The Rise of the Double-crested Cormorant on the Great Lakes
The Double-crested Cormorant (Phalacrocorax auritus), a large fish-eating bird now found throughout the Great Lakes, has undergone dramatic changes in population over the last three decades. Devastated by the effects of toxic chemicals, the number of nesting pairs decreased by 86% from approximately 900 in the early 1950s to a mere 125 in 1973. The cormorant disappeared as a nesting bird on Lakes Michigan and Superior and only about ten pairs remained on Lake Ontario. From 1973 to 1993, however, the
cormorant population increased over 300- fold to more than 38,000 pairs. The cormorant is now more numerous on the Great Lakes than at any time in its previously recorded history..........
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The Cormorant or "Crow-Duck"
There are about 30 species of cormorant worldwide, including six in North America, of which the Double-crested Cormorant is the most common and widespread, and the only species which is commonly seen inland around bodies of fresh water.
The Double-crested Cormorant is a large, greenish-black waterbird with a slender hooked-tip bill, orange facial skin, and webbed feet set well back on its body. It is named for the two small tufts of feathers on either side of its head, which appear on the adults in spring plumage. The bird can frequently be observed standing erect on rocks or posts, sometimes in a spread-eagle posture; or swimming low in the water, often with only its headand neck exposed. On the water, it can be distinguished from loons by the distinct upward angle of its head and bill. Flocks often fly in "V" formations similar to geese. This behaviour and their general colouring likely led to cormorants being called "crow-ducks" by early European settlers.
In North America, the Double-crested Cormorant nests from southwest Alaska, central Alberta, James Bay and Newfoundland south to the Gulf of Mexico. Populations centred in Florida and along much of the Pacific coast are resident there year-round; birds from Alaska, the North American interior and the Atlantic coast migrate south in winter to the Gulf of Mexico, the Bahamas and the Greater Antilles. Populations nesting on the Great Lakes migrate south via two routes: some go directly south along the Mississippi River drainage while others first travel east to the Atlantic coast and then south to the Gulf Coast. Southward migration begins around the end of August, and most birds return to the Great Lakes the following April.
Cormorants are very sociable year-round and almost always nest in colonies. They require undisturbed nesting sites with a convenient food supply, and usually build their nests (consisting of sticks and other drift material) on the ground - on rocks, islets, cliff tops and ledges - or in trees near water. On the Great Lakes, egg laying commences in late April and early May after the birds have gone through courtship and have built their nests. Cormorants usually lay three to four light blue eggs which may be partially
covered with a white chalky covering. The eggs hatch after about 25-28 days of incubation. The young become fully independent about ten weeks after hatching, around mid-August. However, the timing of nesting can vary considerably. In June, for example, it is common to see freshly constructed empty nests, nests with well-incubated eggs, and nests with young, all in the same colony. Both the male and female birds share in nest-building, incubation and feeding of the chicks. The birds have the unusual habit of
incubating their eggs by wrapping the webs of their feet around them. The cormorant's diet consists primarily of fish, but sometimes includes small invertebrates such as crayfish.
In the Great Lakes, the population of the Double-crested Cormorant showed an initial 30-year period of colonization (1920s - 1950s) followed by a 20-year decline (1950s - 1970s) and, most recently, a spectacular 20-year resurgence (1970s - 1990s).............................................
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Historically, it is thought that the Double-crested Cormorant did not nest in the Great Lakes. Archaeological excavations in aboriginal settlements have not shown any evidence of the bird. Although cormorants have nested in Lake of the Woods (in northwestern Ontario) for hundreds of years, the first suspected nesting on the Great Lakes did not occur until 1913, at the far
western end of Lake Superior. From there colonies spread eastward to Lake Nipigon in the 1920s, to Lake Huron and Georgian Bay in the early 1930s and finally to Lakes Ontario and Erie in the late 1930s.
This range expansion coincided with similar cormorant expansions all across North America. However, the order in which the lakes were colonized strongly suggests an eastward expansion of the Great Plains population, rather than a westward spread of the Atlantic population. The cormorant's invasion was successful, as their population increased steadily during the '30s and '40s. By the late 1940s and early 1950s the cormorant had becomeso common that control measures were authorized in some areas of Ontario to reduce suspected competition with commercial and sport fisheries................
