Diet, Growth and Reproductive Success of Double-crested Cormorants (Phalacrocorax auritus) at Dore Lake, Saskatchewan

Abstract

From May through August in 1985 and 1986, boluses of fish regurgitated by Double-crested Cormorants were collected at Dore Lake Saskatchewan, to determine if the birds were feeding on fish with commercial or sport value. The collections were part of a study done in response to complaints from commercial fishermen, sport fishermen and resort owners from Dore Lake, who felt a decrease in their catches of northern pike, walleye and lake whitefish had occurred as a result of high fish consumption by cormorants. The second part of my work involved examination of cormorant growth and reproductive success. Eighteen nests along a transect were marked in May 1986 and mean clutch size, brood size, number of chicks fledged and dates of clutch initiation were compared based on the location of the nest in the colony. Chick growth rates in relation to hatching sequence, were compared within and between nests, and adult-chick interactions at the nest were observed to determine if feeding rates differed among chicks in the same brood. Yellow perch and white sucker made up 78% of the weight of samples collected and ninespine stickleback, yellow perch and spottail shiner made up 80% of the total number of fish collected. No significant difference was found in clutch size, brood size or number of chicks fledged based on nest location but clutches in nests on the edge of the colony were initiated earlier than those in the centre of the colony. Within a nest the first chick to hatch was fed more often and grew faster than its siblings. Both feeding frequency and growth rate decreased with hatching sequence, the later a chick hatched the less often it was fed and the slower it grew. Cormorants were likely not responsible for decreases in the walleye population and lake whitefish and northern pike populations are healthy and their numbers increasing (Sawchyn 1987). Present low numbers of walleye and decreased numbers of lake whitefish and northern pike in the past, are thought to be the result of a combination of factors, the main one being heavy fishing pressure (Sawchyn 1987). The cormorants may be of benefit to the fishermen by preying on yellow perch and white sucker, species that could be competing with the three species of sport or commercial fish. Differences in feeding frequencies and growth rates within a brood and an average of 50% of the chicks in a brood fledging, may indicate a limitation in availability of food in Dore Lake, or in the adult's ability to capture prey. The reasons for earlier clutch initiation on the periphery of the colony are unknown. Microclimatic differences on the edges of the island may provide better areas for nesting; these would be occupied by the first birds to return to Dore Lake in the spring and the centre of the colony would be occupied by birds that arrived later.