The Relationship Between the Abundance of Smallmouth Bass and Double-crested Cormorants in the Eastern Basin of Lake Ontario

Available population and diet data on double-crested cormorant ( Phalacrocorax auritus) and smallmouth bass ( Micropterus dolomieui) numbers, demographics, and exploitation rates were synthesized to examine the relationship between cormorant and smallmouth bass abundance in the U.S. waters of the eastern basin of Lake Ontario. It was found that after the number of cormorants nesting on Little Galloo Island in New York exceeded 3,500 pairs in 1989, survival of young smallmouth bass, not yet of legal size for the sport harvest (< 305 mm), began to decline. Despite production of strong year classes in 1987 and 1988, abundance of smallmouth bass measured from gill net surveys declined to its lowest level by 1995 and remained there through 1998. Stable or increasing catch and harvest rates in other local fisheries along the U.S. shore suggested that declines in smallmouth bass abundance in the eastern basin were not related to water quality. Stable or increasing growth rates for smallmouth bass age 2 and older since the 1980s further indicated that food resource limitation was also not the cause for declines in abundance. Comparisons of estimates of size and age-specific predation on smallmouth bass by cormorants with projected smallmouth bass population size indicated that much of the increased mortality on young smallmouth bass, could be explained by cormorant predation.