Double-crested cormorant predation on yellow perch in the eastern basin of Lake Ontario

Previous work indicated that the abundance of yellow perch (Perca flavescens) in the eastern basin of Lake Ontario declined from 1976 to 1999 despite production of moderate to strong year classes each year during 1991 through 1995. Adult perch stock size failed to increase because of accelerated mortality after the first fall of life. Increases in mortality coincided with a number of ecosystem changes including increased abundance of double-crested cormorants (Phalacrocorax auritus). Otoliths obtained from cormorant pellets collected on Little Galloo Island were used to examine the size and age of perch consumed by cormorants during 1993 to 1994 and 1996 to 1999. Size and age specific diet composition, combined with existing estimates of yellow perch consumed annually by cormorants were compared to perch population projections to evaluate the potential for this new form of predation to induce observed population trends. Perch stock abundance was projected using a range of standing stock estimates front the literature partitioned with age composition data front the eastern basin population. The total length of perch consumed by cormorants ranged from 59 to 236 mm, the majority of which were age-1 (48%), age-2 (20%), and age-3 (20%). Comparisons of age structured predation by cormorants and perch population projections indicated that cormorant predation reduced age-3 perch abundance most. At a high estimate of 65 kg/ha, cormorants it,ere capable of consuming 29% of the age-3 perch stock. This analysis indicated that cormorant predation had the potential to play an important role in regulating perch population levels in the eastern basin during the 1990s.