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Editor's note: This article originally appeared in the March 2006 issue of FLW Outdoors Magazine. Learn more about FLW Outdoors Magazine and how to subscribe by clicking here.
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Cormorants eat fish – a lot of them. An adult cormorant will eat about a pound of fish per day, more when they are feeding young in the nest.

Cormorants – specifically the six species of them, including the most common double-crested cormorant – were once regulars throughout North America. Hunting and egg harvest in the late 19th and early 20th centuries reduced cormorant populations. Environmental contaminants – particularly organochlorine pesticides like DDT that accumulate in fish and, therefore, in fish-eating birds – led to a drastic reduction in cormorant numbers in the mid-20th century. In the Great Lakes, where cormorant counts have been maintained for years, only 200 nesting pairs remained in 1968. Reduction in contaminants since the 1970s and protected migratory bird status facilitated a resurgence of cormorants. In 2000, 115,000 nesting pairs of cormorants were reported from the Great Lakes. The current North American cormorant population is estimated at 2 million birds.

Cormorants are very effective predators. They can dive to 40 feet deep. They eat whatever fish they can catch, with most ranging in size from 3 to 10 inches long.

Older feeding studies consistently concluded that cormorants consumed fish in proportion to their abundance. Gizzard shad dominate the cormorant diet in Southeastern reservoirs. Alewives and emerald shiners are heavily consumed in the Great Lakes, as are the rapidly proliferating round gobies.

But some recent studies indicate that burgeoning cormorant populations can impact sport fish populations, too. In Mississippi, cormorants primarily ate abundant shad, but showed a preference for adult bluegills. Smallmouth bass abundance around Little Galloo Island in Lake Ontario declined after the Island cormorant colony exceeded 3,500 nesting pairs. At the same time, smallmouth bass abundance increased or remained stable in cormorant-free areas of the lake.

In Oneida Lake, N.Y., a new and rapidly expanding cormorant population took a heavy toll on yellow perch and walleyes. Cornell University researchers estimated that cormorants harvested as many yellow perch as did anglers. But the cormorants also consumed younger yellow perch, affecting the number of perch recruiting to the fishery. Cormorants consumed only subadult walleyes and therefore did not compete directly with anglers. But the cormorants consumed approximately 30,000 subadult walleyes. Most of the perch and walleyes eaten by the cormorants were a year or more old. This is important, especially for walleyes, because factors that have the greatest effect on the density of fish populations occur during the first year of the fish’s life. The cormorants were eating into the portion of the population that was destined to recruit to the sport fishery.

The decreasing yellow perch harvest in the Les Cheneaux Islands region of Lake Huron is also attributed to increasing cormorants. But here is where the plot thickens: Heavy angler harvest in the mid-1980s and early 1990s put the yellow perch population on the verge of collapse. In the early 1990s, the once abundant alewife population crashed, a result of heavy predation by overstocked salmonids. Is the scarcity of yellow perch solely the result of the expanding cormorant population, or did the scarcity of alewives create a situation in which cormorants continued to suppress a perch population already imperiled by man’s hand?

Cormorants have been the bane of commercial fishers, fish farmers and recreational anglers for centuries. Authorized cormorant control efforts are now in place where populations have proven to be a nuisance. Although this represents a significant change in management philosophy, cormorant suppression in the foreseeable future will likely continue to be a local, after-the-fact management solution.