DOUBLE-CRESTED CORMORANT IMPACTS ON SPORT FISH:
LITERATURE REVIEW, AGENCY SURVEY, AND STRATEGIES1
JOHN L. TRAPP, U.S. Fish and Wildlife Service, Office of Migratory Bird
Management, 4401 North Fairfax Drive, Arlington, VA 22203, USA
STEPHEN J. LEWIS, U.S. Fish and Wildlife Service, Federal Building, 1
Federal Drive, Fort Snelling, MN 55111-4056, USA <email@example.com>
DIANE M. PENCE, U.S. Fish and Wildlife Service, 300 Westgate Center
Drive, Hadley, MA 01035, USA <firstname.lastname@example.org>
Last Revised: 11 June 1998
1 This is the text of a paper that was presented
at the "Symposium on Double-crested Cormorants: Population Status and
Management Issues in the Midwest," held in conjunction with the 59th
Midwest Fish and Wildlife Conference, Milwaukee, Wisconsin, 9 December
Abstract: In response to concerns expressed by anglers, the U.
S. Fish and Wildlife Service (Service) conducted an extensive review of
published studies done throughout the U. S. and Canada on the impacts of
Double-crested Cormorants (DCCOs; Phalacrocorax auritus) on
sport fish populations in open waters. The literature review indicated
that fish species valued by sport and commercial anglers make up a very
small proportion of the DCCOs diet and that DCCOs have a minor effect on
fish populations relative to sport and commercial fishing, natural
predation, and other mortality factors. The Service sent a questionnaire
to State agencies, soliciting their biological information and
professional opinions on the role of DCCOs in regulating wild fish
populations, affecting sport angler catch, and causing adverse impacts on
tourism and other fish-related economies. Agency attitudes towards DCCO
population control were also assessed. Based on the literature review and
the survey responses, it does not appear that a strategy of reducing DCCO
populations to benefit sport fish is biologically warranted at this time.
Key words: Double-crested Cormorant, Phalacrocorax
auritus, food habits, impacts, population control, sport fish, U.S.
Fish and Wildlife Service
The DCCO has been maligned for most of this century by major segments of
society, especially sport and commercial anglers, who have viewed the DCCO
as a direct competitor for valuable fish resources. Human conflicts with
DCCOs have increased exponentially in the 1980s and 1990s as DCCO
populations have responded to a favorable array of human-caused changes in
environmental conditions that have enhanced recruitment and survivorship,
or simply increased the availability of suitable prey (Weseloh and Collier
As a result of remarkably similar patterns of growth in DCCO
populations and the aquaculture (especially catfish Ictalurus
punctatus) industry beginning in the late 1970s, conflicts became
evident in the late 1980s and intensified during the 1990s (Nettleship and
More recently (beginning about 1992), the Service started to receive
complaints from sport anglers that DCCOs were having negative impacts on
sport fish populations. Although largely anecdotal and therefore difficult
to evaluate, complaints were received from across the country (e.g.,
largemouth bass Micropterus salmoides and crappies Pomoxis spp. in Arkansas and Texas, fingerling trout and salmon in eastern Lake
Ontario, yellow perch Perca flavescens and walleye Stizostedion
vitreum in the Great Lakes, trout in the far west). The geographic
extent of the complaints suggested that there was at least a growing
perception among sport anglers that DCCOs were having a negative impact on
their fishing opportunities. Additionally, charter boat operators and
resort owners have complained of economic impacts, thereby sparking the
interest of politicians.
The objectives of this paper are three-fold: (1) to briefly review the
literature on DCCO food habits in freshwater habitats, (2) to provide a
summary of how State agencies and the Service view the DCCO-sport fish
conflict, and (3) to briefly outline guidelines for dealing with conflicts
between DCCOs and humans (including, but not limited to, sport anglers).
We thank the many officials of State wildlife agencies who responded to
a written request for information about State perspectives on the
DCCO-sport fish conflict. Earlier drafts of this paper benefitted from
reviews by Albert Manville, Paul R. Schmidt, Mark E. Tobin, and two
Our review of the available literature located 25 "major"
studies conducted in 13 States and Provinces, 1923-1994, that reported
results based on a minimum of 30 samples (e.g., regurgitations, pellets,
or individual food items; see Hall 1926, Munro 1927, Lewis 1929, Baillie
1947, Trautman 1951, McLeod and Bondar 1953, Vermeer 1969, O'Meara et al.
