Tag Archives: Fish

Water Birds and Cormorants Lake Ontario

Water Birds and Cormorants Lake Ontario


Double Crested Cormorant studies have dwelt on analyzing pellets to determine the diet of (piscivous = fish eating) Cormorants. One reseacher stated to complete the diet analysis regurgitate and stomach analysis are part of the equation.

Aggressive Interactions. A University of Regina 2,007 study emphasized the substantial increase of Cormorants in the last three decades. These researchers compared Lake Ontario’s Hamilton Harbour, nesting success and aggressive behaviour of Herring Gulls in a colony shared with Cormorants to those in an identical gull-only colony. Results showed “Herring-Gulls nesting with Cormorants engaged in aggressive interactions near their nests than those in a single species colony.”

Gull catches fish

These same researchers conducted a similar study with American White Pelicans in Saskatchewan. In this case, the presence of Cormorants appeared to diffuse Pelican aggression. Their findings showed Cormorants may be capable of displacing other ground – nesting water birds. And, this week The Canadian Press reported in newspapers “White Pelicans risk death flying south toward the Gulf of Mexico Oil Spill (BP).”

Nestings. Tommy Thompson Park, a man-made wilderness park in Toronto has 250,000 park users annually. Their 2,006 Cormorant colony was the largest with 6,125 pair breeding nests. Of the Cormorant 144 chicks diet samples, 90% ate alewife. At this park, there is also a large Black-Crowned Night Heron rookery in the same location as the Cormorants.

Interestingly, a study monitored by the Canadian Wildlife Service in Lake Ontario showed the wetland dependent Black-Crowned Night Heron’s eggs had 30% higher mercury levels than Herring Gulls and Black-Backed Gulls. In 2,003 the “state-endangered” Black-Crowned Night Heron in the Calumet area of the south-west of Lake Michigan has one of the largest breeding colonies. The Black-Crowned Night Heron became the “poster child” for ecological restoration to this area.

A University of Minnesota study in 2,007 of northern Lake Michigan showed nestings of Black-Crowned Night Herons, Great Blue Herons and Cormorants. Here, on the studied island, half of the trees on the island died; yet, half remained alive. Excessive nutrient input which can over-fertilize trees, retard growth and cause morality were cited as issues. And, waterbird guano (seabird droppings) may be causing morality to vegetation by similar over fertilization mechanisms. An Ottawa National Wildlife Refuge study emphasized “Habitat destruction with Cormorant nesting areas, and, the need to protect vegetation for the co-nesting of other waterbird species.”

Foraging Ecology. Northern Lake Michigan 2,005 data suggest Cormorants in the region feed primarily on forage fish and crayfish. In 1999 – 2,002 this area re-kindled a Small Mouth Bass population. Lac (lake) La Biche in northern Alberta changed from ” a system with Walleye as the top fish-eating predator to one where double crested Cormorants are at the top of the aquatic food chain.” As a result, due to egg oiling and culling the Cormorant population has decreased. This 2,006 decrease in cormorant consumption = 1 million kg fish not consummed by cormorants. The ecosystem model asserts potential restoration of Walleye dominance.

Prey Consumption. In Lake Huron, especially the North Channel and Georgian Bay Cormorants were abundant, but in 2,004 they declined. The sharp decline in alewife may have been the primary cause of the Cormorant decline. But, Cormorants increased dramatically in Lake Ontario. One 2,007 study examined Northern Pike and Muskellunge recovered in Cormorant pellets. This pellet analysis showed 1.1% (60,000 fish) were Esocid (pike) and they were predominant in the Cormorant’s diet.


Another 1997 study gives another viewpoint as they give evidence of Secondary Consumption by Cormorants. The researchers suggest “invertebrate prey were present in the digestive tracts of fish that were consumed by Cormorants. Gastropods (consumed first by Pumpkin seed fish), and, between the presence of Dacapods and fish species that are known as crayfish predators, notably Rock Bass and Small Mouth Bass.”

Prey consumption was discussed in a 2,006 food study of Cormorants. Here, of the 228 stomachs examined, 77% contained food. the most plentiful food was Emerald Shiners, Round Gobies , and Gizzard Shad. Although Emerald Shiners were he most abundant prey, 67% of cormorant consumption consisted of Gizzard Shad, White Perch and Freshwater Drum.


Hope you enjoy the information and pictures today

Doug Worrall