Double-crested Cormorant culling in the St Lawrence River Estuary

The St. Lawrence River Double-crested Cormorant (Phalacrocorax auritus) population grew from 6,155 breeding pairs in 1979 to 17,854 in 1988. Ground-nesting birds made up 54% of the population (374+/-70SE in 21 colonies) and tree-nesting ones the remainder (92+/-338SE in seven colonies). Deterministic modeling led to a forecast of 27,000 pairs in 1993. In order to halt further damage to unique and restricted insular forest ecosystems, a five-year culling program was introduced in 1989 with the following goals: to reduce the breeding population to 10,000 pairs and to discourage tree-nesting. Modeling revealed that these goals could only be attained by lowering recruitment (egg-spraying) in accessible ground nests and by reducing the number of adults in inaccessible forest habitats al the same time. The dual approach-spraying 75% of all nests (5,051+/-279SE/y) with inert mineral oil in ground-nesting colonies and culling (1,979+/-66SE/y) breeding birds in arboreal ones-has since reversed population trends. By 1991, three years after the beginning of the program, the estuarine population fell to 12,000 pairs. Periodic control measures Mill be needed at the end of the five-year program to maintain the population at the 10,000 pairs level.