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Issues |
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Issues |
Identification
The double-crested cormorant is the only cormorant
species found in the Great Lakes region. It is a medium to large
(typical length 70 – 90 cm, body mass 1.2 – 2.5 kg) greenish-black
coloured waterbird. The bill is long (5-8 cm), slender and hooked at the
end. The bird has brilliant turquoise eyes, a golden throat patch, and a
cobalt-blue mouth for a short time prior to and during the breeding
season. Range
In Canada, cormorants breed as far west as British
Columbia and easterly to the Atlantic Coast including Newfoundland. In
Ontario, they are found throughout the Great Lakes and have a breeding
range that extends north to southern James Bay. Populations nesting in
Ontario or on the Great Lakes migrate south along the Mississippi River
drainage or travel east to the Atlantic coast and then south to the Gulf
coast Diet and Foraging
Cormorants are fish-eating birds that typically feed on
slow-moving or schooling fish that are up to approximately 15cm in
length. They are an opportunistic, generalist feeder, normally preying
on abundant and easy to catch fish species. Cormorants are known to be
opportunistic to the extent they will congregate at specific times and
areas where fish concentrations are high, such as spawning grounds,
stocking release sites, and aquaculture facilities; however they are
also known to feed solitarily. Cormorants have been observed to forage
in cooperative flocks in order to force schools of fish into shallow
bays or inlets to enhance foraging success. The diet of cormorants has
been studied extensively in North America. Research suggests that the
majority of their diet is composed of small fish that are of relatively
low economic and commercial value. In the Great Lakes region these
species primarily include: alewife, rainbow smelt, gizzard shad, and to
a lesser extent, sticklebacks, white suckers, pumpkinseeds, rock bass,
and more recently, round goby. History
Double-crested cormorants were known to be present in
north-western Ontario on Lake of the Woods as early as the 1790s.
Cormorant populations expanded across the Great Lakes between the 1900s
and the 1950s. Nevertheless, cormorant colonization was documented
moving eastward across Ontario from the early 1900s to the late 1930s.
Concerns for recreational and commercial fisheries prompted Ontario to
implement a cormorant control program in the late 1950s in specific
areas of the Great Lakes. These early control attempts only slowed the
growth of the population during the 1950s and did not significantly
reduce the Great Lakes population. Despite their range expansion and prolific nature,
cormorant populations experienced a dramatic decline throughout the
1960s and early 1970s. The Great Lakes-wide population dwindled to
approximately 130 nesting pairs by the early 1970s and breeding birds
had completely disappeared from the U.S. portion of the Great Lakes. In
the late 1960s, biologists discovered that this drastic population
decline was the result of reduced recruitment due to eggshell thinning
and reproductive failure caused by high levels of toxic contaminants in
the Great Lakes. As a result, some U.S. states (i.e., Wisconsin,
Illinois, Michigan) listed cormorants under their endangered species
legislation in the 1970s and early 1980s. New regulations, enhanced enforcement and public
awareness surrounding toxic contaminants resulted in significantly
reduced levels of toxic chemicals (i.e., reduced by 80%) in the Great
Lakes between 1971 and 1989. As a result, reproductive success returned
to relatively normal levels for cormorants and their populations began a
radical recovery in the mid to late 1970s. From 1973 to 1993, cormorant
numbers increased by nearly 300-fold to 38,000 nesting pairs and 80 new
colonies across the Great Lakes. The dramatic recovery of cormorant populations cannot
be solely attributed to decreased contamination in the Great Lakes.
Population increases have been much higher than the rate of increase
during the initial range expansion into the lower Great Lakes during the
1930s and 1940s. A rich and abundant food supply in breeding and
wintering areas also played a significant role in their dramatic
recovery. Alewife and rainbow smelt were the primary food sources for
cormorants in the Great Lakes and these prey fish stocks experienced
significant population increases in response to the decline in large
predatory fish during this time. Increased aquaculture and abundant food
supply at cormorant wintering grounds (i.e., the southern United States)
likely contributed to an increase in over-winter survival of cormorants
as well. Throughout the 1990s cormorant populations continued to
increase across the Great Lakes and expand into previously unoccupied
areas including some inland lakes.
The highest recorded cormorant populations in the history of the Great
Lakes were recorded during the late 1990s and early 2000s. Over the past
five years some regions of the Great Lakes have begun to show signs that
cormorant populations are leveling off. Some areas of the Great Lakes
basin have experienced significant decreases in cormorant numbers over
the past five years, which may suggest that a new biological carrying
capacity is being reached. Inland Lakes,
Southern Region
Over the past several years there has been an increase
in the number of foraging and nesting cormorants on inland lakes. In
1987, the Ontario Breeding Bird Atlas reported no confirmed cormorant
breeding outside of the Great Lakes in the Southern Region. Cormorants
have since expanded to establish a number of new breeding colonies on
inland water bodies, however the vast majority of foraging and nesting
still occurs along the Great Lakes coastline. To date, approximately 20
inland water bodies have confirmed cormorant nesting colonies in
Southern Region.