1982, Ludwig 1984, Gallant 1986, Craven and Lev 1987, Haws 1987, Hobson et
al. 1989, Ludwig et al. 1989, Neumann 1992, Campo et al. 1993, Karwowski
1994, MacNeil 1994).
Survey of State Agencies
In August 1996, a letter was sent to the directors of all 50 State fish
and wildlife agencies. The letter solicited their insight on the severity
of the DCCO-sport fish conflict in their respective States and their
suggestions for resolving this controversy in a suitable manner. Enclosed
with the letter was a list of 10 questions to help focus their thoughts
and to suggest the types of information that would be most useful to the
States were not required to respond to all of the questions, and many
chose not to do so. Also, rather than directly addressing each of the
questions, some States responded in a narrative style. In such instances,
we tried to collate each opinion expressed with the most pertinent
question. We made every effort to accurately reflect the perspectives
presented by the individual States.
What did we learn from this synthesis?--A minimum of 75
species of fish representing 22 families were detected as prey items, but
only 29 species ever comprised more than 10% of the diet at a specific
site. These results confirm that the DCCO is an opportunistic piscivore,
feeding on a wide diversity of prey. In a given situation, it tends to
prey on those species that are most abundant and most easily captured. The
ease with which a fish can be caught depends on a number of factors,
including distribution (vertical and horizontal), habitat (open water vs
vegetated zones), relative abundance, behavior, and physical condition of
the fish. Thus, the composition of the DCCOs diet varies considerably from
site to site, depending on the fish species that are most readily
Eighteen species were detected at 5 or more sites, but only 5 species
(alewife Alosa pseudoharengus, brook stickleback Culaea
inconstans, ninespine stickleback Gasterosteus aculeatus,
yellow perch, and slimy sculpin Cottus cognatus) consistently
comprised > 10% of the diet, and of these, only the yellow perch is
regularly sought by sport anglers. The consistency of yellow perch in the
diet probably reflects its widespread distribution and abundance rather
than a preference.
On average, prey species represented major (i.e., > 10%) components
of the diet in 29% (74) of the 252 instances in which detected (Table
1). The frequency with which three families of fish occurred as
major components of the diet differed significantly (P 0.05, X2)
from expected values: herrings (alewives and shad Dorosoma spp.,
specifically) and sticklebacks occurred as major prey items more than
twice as frequently as expected, while salmonids (trout, salmon, and
allies) were repesented in the diet as major prey items only half as
frequently as expected (Table 1). Members of all other families (including
sunfish and perches) occurred as major and minor components of the diet in
about the proportions expected.
What didn't we learn from this synthesis?--The collective
studies revealed little about food preferences of DCCOs. That would
require simultaneous information on the relative abundance of all
potential prey species at a given site, but such information is
universally lacking in these studies.
Similarly, the collective studies revealed little about the impacts of
DCCOs on fish populations. That would require quantitative information
about the numerical abundance of the prey species and detailed knowledge
of the effects of all other factors (both biotic and abiotic) that can
affect fish populations.
Subsequent to this synthesis, several studies (Blackwell et al. 1995,
Ross and Johnson 1995, Fowle et al. in review) have reported potentially
severe localized predation on sport fish populations by DCCOs. These were
usually situations in which a fishery had been intensively managed to
benefit sport anglers. Similar findings have been reported from Europe
(Russell et al. 1996) for the closely related great cormorant (P. carbo).
Survey of State Agencies
Responses were received from 25 State agencies distributed within
geographic regions as follows: Northeast (Connecticut, Maine, New
Hampshire, New York, Pennsylvania, Vermont), Southeast (Alabama, Arkansas,
Kentucky, Louisiana, South Carolina, Virginia), Midwest (Illinois,
Indiana, Michigan, Minnesota, Missouri, Ohio, Wisconsin), Prairies
(Montana, North Dakota), Southwest (Arizona), and Far West (Alaska,
A summary of responses to each of the questions posed to the State
agencies is provided in the following 10 paragraphs.
