. The suitability of using a two-sourceisotopic mixing model in order to quantify the extent of freshwater feeding in piscivorousbirds is discussed.

7. Although the results indicate long-termresidency and feeding in freshwater systems, they do not indicate whether birds werefeeding regularly at the sites at which they were shot, or the composition of the diet. Itis recommended that further studies using telemetry and multiple isotope analyses becarried out in order to address these issues.

Reference:

S. Bearhop, D.R.Thompson, S. Waldron, I.C. Russell, G. Alexander and R.W. Furness, 1999.Stable isotopes indicate the extent of freshwater feeding by cormorants Phalacrocoraxcarbo shot at inland fisheries in England. Journal of Applied Ecology, 36: 75-84.

S. Bearhop, D.R.Thompson, S. Waldron, I.C. Russell*, G. Alexander and R.W. Furness

Journal of Applied Ecology, 36: 75-84