Audubon goes on to describe with astonishing accuracy the anatomy, distribution, habits, and nesting behavior of Double-crested Cormorants, including a vivid description of “…the birds on their nests, all over the rock, which was completely white-washed with their excrement, that emitted a disagreeable odour to a great distance”.
Although Double-crested Cormorants were common coastal birds at the beginning of the 20th century, cormorants were rare on Lake Champlain as recently as the 1970’s. The Vermont Breeding Bird Atlas recorded only a single nesting pair of cormorants in 1981 and the New York Breeding Bird Atlas found only 6 nesting blocks in 1984, mostly on Long Island. However, the New York atlas noted that one colony was established on the Four Brothers Islands in Lake Champlain, a harbinger of events to come. Apparently, DDT exerted the same deleterious effect on cormorant egg development that was observed in raptors, causing the cormorant population nationwide to crash. After DDT was banned in 1972 the door was open for cormorants to return to their natural breeding sites and beyond. Furthermore, the expansion of catfish aquaculture in the South supplied a rich, new food source for cormorants on their wintering grounds, leading to improved survival and healthier birds during the breeding season.
After DDT was banned Double-crested Cormorants expanded their range to include Lake Champlain. The cormorant population increased rapidly for more than two decades until it peaked in 1999. Since 1999 the population has remained stable at about 9000 nesting pairs, occupying 4500 nests annually. In Vermont one of the largest cormorant colonies on Lake Champlain historically was established on Young Island just offshore from Grand Isle. Young Island was donated to the state of Vermont by a local physician several years ago and is managed by the Vermont Fish and Wildlife Department. In 1999 the Department obtained a depredation permit from the USFW Service, allowing the state to oil the eggs of nesting cormorants to suffocate them and to shoot cormorants if necessary. It worked. The eradication program reduced the number of successful cormorant nests on Young Island significantly. An unintended consequence of this program, though, was that nesting cormorants dispersed to other Champlain islands to breed, in particular the Four Brothers Islands in New York. The total number of nesting cormorants on the lake hardly budged; they were simply redistributed. This observation suggests that attempts to control the cormorant population on the lake, must take into account the fact that disrupting cormorant nests on one island will likely cause cormorants to move their colony elsewhere. To effectively control cormorants on Lake Champlain, a lake-wide management program will be necessary.
Pressure to reduce the number of cormorants on the lake comes primarily from sport fishermen, who believe that cormorants threaten fish stocks in Lake Champlain. Cormorants are indeed large, fish-eating birds that consume about a pound of fish each day to meet their energy requirements. Although cormorants are opportunistic feeders, consuming whatever fish are available, they prefer smaller fish in the 3-6 inch class that swim in shallow water. In Lake Champlain that means Yellow Perch, which are present in abundance and often form schools in shallow water. Cormorants are excellent divers, propelled by their webbed feet, but prefer fish that swim in water less than 20 feet deep. Yellow Perch, Walleyes, and Smallmouth Bass, all fill the bill, so to speak. Trout and salmon on the other hand, species of great interest to recreational fishermen, are seldom found in the stomach contents of cormorants, the exception being those occasions when large numbers of small trout are released during stocking. Incidentally, it should be noted that Lake Trout and Landlocked Salmon do not reproduce in Lake Champlain in sufficient numbers to produce a sustainable population. It is largely a “put-and-take” fishery. The Vermont Fish and Wildlife Department “puts” and the fishermen “take”.
Several studies have attempted to measure the effect of cormorants on fish stocks with ambiguous results. In fact, separating the impact of cormorants on fisheries from other adverse factors such as pollution, toxins, algae blooms, weed growth, predatory fish such as lamprey, dams and other barriers to reproduction, invasive species such as zebra mussels, climate change, a shifting food web, and the “take” by sport fishermen themselves has proven to be difficult, if not impossible. Studies performed on Lake Oneida in New York have been cited to support the view that cormorants damage fish populations, in this case Yellow Perch. However, Lake Oneida is only 22 miles long and 22 feet deep on average, hardly comparable to Lake Champlain. At the other end of the spectrum, some, but not all studies from Lake Huron have been reported, which suggest that cormorants played a role in damaging the fishery there. Lake Huron is huge-over 200 miles long and 150 miles wide. Yet its fish populations, including introduced Chinook Salmon, have struggled for years for a variety of known and unknown reasons, only one of which is the burgeoning cormorant population. Whether the results of studies performed on Lake Huron are applicable to Lake Champlain is doubtful, if not misleading. Do cormorants have an impact on fish stocks? Yes. Does this impact adversely affect fish populations disproportionately on Lake Champlain? Unknown.
Another complicating factor has been the recent appearance of large schools of alewives in Lake Champlain. Cormorants colonizing the Four Brothers Islands have been forced to forage in the deeper waters of the lake where perch are seldom found, but alewives are abundant. Recent analyses of the stomach contents from cormorants nesting on the Four Brothers Islands have turned up predominantly alewives. Alewives are an invasive species in Lake Champlain, whose populations are subject to wide variation in numbers as well as temperature-related “die offs” as occurred in 2008. Although salmonids (trout and salmon) feed on alewives voraciously, the presence of the enzyme thiaminase in alewives could result in a deficiency of thiamine in predatory trout and salmon and their eggs. In this sense predation of alewives by cormorants may not be such a bad thing.