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The Concern
Many fish-eating birds, and cormorants in particular, arouse suspicion and even hostility among fish harvesters, who believe that these birds reduce the numbers of commercially and recreationally valuable fish. Anglers and commercial harvesters claim that cormorants consume large quantities of desirable fish. Studies have repeatedly shown that in a natural environment,
cormorants feed primarily on small, largely non-commercial, shallow-water fish. On the Great Lakes, these include abundant species such as Alewife (Alosa pseudoharengus), Rainbow Smelt (Osmerus mordax) and Yellow Perch (Perca flavescens). The birds also take much smaller numbers of White Suckers (Catostomus commersoni), Pumpkinseed, crappie, bass (Centrarchidae) and sticklebacks. However, as cormorant numbers continued to steadily increase in the 1940s, fish harvesters in Ontario became
increasingly concerned. By 1946 they were calling for a cormorant control program..............
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Control Measures
In response to the growing cormorant numbers, a control policy was launched, primarily on Georgian Bay. This remained in effect until 1966. Early local attempts at control included the destruction of all eggs at various colonies. This proved unsuccessful, because the birds responded by laying more eggs, often on different islets, where they could raise their brood before the new site was discovered. Later methods of control avoided this problem by spraying the eggs with a solution of formaldehyde and soap. This cut off the air supply to the developing embryo, causing it to suffocate, but left the eggs intact. This method destroyed all reproduction for the year because by the time a cormorant pair had incubated the sprayed (but intact) clutch for the full incubation period or longer, it was too late in the season for them to attempt another nesting.
Unsanctioned control by fish harvesters also likely began during this period and included organized annual destruction of colonies by shooting adults, and destroying eggs, nests and young. Both sanctioned and unsanctioned controls largely ended by 1960. At best, the controls only slowed the growth of the cormorant population during the 1940s and 1950s; they probably did not reduce its size appreciably....................
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Decline: 1950s - 1970s
Reproductive Failure
Despite the end of the control measures, the Double-crested Cormorant population declined dramatically throughout the 1960s and early 1970s. Evidently some other factor was at work. By 1973, the cormorant population in the Great Lakes had declined by 86% and breeding birds had vanished from Lakes Michigan and Superior. During the late 1960s, scientists discovered that the eggshells of cormorants nesting on the Great Lakes had been thinning since about 1955. By the early 1970s, eggshells were nearly 30% thinner than normal. This had a devastating impact on the cormorant population. Thin-shelled eggs could not withstand the weght of the incubating
bird and would break before reaching term, killing the embryo. Not surprisingly, scientists also discovered that reproductive success -- the number of chicks raised successfully -- had declined from a "normal" level of about two chicks per pair to only 0 - 0.2 chicks per pair. For cormorants to maintain a steady population, the number of young produced each year and eventually entering the breeding population must match the number of adult deaths. However, this production rate was far too low to balance adult mortality rates, which probably accounted for most of the dramatic decline
in population levels. Clearly something was severely wrong with the cormorants on the Great Lakesssssssssssssssssss
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The Role of Toxic Contaminants
During the 1960s it was suspected that the declining cormorant population was related to the high levels of toxic contaminants, particularly DDE and PCBs, then present in the Great Lakes. The symptoms of the decline -- widespread reproductive failure associated with moderate to severe thinning of eggshells, and high frequency of egg breakage -- are all characteristic of DDE contamination. Residues of both DDE and PCBs in cormorant eggs from
Lake Huron were found to be the highest known for this species in Canada from 1968 to 1972. In 1972 researchers discovered that 95% of the eggs in the Lake Huron colonies had broken or disappeared by the end of the incubation period. Toxic contamination was the most likely cause of the widespread reproductive failure among cormorants on the Great Lakes during this period. Similar declines and reproductive failure were noted in other parts of the cormorants range, including Alberta, Minnesota and Wisconsin..............
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Resurgence: 1970s - 1990s
The Cormoran Explosion
In the mid-1970s, cormorant numbers began a dramatic recovery. From 1973 to 1991, their numbers increased more than 300-fold. During this 18-year period, the average annual rate of increase was approximately 35%, meaning that the cormorant population was doubling every three years. From 1973 to 1993, more than 80 new cormorant colonies were established, including several sites which had been abandoned since the 1970s or earlier. In 1993, the total number of colonies on the Great Lakes was over 100. Eggshell
thickness has returned to more normal levels, as has reproductive success. In Lake Ontario, for example, an average of 1.9 young per pair was produced in 1990...................
