Ministry of Natural Resources
Double-crested cormorants: Their distribution and potential impact on fisheries in northwestern Ontario Scott Parker Double-crested cormorants (Phalocrocorax auritus, henceforth cormorants) are large, fish-eating birds found on numerous waterbodies throughout Ontario 1. The cormorant has undergone dramatic fluctuations in population and range expansion throughout the past century. Most significant is the population explosion and re-invasion of the Great Lakes region and surrounding waterbodies since the mid 1970s 1,2. Their consumption of fish as prey as well as their impact on the environment has resulted in widespread public concern. Many anglers, tourist outfitters, and commercial fishers are concerned that cormorants deplete fish populations through predation, or that they remove a disproportionate amount of prey biomass that supports game fish populations, such as walleye (Sander vitreus). Similar concerns have been raised regarding the increasing movement and establishment of breeding colonies of cormorants on inland waterbodies in northwestern Ontario. As such, cormorant management has become a contentious issue among stakeholders and resource users in the region. Cormorants are found across Canada, from the Pacific coast to the Atlantic Ocean, and from as far north as James Bay and south throughout the Great Lakes 3. Cormorants were known to be present in northwestern Ontario on Lake of the Woods as early as the 1700s. It is believed that cormorants first colonized the Great Lakes region early in the 20th century with the first reports of nesting birds in 1913 at the western end of Lake Superior 2,4. Cormorant numbers in the Great Lakes region increased steadily until the 1950s. During the 1960s and 1970s cormorant populations experienced devastating declines and were almost extirpated as a result of toxic contaminants, specifically the pesticide DDT (dichlorodiphenyltrichloroethane). Cormorants were especially susceptible to the biomagnification of DDT that, when metabolized, produced DDE (dichlorodiphenyltrichloroethylene). The high level of contamination was related to eggshell thinning, hatching deformities, and extensive reproductive failure throughout the Great Lakes region and elsewhere 3. By 1973, the entire Great Lakes population declined to approximately 130 breeding pairs and were extirpated completely from Lake Superior 3,5. Since then, populations in the Great Lakes region have increased dramatically over the past three decades.
Aquatic Update 2011-5
Cormorants are migratory and are present in the Great Lakes region from approximately mid-April until September 3,5. They generally do not reach more northerly latitudes (e.g. Canadian prairies) until lateApril or early May 6. Dense colonies nest in trees or on the ground on islands and peninsulas in close proximity to feeding areas. Cormorants typically forage during the day in shallow water that is less than 10 m deep and within 5 km of shore. Foraging is usually solitary, but they have been observed fishing in large cooperative flocks 6. There has been extensive research on the diet of cormorants in North America 6,7,8,9,10,11. Cormorants typically feed on slow-moving or schooling fish that are approximately 15 cm in length or smaller 7,9,12,13. They are a generalist feeder preying on the most abundant or readily available fish species present 9. The majority of studies have shown that cormorants feed primarily on species of little to no commercial value such as alewife (Alosa pseudoharengus), rainbow smelt (Osmerus mordax), gizzard shad (Dorsoma cepadianum), and various Cyprinid (minnow) species 5,6,9,10,11,14. However, cormorants will feed on sport fish such as yellow perch (Perca flavescens), walleye (Sander vitreus), and smallmouth bass (Micropterus dolomieu), where available 15,16,17,18,19. There is an abundance of contradictory evidence regarding the impact of cormorants on recreational fisheries. Some research suggests that cormorants may have been a factor in the localized decline or suppression of recreational fisheries 15,16,17,18,19,20. Other research indicates that cormorant predation is not a significant contribution to the overall mortality of fish populations 10,21,22,23,24,25. Declines in yellow perch and smallmouth bass stocks in the eastern basin of Lake Ontario been attributed to increasing cormorant numbers 15,17. Similar declines in yellow perch abundance in northern Lake Huron, coincided with increasing cormorant numbers 16. However, in both instances, other factors such as overfishing likely contributed to fishery declines 16. Other studies acknowledge the extent of cormorant predation but fail to relate cormorants to recreational fishery declines. In the Les Cheneaux Islands of Lake Huron, cormorant consumption of yellow perch had little effect on its population size or the recreational