Characteristics of a Rapidly Increasing Colony of Double-Crested Cormorants ( Phalacrocorax auritus) in Lake Ontario: Population Size, Reproductive Parameters and Band Recoveries

Abstract

The double-crested cormorant ( Phalacrocorax auritus) is now a prominent component of the Great Lakes ecosystem. Lake Ontario supported 24% of the breeding population in 1991, and the largest colony—5,428 nests on Little Galloo Island (LGI). Increases at LGI averaged 36% per annum since colonization in 1974. This dramatic increase is attributed to three main factors: 1) reduced levels of organochlorine contaminants in the diet, particularly DDE which caused eggshell thinning, leading to egg breakage and total reproductive failure; 2) relaxation of human persecution, which kept overall Great Lakes cormorant numbers low earlier this century; and 3) increased availability of forage-base fish, particularly alewife ( Alosa pseudoharengus). Cormorant increases at LGI since 1983 were correlated significantly with abundance indices of sexually mature alewife three years previously. Most cormorants first breed when 3 years old, and alewife are important in the diet in Lake Ontario. Since the DDT-era, alewife availability has probably been the key factor affecting cormorant reproductive output, fledging condition, and post-fledging survival on Lake Ontario, and hence subsequent recruitment rates. LGI cormorants winter along Atlantic coasts of the southeastern U.S. and in the lower Mississippi Valley. Improved over-winter survival due to exploitation of farmed channel catfish ( Ictalurus punctatus), may also have increased recruitment rates of cormorants.