1. Considering all of the environmental (physical and biological)
factors that can affect fish populations in public waters, including
consumption by commercial and sport anglers, what is the relative role or
importance of DCCO predation?--None of the 13 States responding to
this question was able to provide satisfactory information about the
relative role of DCCOs in aquatic ecosystems. Eight States (Arkansas, New
Hampshire, Louisiana, Indiana, Kentucky, Minnesota, Pennsylvania, South
Carolina) reported a lack of documented evidence that DCCO predation was
currently a significant factor limiting sport fish populations. Four
States provided anecdotal evidence about the possible role of DCCOs: (1)
Oregon reported that elimination of human harvest of coho salmon for 3
years did not result in a recovery of the population, suggesting that
other factors (possibly including DCCO predation) are currently more
important in controlling coho populations; (2) North Dakota believed that
the relative impacts of DCCOs on prairie lakes and reservoirs managed for
sport fish was "significant;" (3) Maine thought that DCCO
predation on newly stocked salmonids was "probably high" in some
situations; and (4) Arizona noted the beginnings of a problem at
put-and-take trout lakes receiving heavy recreational fishing, where the
number of DCCOs has increased over the past 10 years from none to about 40
birds/day. Michigan noted that DCCO predation was responsible for about
1/5th of the total annual mortality of yellow perch at one location (see
Diana et al. 1997).
2. Is there any documented evidence that increased DCCO
populations have depleted the supply of fish available to sport anglers?--Twenty-one
States responded to this question, and most (15) reported a lack of
evidence that DCCOs have depleted the supply of fish available to sport
anglers. Four of these States qualified their responses (e.g., Arkansas
was concerned that predation on forage fish could deplete the prey base
for largemouth bass and crappie; California suggested that DCCO predation
could affect fish populations at hatcheries; Montana noted that there may
be localized situations where fisheries are impacted by concentrations of
DCCOs; and Michigan indicated that DCCOs had certainly caused some
reductions--but not depletions--in the numbers of some species available
to sport anglers). Six States reported impacts to sport fish (but not
necessarily depletion of supply of fish available to sport anglers) of a
largely anecdotal nature: Maine reported documented predation on Atlantic
salmon Salmo alar; Arkansas noted a decrease in "return-to-creel"
harvest indices; Alabama noted that unsuccessful attempts by DCCOs to
catch bluegills Lepomis macrochirus resulted in severe wounding
of a majority of these fish in a pond; New Hampshire reported "seemingly
strong circumstantial evidence" of predation on stocked brown trout Salmo trutta; North Dakota indicated that "problems of DCCO
predation are compounded during migrations that coincide with State
stocking efforts, with DCCOs targeting spring-stocked trout which take 1-3
weeks to acclimate to the new environment;" and Virginia thought that
increased DCCO populations have "undoubtedly" had some local
impacts on fisheries.
3. Is there any documented evidence that increased DCCO
populations have affected local economies associated with the sport
fishing or tourism industries?--None of the 12 responding States
reported any documented evidence that DCCOs had affected local economies
associated with the sport fishing or tourism industry. Oregon noted that "we
can document the economic effect of decreased salmon populations through
closure of commercial fisheries and formerly popular sport charter
fisheries, and declining sales of salmon harvest tags, but we believe that
DCCO predation is only one of a number of causative factors which in total
are responsible." North Dakota replied that potential losses to DCCOs
was "an economic drain" to local communities and sport anglers,
but did not provide details.
4. Is there any reason to believe that a widespread DCCO control
program would significantly increase the supply of fish available to sport
anglers?--Of the 14 States answering this question, 8 (Arizona,
California, Connecticut, Kentucky, Louisiana, Michigan, Minnesota,
Pennsylvania) responded that a widespread DCCO control program was not
justified, as there was no evidence that it would increase the supply of
fish available to sport anglers. A variety of responses was received from
the remaining 6 States: 2 (Maine and South Carolina) indicated that
widespread control might be appropriate for protecting newly stocked
hatchery fish; North Dakota thought that widespread DCCO control "would
significantly increase game fish" populations; Arkansas suggested
that decreasing the DCCO population "might possibly" increase
the amount of forage available to sport fish; Oregon replied that it did
not have enough information to judge whether widespread control would be
an appropriate approach; and Montana reported that any potential
solutions, including widespread control, should be carefully evaluated
before being implemented.
