New Study Says Cormorants and Anglers Do Not Compete for the Same Fish

by Steve Chupack

Coromorants nesting in tree

he bad news is that the cormorant population continues to show very substantial growth, and is taking over new territory. The good news is that the wave-skipping black birds are not in direct competition with anglers for the same meal.

Researchers at the University of Vermont have released the findings of an important two year study examining population dynamics and feeding habits of Double- crested Cormorants (Phalarcrocorax auritus).

Among the new findings were that the number of nesting pairs increased from 2235 in 1995 to 3079, a 38% jump. Nesting sites increased from six islands in 1995, to eight in 1996. The study also concluded that small yellow perch, rather than more prized catches, make up over 80% of the cormorant diet of about a pound of fish per day. The study was conducted by Margaret Fowle as part of her M.S. thesis project, under the direction of Professor David Capen in the School of Natural Resources, in cooperation with N. J.. Buckley in the Department of Biology. It was funded by the United States Fish and Wildlife Service.

The study’s primary purpose was to estimate the size of the cormorant population and establish models for predicting future growth. The research focused on Young Island, land owned by the State of Vermont, located about a mile off Grand Isle. The method to determine the current and projected population included counting the number of nests, and factoring in complex variables such as number of young per nest (the researchers decided it was 2.54), number of birds of breeding age, and survival rate of chicks. Finding out what Cormorants were eating required a more messy method: examining the regurgitant of nestlings. Turns out, that when a researcher approaches a nest, the just-fed chicks do their part for science by throwing up. It’s just their natural defense mechanism,’’ says researcher Fowle, ” we didn’t take it personally.”

The finding about the growing population will surprise few lake watchers. (You can just hear the good-natured barbs at the bait shop: “Yeap, those fellas at the university sure do like to count things”).

However, the finding on what cormorants seek out as their fish of choice, small yellow perch, may not be so readily accepted. There is a firmly held belief among some who fish the lake that cormorants are consuming large amounts of valuable sport and commercial fish. The study’s findings do not support this claim. For example, the average length of yellow perch taken by anglers is reported to be about seven-and-a-half inches, considerably bigger than the average size consumed by cormorants (four-and a quarter inches). The study concludes: “Thus in terms of size [of fish taken], direct competition between cormorants and anglers on Lake Champlain appears to be minimal.”

An estimated 2 million pounds of fish were eaten by the bird in 1996, and researchers calculate that about 1.66 million pounds (83%) were yellow perch, practically all in the small, bait-size category. The small proportion of salmon family fish, smallmouth bass, walleye or other valuable sport and commercial fish consumed by cormorants, less than 2%, was not a big surprise to researchers.

The study revealed that, based on official reports of catches ofYellow Perch, people take 50% less fish out of Lake Champlain than cormorants do. Is this a cause for concern? Unfortunately, nobody knows how many yellow perch are in the lake, so the absence of good information makes for great debate. “In fact,” acknowledges Professor Capen, “nobody really knows what the long-term effect might be if cormorants continue to consume small yellow perch at their present rate”. Given current information, the supply of yellow perch appears to be so abundant he believes that what the cormorants eat doesn’t diminish in any substantial way what’s left for the sport or commercial market.

Just how serious is the population problem? There are seventy-two islands in Lake Champlain, eight of which to date have become permanent homes to the cormorant: The Four Brothers, Young, Bixby, Shad, and Mud. Young Island has lost all its trees as a result of the cormorant’s nesting habits and associated destructive effects on the island.

It is estimated that the number of breeding adults has grown from about 500 pair in 1989, to 1100 pair in 1994, and may reach a projected 7100 pair in the year 2000. This phenomenal growth is all the more dramatic when it is put in the perspective that there actually was a dramatic decline in the cormorant population regionally and in the nation in the period from about 1950 to the early 1970’s. The bird was given protected status in the Migratory Bird Act in the early 1970’s.

The next issue of Harbor Watch will consider these questions. Why has there been such a dramatic increase in the cormorant population? And, what about the gull population? Who’s concerned, and why? What’s being done to assess and address the concerns?

If you have questions, suggestions, or an opinion on cormorants or gulls that you would like to share with Harbor Watch readers, you may reach us at:



Birds Devastate Island Habitat

This is the second and concluding part of our look at the impact of the dramatic increase in the population of the Double-crested Cormorant and Ring-billed Gull on Young Island, in Lake Champlain. Both articles are based on work done by researchers at the University of Vermont.

by Steve Chupack

Cormorants seem to be one of the least liked birds, this picture sort of sums it up!

his is the second and concluding part of our look at the impact of the dramatic increase in the population of the Double-crested Cormorant and Ring-billed Gull on Young Island, in Lake Champlain. Both articles are based on work done by researchers at the University of Vermont.