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Decreasing Toxic Chemicals
At the same time that reproductive success and population size were improving, contaminant levels were falling. The most regularly monitored sites (colonies in Lake Huron) showed DDE and PCB levels in cormorant eggs decreasing by more than 80% between 1971 and 1989. Similarly significant reductions in one or both of these compounds have been recorded in several other species of Great Lakes wildlife, including Herring Gulls, Common and Caspian Terns, Ospreys and Lake Trout. The rapid decline of contaminant levels in the mid and late 1970s was due mainly to regulations that were implemented in the early 1970s restricting the use and production
of DDT and related pesticides..........
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Cormorant Numbers on the Canadian Great Lakes
The cormorant populations on Lakes Ontario and Erie and the Canadian portions of Lakes Huron and Superior show an increase, population levels and growth rates are much greater in Lakes Huron and Ontario than in Lakes Erie and Superior. These differences are probably related to the number of possible nesting sites and the availability of food in each lake. Lake Erie is a very productive lake, providing the cormorants with an abundant supply of fish, however the lake has very few uninhabited islands where the birds
could nest. At present, cormorants nest only on four islands in Lake Erie. In contrast, Lake Ontario, and particularly Lake Huron, have many potential nesting islands. Neither lake is as productive as Lake Erie but there is still enough fish to support a large cormorant population. Lake Superior, has an abundance of isolated islands. However, it is much less productive than the other Great Lakes, supporting fewer fish and only a small population of
cormorants..............
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Increasing Food Supply
The dramatic decline in contaminant levels from the mid-1970s to the present day is unlikely the sole factor responsible for the increase in cormorant numbers. Cormorants showed a much slower rate of increase during their initial invasion of the Great Lakes in the 1930s and 1940s, when contaminants would not have affected their population. The dramatic cormorant increase was probably augmented by a rise in the numbers of smaller fish, such as Rainbow Smelt and Alewife, which serve as the bird's primary food source. Since the 1970s, these smaller prey fish have been much more abundant than they were 30 to 40 years earlier. Even though Smelt and Alewife declined during the 1980s, their numbers were still great enough to provide an abundant food supply for the cormorants. These smaller prey fish tend to occur in large schools and inhabit relatively shallow water, providing an ideal food source for the cormorants. Elsewhere around the world, increases in the populations of several fish-eating birds have also been associated with changes in prey abundance.................
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PCBs and Deformities
Polychlorinated biphenyls (PCBs) have been in use since 1929. There are 209 types of PCBs - a small number of these have toxic properties, and are thought to account for the bulk of PCB contamination in animals. The low flammability of PCBs made them useful as fire retardants in insulating and heat-exchanging fluids used in electrical transformers and capacitors. The same property made them useful as lubricating oils. They were also used as
plasticizers and waterproofing agents and in inking processes used to produce carbonless copy paper.
In Canada, uses of PCBs were voluntarily reduced by industrial producers in 1971, and regulated in 1977. Importation of all electrical equipment containing PCBs was banned after 1980, and use of these chemicals was restricted to existing equipment. Transport of PCBs was regulated under the Transportation of Dangerous Goods Act in 1985, and storage of PCBs was controlled in 1988.
In the early 1970s, deformities in several types of waterbirds began to be reported throughout the Great Lakes. These included crossed bills, club feet, extra digits, and eye and skeletal deformities. Bill malformations are one deformity which is clearly developmental (i.e., begun as the embryo develops within the egg) rather than the result of accidents or trauma after the bird has hatched.
Therefore, bill malformations are considered reliable indicators of misfunctions in the normal developmental process and there is strong evidence that PCBs may be responsible.
From 1979 to 1987, the frequency of bill defects in Double crested Cormorant chicks on the Canadian Great Lakes ranged from 0 to 6.2 (average = 3.9) in every 10,000 chicks. This is a higher frequency than that found in relatively uncontaminated areas, such as the Canadian prairies where defects are only 0.6 per 10,000; but much lower than that found in extremely contaminated areas, such as Green Bay in Lake Michigan, where deformities approached 52 per 10,000 chicks. From 1988 to 1992, the frequency of bill defects on
the Canadian Great Lakes ranged from 0 to 3.2 (average = 1.4) per 10,000; clearly the rate of bill deformities is decreasing in some areas.ome areas....reas................
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How Much Fish Do Cormorants Eat?
Do They Compete with Sport Fish?
To determine how much fish cormorants eat in a season we have to know how much they eat each day, how long they are present on the Great Lakes and how many cormorants there are in total.