Lakes in red have cormorant
populations. The largest inland nesting colonies in Southern Region
occur on larger water bodies adjacent to the Great Lakes, including Lake
Simcoe, Lake Couchiching, and Rice Lake. Small groups (1 – 10 birds)
of transient foraging cormorants are commonly seen throughout smaller
lakes from April to October. Larger (50+ birds) resident foraging flocks
are known to occur on Lake Simcoe, Lake Couchiching, the Muskoka Lakes,
the Kawartha Lakes, Rice Lake, Golden Lake, and Lake Opeongo. Concerns about
cormorants in Ontario
Double-crested cormorants are a native species and a
natural part of Ontario’s ecosystem. Over the past several years,
increasing cormorant populations in Ontario have led to growing concerns
from the public, including requests for control programs. These concerns
include impacts on fisheries, vegetation, other species, and human
socioeconomic values. Although cormorants are a natural and valuable
component of the ecosystem their rapid range expansion, population
growth and large numbers in some areas have caused real and/or perceived
conflicts with some stakeholders and ecological impacts to ecosystems at
varying scales. Impacts on Fish
There is a perception by some stakeholder groups and
individuals that fish predation by cormorants is detrimental to
recreational and commercial fisheries in Ontario and throughout the
United States. Cormorants are opportunistic feeders that prey on the
most abundant and easy to catch fish. Bioenergetics studies and
supporting literature suggest that cormorants consume approximately 20
to 25% of their body mass in fish daily Many other birds (e.g., mergansers, scaup, herons,
kingfishers, gulls, terns, loons, etc.) are also capable of consuming
large quantities of fish; however cormorants generally forage with more
concentrated intensity in local areas (i.e., < 5 km of nesting
colony) than other resident birds (e.g., herons, egrets, gulls,
kingfishers, loons, etc.) and for longer periods of time than other
migrant birds. Migrant red-breasted mergansers were estimated to consume
more annual lake-wide fish biomass than cormorants on Lake Erie in the
mid-1990s; however cormorants were responsible for the greatest total
fish consumption in the western basin, suggesting a more concentrated
local fish consumption by cormorants in the areas surrounding their major
nesting colonies. Gulls are also extremely abundant in the Great Lakes
region; however impacts to fisheries remain of relatively little public
concern because their feeding is limited to surface waters and they
consume a wide range of food items. Cormorants are often condemned by some members of the
recreational and commercial fishing community as being one of the main
causes for the decline in some Great Lakes fish populations. Various
studies have shown that cormorants can have an impact on fish abundance
and production; however, there has been no evidence to suggest that
cormorants have been responsible for the demise of any species in the
Great Lakes-wide fish community. Direct impacts from cormorant predation
on larger game fish may be almost non-existent, as cormorants generally
consume fish that are up to approximately 15cm in length. In some cases,
there have been measurable impacts to younger age class sport fish.
Rudstam et al. (2004) and Van De Valk et al. conducted
studies on walleye and yellow perch stocks in Oneida Lake, New York, and
found that while adult walleye exploitation was attributed to angling,
the exploitation of sub-adult walleye was almost entirely attributed to
cormorant predation. Lantry et al. (2002) also reported similar
findings indicating that cormorant predation on 3-5 year old age classes
of smallmouth bass was substantial enough to cause observed declines in
local populations in eastern Lake Ontario. Increased rates of mortality
in younger year classes may severely limit the numbers of adult stock
recruited to the fishery. Belyea et al. (1999) noted that
cormorant predation accounted for a greater proportion of mortality in
young yellow perch, while angling accounted for a greater proportion of
mortality in older yellow perch, but ultimately concluded that cormorant
predation had a minimal impact on\ local Les Cheneaux Island perch
populations during their study. In general, cormorants will seldom take valuable
recreational or commercial fish species, such as walleye, smallmouth
bass, lake whitefish, lake trout, and other salmonids despite recent
population increases. Recreational and commercial fish species are
rarely found to be directly impacted by foraging cormorants, except in
circumstances where the scale of assessment is small and/or the location
is relatively unproductive for forage fish. For example, there is
concern in some Algonquin Park lakes that foraging flocks of cormorants
may cause
significant impacts to brook trout (Salvelinus fontinalis)
populations, because these lakes are small, isolated and relatively
unproductive. Many cormorants that nest and feed in the Great Lakes
spend winter in the Delta region of the Mississippi, which is the center
of the catfish aquaculture industry. Glahn et al. (1995) noted
that the growth of the southern U.S. aquaculture industry has been
paralleled by burgeoning cormorant populations. Researchers have long
speculated that cormorants are experiencing improved body condition and
increased in winter survival due to the artificially inflated forage
base provided by catfish aquaculture facilities. Impacts on
Vegetation
Cormorant colonies can cause significant negative
impacts to vegetation at nesting and roosting sites. These localized
impacts can include physical and/or chemical damage to vegetation and
soil. “Whitewashing” from the deposition of guano (feces) and
associated ecological changes to their surrounding habitat is often
cited as the major concern for vegetation at cormorant colonies |
