CHARACTERISTICS OF A RAPIDLY INCREASING COLONY OF DOUBLE-CRESTED CORMORANTS (PHALACROCORAX-AURITUS) IN LAKE-ONTARIO - POPULATION-SIZE, REPRODUCTIVE PARAMETERS AND BAND RECOVERIES

The double-crested cormorant (Phalacrocorax auritus) is now a prominen t component of the Great Lakes ecosystem. Lake Ontario supported 24% o f 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 t hree 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 p ersecution, which kept overall Great Lakes cormorant numbers low earli er this century; and 3) increased availability of forage-base fish, pa rticularly alewife (Alosa pseudoharengus). Cormorant increases at LGI since 1983 were correlated significantly with abundance indices of sex ually mature alewife three years previously. Most cormorants first bre ed when 3 years old, and alewife are important in the diet in Lake Ont ario. Since the DDT-era, alewife availability has probably been the ke y factor affecting cormorant reproductive output, fledging condition, and post-fledging survival on Lake Ontario, and hence subsequent recru itment rates. LGI cormorants winter along Atlantic coasts of the south eastern U.S. and in the lower Mississippi Valley. Improved over-winter survival due to exploitation of farmed channel catfish (Ictalurus pun ctatus), may also have increased recruitment rates of cormorants.