Restoring Fish Populations leads to tough Choice for Great Lakes Gulls
You might think that stocking the Great Lakes with things like trout and salmon would be good for the herring gull. The birds often eat from the water, so it would be natural to assume that more fish would mean better dining. But a new report published in the journal Ecology by the Ecological Society of America says that the addition of species such as exotic salmon and trout to the area has not been good for the birds, demonstrating that fishery management actions can sometimes have very unexpected outcomes.
Craig Hebert (National Wildlife Research Centre in Ottawa, Canada) and his team analyzed 25 years of data on the gulls and found that throughout the Great Lakes region, the birds; diet had changed through time. Measurements of fatty acids in eggs showed an increase in a type of transfat that mostly comes from food produced by humans.
“It seems that the birds are being forced to make a dietary shift from fish to terrestrial food, including garbage,” says Hebert.
Although no one is certain why the birds are eating more garbage, evidence points to fish stocking. When exotic salmon and trout have been added to the waters, the birds seem to be out competed for their favorite prey of smaller fish, such as alewives.
Herring gulls, which differ from the ring-billed gulls that often populate North American beaches and parking lots, are by no means endangered.
But the birds have long been used as monitors of environmental conditions on the Laurentian Great Lakes. Their eggs are collected annually and analyzed for insights into how the region’s food webs and contaminant levels are changing.
Gulls are top predators in this system. When fish are unavailable, the birds turn to land instead for their foraging. When given a choice between prey fish and garbage, the birds readily chose the fish. Thus scientists assume that they only eat garbage when the prey fish numbers are low.
Prey fish abundance in the lakes has declined over the last few decades. Although multiple factors may be at play, predation by piscivorous fish appears to be the one factor that was universally important across all five Great Lakes because of massive fish stocking. The stocking was done to create recreational activities, and to reduce populations of exotic prey fish in the hope of restoring populations of native fishes.
“The effects on other species that are more closely tied to the water, including predator fish and other birds, may be more severe,” says Hebert. “For example, terns can only eat fish, so their diet may be affected by this, too. They don’t have the option of eating food found on land, such as human garbage.”
The Ecological Society of America is the world's largest professional organization of ecologists, representing 10,000 scientists in the United States and around the globe. Since its founding in 1915, ESA has promoted the responsible application of ecological principles to the solution of environmental problems through ESA reports, journals, research, and expert testimony to Congress. ESA publishes four print journals and one online-only, open-access journal Ecosphere and convenes an annual scientific conference. Visit the ESA website at www.esa.org or find experts in ecological science at www.esa.org/pao/rrt.
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