Last Week: Review of Research Findings

Last week’s issue (May 22-29) presented highlights of information from recent research on the cormorant population of Young Island. Margaret Fowle, working under the direction of Professor David Capen at the University of Vermont’s School of Natural Resources, conducted field studies on the island in the late spring of 1995 and 1996. Among her major findings were: (1) the number of breeding adults has grown from about 500 pairs in 1989, to 1100 pairs in 1994, and may reach a projected 7100 pairs in the year 2000. Thus, this small island west of Grand Isle could be the home for nearly 15,000 adult birds in a few more years. The number of nesting pairs grew by just under 38% between 1995 and 1996. The cormorants colonized another two islands,

bringing the total to six. (2) Small Yellow Perch, rather than more prized catches sought by anglers, make up over 80% of the comorant diet. Though birds and humans are not competing directly for the same fish, nobody knows what the long term implications are if this pattern continues.

1989 Gull Study Revisited

This week we take another look at Young Island; this time at the impact of the Ring-billed Gull on the habitat of the island. The information comes primarily from the 1989 Master of Science thesis by Alicia Daniel, currently with the Department of Botany at the University of Vermont. The research findings and model used to project future impact were based on her field work on Young Island. Although there have been no formal follow-up studies, both the findings and model remain valid, according to Daniel and Capen. If anything, there is new evidence to support the researcher’s hypothesis. Fowle focused on the cormorant population and their diet. Daniel examined the impact of the gulls on the vegetation and soils. Together, these studies provide an excellent, comprehensive picture of the birds and their impact on Young Island as well as possible implications for other sites the birds have colonized. Daniel’s methods included testing soil and guano samples, counting birds and nests, mapping and evaluating vegetation, and the use of aerial photographs.

Why Young Island Was Ideal Site to Study the Impact

Daniel selected Young Island because it offered an excellent opportunity to study the effects of severe natural pollution. Previous studies elsewhere had documented what happens when ecosystems are exposed to man-made pollution, such as that caused by irradiation and smoke plumes from iron smelters. Daniel wondered what the response of a wooded ecosystem would be to increasing levels of biological pollution. As a field laboratory for this purpose, Young Island was perfect. The gull population increased from 100 nesting pairs in 1951 to almost 13,000 pairs in 1989. They expanded from one corner of the island to over half of its total area during this period. Her eyes and nose told her, in her first explorations of the island, there was no shortage of biological pollution.

Predicting the Island’s Declining Ecosystem

The question Daniel set out to answer was whether a similar pattern, or model, would occur if the polluting agents were natural and biological. Her findings on Young Island, with its huge gull colony, corroborated three of the five predicted outcomes.

Specifically, (1) large plants, trees, and shrubs of all species have died; and, (2) the structure of the forest ecosystem has changed to one dominated by small scattered shrubs and herbs, including weedy species not previously present. (3) The large amounts of guano deposited by the birds results in toxic concentrations of some pollutants that may be limiting many plant species. (4) She did not find evidence that soil erosion had increased, (5) or that the run-off of soil and nutrients had disrupted interconnected aquatic systems.

Daniel concluded, in 1989, that “under the aggressive expansion of Ring-billed Gulls and Double-crested Cormorants, Young Island is rapidly losing its former diversity of flora and fauna, including nesting trees for Black-crowned Night Herons and Cattle Egrets. I found no way that the process could be reversed or even slowed as long as Ring-billed Gulls and Double-crested Cormorants continue to occupy the island.”

Gull Population and Impact

Ring-billed Gulls first set up nesting colonies on three Lake Champlain Islands in 1951, feeding on abundant new food sources provided by humans, such as garbage in landfills and leftovers from fast-food establishments. In May of 1989 there were 12,995 Ring-billed Bull nests on Young Island. Since each nest was tended by an adult pair, the total came to almost 26,000 birds.

Young Island is 1000 meters off the west shore of Grand Isle, and quite small. It is about 250 meters long and about 150 meters wide. In 1986 it was estimated that the gull colony had been growing in recent years at a rate of 17% per year. Other species on the island in 1989 included 221 pairs of Herring Gulls, two pairs of Great Black-backed Gulls, and 395 pairs of Double-crested Cormorants. Gulls drastically changed the island environment. Their droppings overfertilize vegetation and the birds trample and pull it out. They drive out other nesting birds by their behavior which has been described as “adaptable, opportunistic, omnivorous, gregarious, and prolific.” Why have the cormorant and gull populations increased so greatly? Major contributing factors include dramatic decrease in the use of pesticides, few natural enemies, protection under the Migratory Bird Act, plentiful food supply, and higher survival rates of young.