The average cormorant weighs approximately 1.9 kg (4.2 lbs) and will eat about 25% of its weight in fish each day or about 0.48 kg (1.0 lbs). Most adult cormorants reside on the Great Lakes from about mid-April to late August or early September (about 135 days). During that time, one adult cormorant will eat about 65 kg (143 lbs.) of fish. Most young cormorants, on Lake Ontario, hatch in late May but do not really start eating their "pound" of fish per day until about mid-June. Most young cormorants remain on the Great Lakes until mid-late September, or for about 100 days. In 1991, for example, over 40,000 cormorants (adults and young) lived on Lake Ontario and consumed about 2.25 million kg (5 million lbs.). That sounds like a lot of fish - until you consider how many smaller prey fish and larger sport fish there are in the lake. Fisheries biologists estimated that there are 418 million kg (920 million
lbs) of the smaller prey fish in Lake Ontario. Salmon and trout consumed approximately 56 million kg (123 million lbs) of these prey fish. Hence, the sport fish took about 13.4% of the prey fish and the cormorants took 0.5%.
To look at the same question in another light, statistics from the Glenora Fisheries Research Station (Ontario Ministry of Natural Resources) are used. In 1991, for example, there were about 6.1 million sport fish in eastern Lake Ontario. The average annual food consumption by a single Lake Trout is 6.5 kg. The average annual intake of a cormorant on Lake Ontario is 65 kg. In other words one cormorant eats about as much as ten Lake Trout. Approximately 30,000 cormorants fed in eastern Lake Ontario in 1991. Thus, these birds would eat the same amount of fish as 300,000 Lake Trout.
Since there are about 6.1 million sport fish in eastern Lake Ontario, this means the cormorants are eating the same amount of fish as only 5% of the sport fish. Hence, scientists and fish managers conclude that the amount of fish which cormorants consume in eastern Lake Ontario is not a serious threat to the sport fish. .............................
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The Future
Can they increase forever?
The Double-crested Cormorant clearly demonstrates the "population explosion" phenomenon. This can be expected in any species occupying a large new habitat, such as the Great Lakes. Today, the lakes are even more attractive to these birds: legislation now protects cormorants, commercial fishing has decreased, human persecution has lessened and levels of toxic chemicals are lower.
If the current rate of population growth continues, the Great Lakes cormorant population would increase to more than three-quarters of a million birds by the year 2000. However, this is unlikely to occur. Most animals which colonize new areas show a period of initial, unrestricted growth, just as we have seen with cormorants over the last 10-15 years. Such growth, however, can not be sustained forever. Eventually, the population outstrips its food
supply, outgrows its habitat (or nesting areas), or is reduced by disease or predation. When this happens, the cormorant population can be expected to drop in numbers, until it stabilizes itself again.
In recent years, the explosive growth of the cormorant population has definitely slowed. The largest colony on the Great Lakes, at Little Galloo Island in Lake Ontario, showed a 31% decrease in numbers of breeding pairs between 1992 and 1994. In 1994, the cormorant population of Lake Ontario decreased by 6%. This was the first such decline in over 15 years. In 1992, Newcastle Disease Virus killed up to 30% of the young cormorants in several colonies. These population declines and disease outbreaks are signs that
other factors are starting to have an impact on the population. Thus it seems unlikely that the cormorant population will continue to grow as it hs during the 1980s and early 1990s.
According to fisheries biologists, stocks of the smaller prey fish have been decreasing dramatically in recent years. This decrease in the cormorants' food supply is probably already contributing to their decline on Lake Ontario...........................ario.
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Too many Cormorants?
Fish harvesters have raised concerns about the rapid increase of cormorants and its potential effect on fish numbers in the Great Lakes. They fear that: 1) cormorants are competing with fish harvesters for large, major sport fish like Lake Trout and salmon; 2) the birds feed on the same prey fish which the large predatory fish need for food; and, 3) cormorants feed on and deplete local supplies of pan fish, such as perch and bass.......
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There is also concern about the effect of cormorants on the vegetation in their nesting grounds. Cormorants can damage vegetation by stripping leaves from trees. The combined weight of the birds and their nests can even break branches. But perhaps most importantly, their excrement, which rains down to the ground from their nests, kills the ground vegetation and eventually kills the nest tree. In some cases, the loss of these trees can lead to increased erosion. This is of particular concern on sandspits and barrier beaches which protect interior wetlands. In other areas, the vegetation may be of unusual natural significance, such as the islands in western Lake Erie which are forested by rare stands of Carolinian woodlands. The large cormorant colonies there could seriously impact or even destroy this vegetation.
