CORMORANT (PHALACROCORAX-CARBO [L]) POPULATIONS AND PATTERNS OF ABUNDANCE AT BREEDING AND FEEDING SITES IN NORTHERN-IRELAND, WITH PARTICULAR REFERENCE TO LOUGH-NEAGH

Breeding cormorants, Phalacrocorax carbo, in Northern Ireland remain s easonally dependent on a coastal environment where they can be censuse d with accuracy, but numbers in other habitats and at other times of t he year are less certain. This study establishes the long term and reg ional patterns of abundance at breeding and feeding localities, which might in tum be related to diet. Birds were regularly observed and cou nted at a variety of feeding sites, and some aspects of their breeding success and fledgling diet evaluated at the largest N.Ireland breedin g colony. Numbers of breeding birds increased dramatically over a peri od of eight years but recently show signs of declining. There is likel y to be a dynamic relationship between populations of a tapeworm (Ligu la intestinalis L.), the numbers of roach (Rutilus rutilus [L.]) and c ormorant populations feeding at Lough Neagh, the largest lake in the B ritish Isles. While roach are not a major component in the diet of fle dglings, the large numbers of cormorants feeding at the perimeter of L . Neagh suggest that changes in roach populations will affect birds mo st acutely during or following the winter. We suggest that this might result in a reduction in the proportion of the population volunteering to breed in the subsequent season, but require further data. The long er term effects of one fish species on cormorant populations are unlik ely to be critical, since these birds are highly opportunist. In other habitats cormorant numbers are either very stable (estuary) or variab le (river) depending on the seasonal and annual availability of their prey. There is no evidence for a systematic seasonal shift in habitat, as suggested by other studies.