Cormorant Impact on Soils and Vegetation

Only one cormorant nest was documented on the island in 1981.The nearly 400 nests just eight years later had a number of adverse affects, including the likely displacement of smaller colonies of tree nesters, 35 pairs of Black-crowned Night Herons and 7 pairs of Cattle Egrets.Damage inflicted by cormorants includes breaking of tree limbs under the weight of the birds and their nests; depositing guano at the base of the trees, and whitewashing leaves with guano. The coating, which Daniel describes as making them look as if they have been painted white, reduces photosynthesis to near zero. The prediction by Daniel has come true: “the arrival of the cormorants to the last stand of uncolonized trees signals the beginning of a rapid (less than ten years) expatriation from the island of the Black-crowned Night Herons and Cattle Egrets.” The more serious long-term effects she predicted have also occurred, as they have hastened the death of the few remaining trees.

Who Cares?

Just about anyone with an interest in Lake Champlain is concerned, some more than others. Esthetically, the treeless islands are an eyesore to those who remember islands before the birds took over. Environmentalists who put a premium on bio-diversity see the crowding out of other species as a real loss. People who fish commercially, for sport or recreation are worried. Boaters and owners of island or shore property fear the devaluation of their land. The Department of Fish and Wildlife, businesses with a special interest in tourism, researchers and naturalists, are all on the list. (There is virtually no quantifiable evidence that cormorants or gulls constitute a health risk to humans).

Future Outlook For Young Island and Other Islands

Daniel recommended some management practices to the State’s Department of Fish and Wildlife. These concerned enhancing waterfowl nesting, maintaining an optimal balance among the current nesting species on the island, halting the decline of vegetation on the island to preserve an existing layered canopy structure of herbs, shrubs, and trees, and identifying best methods for minimizing public disturbance of the bird nesting areas. The expansion of the gull and cormorant population has proceeded at such a pace that the recommendations were either not practical to implement, or became moot as the birds took over the island. For example, Daniel pointed out that the acidity of the soil might be treated with applications of lime. However, as long as the birds are around to coat any new leaves with guano, she saw altering the chemistry of the soil to make it more hospitable to trees as a futile intervention.

Daniel’s long-term outlook for Young Island was optimistic – but it is a rather long time-frame. “Although I believe Young Island will continue on a downward spiral for the next couple of decades, losing the remainder of its native and woody vegetation, I am more hopeful that its recovery —in the event that the ring-billed Gulls and cormorants abandon this site — will be measured in decades and not millennia.” There is little evidence that the gulls will be leaving soon on their own. However, it is worth noting that the gull population probably peaked at about 13,000 nesting pairs in 1989. Experts believe the number dropped to about 10,000 pairs in 1995. Whether this downward direction is a trend, nobody knows, but the bird counters will be paying close attention to this indicator.

Options for the Future: Can, Should the Bird Problem Be”Managed”?

Some wildlife biologists argue that the gull and cormorant populations did not “naturally” explode. Those who hold this view say it is the result of complex interactions, most if not all of which can be traced to some previous action or intervention by humans. They would cite the massive amounts of human food waste that enterers the food chain, for example, as well as the restoration of environments less polluted with insecticides and other toxins. Interventionists argue not to do something aggressive soon to control the bird population only makes inevitable further destruction of valuable island and shoreline habitat.

To this point, there have been limited attempts to manage the cormorant population. The Vermont Fish and Wildlife Department and U.S. Department of Agriculture experimented in 1995 and 1996 with methods of discouraging the expansion of cormorant nesting colonies after cormorants established several new colonies. Tree nests were removed early in the summer and although the birds continued to roost throughout the summer, they did not rebuild the nests. Similar small-scale nest removals will be conducted again this year.

The Department of Fish and Wildlife has created a committee to review a wide range of options. Best bet of those most familiar with the bird problem is that funding will be sought to underwrite a major study that will specifically focus on options the State should consider. It will not be easy to reach consensus, as a number of parties and interests must be involved.

This issue will not go away, and may become increasingly controversial. This would almost certainly occur, for example, if gulls and cormorants succeed in colonizing additional islands or shoreline property with greater real estate value than Young Island or other islands they have occupied.

Appreciation is expressed to UVM researchers Dave Capen, Margaret Fowle, N.J. Buckley, and Alicia Daniel for making available to Harbor Watch the research upon which our two articles were based.