Less controversial is the effect of cormorants on vegetation. Cormorants build their nests in trees and on the ground by breaking off twigs and branches. Even more damage results from the ammonium-rich guano released by roosting birds. Their excrement coats the trees, rocks, and ground vegetation, defoliating the trees and killing the vegetation in short order. One need only visit the Four Brothers Islands to see the results of cormorants roosting and nesting there. One of the islands (Island D) where cormorants have been allowed to roost in the past consists of white-washed rocks and dead trees. Islands left untouched by cormorants support lush vegetation.
Although there is little evidence that cormorants are responsible for widespread pollution in the lake, comparable say, to agricultural runoff, local effects on inhabited islands may be considerable and in theory could pose a public health problem, not to mention the “disagreeable odour” described by Audubon. By permit the USDA Wildlife Division is allowed to mitigate cormorant-related damage when requested to do so by private landowners, using both lethal and non-lethal methods. Although this represents a band aid approach to the problem of cormorant population control, the mitigation of cormorant-related damage to private property, not to mention public health concerns, seems reasonable.
Another cormorant conundrum relates to interspecies competition. Birds are prisoners of their habitat. Lose the habitat; lose the birds. Given the adverse effect of cormorant guano on vegetation, concern about the loss of habitat suitable for other colonial birds and their nests is understandable. What species might be affected? Well, gulls for one. But gulls on Lake Champlain are thriving. In fact, Ring-billed Gulls are also targeted for population reduction by the Vermont Fish and Wildlife Department and the USFW Service. Interestingly enough, gulls prey on the eggs of Double-crested Cormorants and seem to cohabitate with them quite well.
Terns? Common Tern populations have rebounded modestly on selected Lake Champlain islands under the watchful eye of Audubon Vermont’s Mark LaBarr, the leader of the Common Tern Restoration Project. Caspian Terns are uncommon on Lake Champlain, but seem to have gained a foothold in recent years. Restoring Common Terns to the lake has required preserving suitable nesting conditions, including the exclusion of cormorants and gulls from nesting sites, and fending off predators such as Great Horned Owls, Black-crowned Night Herons, Ruddy Turnstones, and mink. The success of this project demonstrates that an intensive focus on restoring and maintaining selected bird populations works.
Herons and egrets? Regionally, Great Blue Heron, Black-crowned Night Heron, and Great Egret populations seem to be stable, although Black-crowned Night Herons are a species of moderate concern nationally. Cattle Egrets? Cattle Egrets are non-natives in North America, but the population is thriving nationally, having spread from Florida, where the first colony was established in 1952, to many of the coastal northeastern and southeastern states. In Vermont the small population of Cattle Egrets nesting on Lake Champlain is holding on tenaciously despite many obstacles to its growth.
Interspecies competition is an issue (and works both for and against cormorants), but does not justify a wholesale reduction in cormorant numbers. In fact, prior to 1999 the majority of cormorant nests in Vermont were established on Young Island until the Vermont Fish and Wildlife Department intervened, causing widespread dispersal of nesting cormorants to other Lake Champlain Islands. Perhaps the issue of habitat destruction and interspecies competition would be less problematical today if this intervention had not occurred. Protecting species of concern where they exist or have existed as exemplified by the Common Tern Restoration Project, rather than a drastic reduction in the population of perceived “nuisance” birds, might be a reasonable alternative approach to preserving biodiversity on Lake Champlain.
The USFW Service is developing a draft proposal to reduce the size of the cormorant population on Lake Champlain to 3000-6000 birds from the current 14000-18000 cormorants according to the Burlington Free Press (July 11, 2010). The Vermont Fish and Wildlife Department supports the most aggressive approach enthusiastically, reflecting the Department’s traditional bias toward the concerns of fishermen. Nationally, the USFW Service is prepared to kill over 200,000 cormorants annually.
Is this legal? From a legal standpoint, Double-crested Cormorants are covered under the International Migratory Bird Treaty of 1918 and its amendments. This treaty is responsible in large part for outlawing the practice of hunting Snowy Egrets and Great Egrets for their plumage and preventing the widespread massacre of shorebirds by hunters. Not before Eskimo Curlews were hunted to extinction, though. The treaty has been effective in protecting migratory birds threatened by overhunting or loss of habitat. In theory, the treaty should also protect birds such as cormorants whose populations have been successfully restored. Indeed, in Canada the return of cormorants to their native haunts is viewed as a huge success story. However, in the United States economic considerations often receive higher priority than maintaining biodiversity. The International Migratory Bird Treaty allows states wide latitude in managing bird populations within their borders and provides no enforcement mechanisms when states violate the intent if not the provisions of the treaty. Thus, it is unlikely that the USFWS plan, when it eventually comes to light, will be subject to negotiation with other signatories under the terms of the International Migratory Bird Treaty.
The cormorant conundrum produces more questions than answers. Consider these questions. Where exactly will cormorants be allowed to nest on Lake Champlain without harassment? Will there be unintended consequences associated with attempts to reduce the cormorant population such as disturbance of other nesting birds? Will killing cormorants really improve the nesting success of more valued birds like herons, terns, and egrets? Is an industrial-scale program to kill cormorants a cost-effective use of scarce resources? What will the cost of this program be now and in the future? Is the knowledge base about cormorants and their impact on the Lake Champlain fishery sufficiently robust to determine with confidence that drastic population reduction will improve selected fish stocks? Or are we simply removing a visible, unpopular, predatory bird from the ecosystem that has become a scapegoat for many of Lake Champlain’s problems? At the very least careful public scrutiny of the USFWS draft proposal by all of the lake’s stakeholders is essential before the cormorant control program proposed by the USFWS is implemented.