These unresolved issues have spawned suggestions that it is time to implement anther control program. However, because the cormorant colonies are so widely distributed, and their numbers so large, it would be difficult to implement an effective large-scale control program using known methods.
The cormorant is a native species in Canada. There is a biological principle that states that the greater the number of different organisms an area can support, i.e. the more diverse the wildlife, the better the area. This is known as biodiversity. The richest areas in the world for biodiversity are the tropics...moist, warm areas the year 'round. The temperate zone areas, where we live and where the Great Lakes are located, are comparatively poor in biodiversity. We have lost many species from the Great Lakes already...the Atlantic Salmon, the Blue Pike, the Passenger Pigeon, the Whooping Crane...we don't need to lose anymore..........................
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The return of the cormorant to the Great Lakes has been a tremendous success story. The species almost vanished from the Great Lakes due to the effects of DDE and other toxic chemicals. Through voluntary and legislated controls, levels of this compound and other toxic substances have declined dramatically in the Great Lakes - to a point where the cormorant population can again breed successfully. The cormorant has finally been re-established as an integral component of the Great Lakes ecosystem.
The cormorants' return has been very successful, perhaps even too sucessful. Some interest groups are voicing concern over cormorant numbers, and the problems they are creating. While biologists have shown that some of these concerns are not well founded, others require further research. However, in recent years, the cormorant population has not grown as rapidly as it once did. In fact, biologists suspect that the birds may evon Lake Ontario, their numbers declined by 6% between 1993 and 1994. This was likely in response to a decrease in the numbers of alewife - a small fish
which serves as their main food source.
Finally, changes in reproductive success and the incidence of deformities in Great Lakes waterbirds are two very strong indicators of ecosystem health. Although cormorants are now reproducing at normal levels, the fact that deformities are still occurring suggests that contaminant-related health problems still persist in Great Lakes wildlife. While we have made great headway in cleaning up the Great Lakes, there is still considerable work
to be done before we can be assured that the Great Lakes Basin is a healthy environment for all species.....................
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DDE and Eggshell Thinning
DDT came into widespread use as an insecticide in the late 1940s. The chemical is regulated by Agriculture Canada under the Pest Control Products Act and most uses were banned in 1974. Registration of all DDT products was discontinued in 1985; however, the use and sale of existing stocks of DDT products were allowed until Dec. 31, 1990. DDE (Dichlorodiphenyldichloroethylene) is a "metabolite" or breakdown product of DDT (dichlorodiphenyltrichloroethane). DDE is produced in most animals
when the body tries to rid itself of DDT. It is the levels of DDE, and not DDT, which are routinely measured in animals. DDE is the most fat-soluble of the DDT breakdown products, and thus the most commonly found compound in animals.
It is this storage of DDE in the animal's body fat which makes the effect of the pesticide so insidious. When a predator, such as a fish-eating bird, consumes contaminated prey, the chemicals in the fish's tissues are stored in the bird's body. Over a life-time of consuming thousands of fish, the birds can accumuate high levels of chemicals in their bodies.
Reduced productivity and eggshell thinning were the first problems related to contaminants identified in birds on the Great Lakes. Eggshell thinning is strongly correlated with DDE levels -- as DDE levels increase, so does the degree of thinning. Thinning is caused by the presence of DDE in female birds. Eggshells are made of calcium carbonate; the calcium is obtained in part from the bird's bones and transferred to the eggshell. DDE inhibits the action of the enzyme needed in the transfer of this compound. As a result,
the eggshell does not contain as much calcium carbonate as it should. It is thinner than normal, and cannot support the weight of an incubating bird. Cormorants are very sensitive to the effects of DDE, and are particularly sensitive to the resulting eggshell thinning because they incubate their eggs by wrapping the webs of their feet around them -- in effect, standing on them.tanding on them............
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For further reading
* Anderson, D.W. and J.J. Hickey. 1972. Eggshell changes in
certain North American birds. Proceedings of the 15th International
Ornithological Congress. 15: 514-540.
* Christie, W.J. 1972. Lake Ontario: Effects of Exploitation,
Introductions, and Eutrophication on the Salmonid Community.
Journal of the Fisheries Research Board of Canada 29:913-929.
* Great Lakes Fishery Commission. 1992. Status of the Lake
Ontario offshore pelagic fish community and related ecosystem in 1992.
Lake Ontario Committee. Kingston, Ontario, 28-29 July 1992. 33 pp.
* Lewis, H.F. 1929. The Natural History of the Double-crested
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