5. Is there any reason to believe that localized DCCO control
programs would significantly increase the supply of fish available to
sport anglers?--Replies to this question generally mirrored those
received in response to the previous question. Of the 14 States
responding, 8 (Connecticut, Illinois, Kentucky, Louisiana, Michigan,
Minnesota, Oregon, Pennsylvania) implied that they did not support
localized population control because of uncertainties about its effects on
sport fish populations. The remaining 6 States submitted a variety of
responses: 3 (Maine, New Hampshire, and South Carolina) suggested that
localized control might be necessary to protect newly stocked hatchery
fish; North Dakota believed that local DCCO control "would
significantly increase game fish" populations; Arkansas suggested
that DCCO control "might possibly" increase the amount of forage
available to sport fish; and Arizona indicated that local control "may"
be necessary if data are collected that indicate local impacts.
6. Are there any circumstances under which your agency would
support or endorse DCCO control programs on public lands or waters?--This
question elicited a wide variety of responses from the 18 States that
Reduce sport fish impacts (when documented)--4 States
(Arkansas, Louisiana, Michigan, and North Dakota);
Control disease outbreaks--3 (Kentucky, Minnesota, and
Protect newly stocked hatchery fish--2 (Maine and New
Protect sensitive colonial waterbird populations--2
(Connecticut and Ohio);
Reduce aquaculture impacts--2 (Missouri and Pennsylvania);
Not specified or None--2 (Illinois and South Carolina);
Don't know--2 (Arizona and Oregon); and
Protect endangered or threatened fin fish--1 (California);
7. Assuming that a DCCO control program was judged to be
biologically sound and socially acceptable, could the costs be justified
in an era of Federal and State budget cutting (i.e., how would you rank
DCCO control relative to the other resource management funding needs of
your agency)?--Of the 13 States responding to this question, only 4
(Arizona, Maine, Ohio, Oregon) ranked DCCO control as a high or fairly
high priority relative to other resource management funding needs of the
agency. Maine and Oregon suggested that the expenses of DCCO control could
be justified on the basis of the value of the fish (e.g., hatchery reared
fish, anadromous salmonids) impacted by DCCO predation. Arizona replied
that sport fish restoration (Dingell-Johnson) funds could be used to pay
for the costs of DCCO control. Five of the States responding to this
question (Arkansas, Connecticut, North Dakota, Pennsylvania, South
Carolina) implied that the costs of cormorant control might be justified
in some instances, but did not rank DCCO control relative to other funding
needs of the agency. Finally, 4 States (Kentucky, Louisiana, Michigan,
Minnesota) considered DCCO control to be a fairly low or very low
priority. Michigan commented further, that:
Even if DCCOs were shown to have a major impact on perch
populations, it is doubtful that a control program would be socially
acceptable. It would be like advocating the control of hawk and owl
populations so more pheasants are put in the hunter's bag!
8. Would removing the DCCO from the protection of the Migratory
Bird Treaty Act (MBTA), a measure favored by some sport anglers, be
beneficial or detrimental in helping to resolve the DCCO-fish depredation
issue?--Of the 14 States responding to this question, a majority of 9
(Alaska, Arizona, Connecticut, Kentucky, Louisiana, Michigan, Minnesota,
Missouri, Pennsylvania) did not favor removal of the DCCO from the
protection of the MBTA. Selected comments from this group of States
variously described the proposed action as (1) "inconsistent given
the best scientific information currently available" on the impacts
of DCCOs on sport fisheries (Alaska), (2) "unwise"
(Pennsylvania), (3) "a bad precedent" (Michigan), (4) "not
. . . prudent" (Missouri), and (5) "send[ing the wrong] message
to anglers" (Minnesota). Five States (Arkansas, Maine, North Dakota,
Oregon, South Carolina) implied that removal of the DCCO from protection
of the MBTA "might be helpful" if it was determined that DCCO
predation was a significant limiting factor for sport fish populations.
9. What is the single most beneficial and cost-effective action
the Service could take to resolve this controversy?--The 16 responses
reflected a diversity of opinions. Five States (Alaska, Michigan,
Minnesota, Montana, Oregon) indicated the need for additional
site-specific research to assess the impacts of DCCOs on sport fish
populations and their role in the transmission of fish diseases; 3 States
(Kentucky, North Dakota, and South Carolina) believed that additional
management authority should be given to individual States; 2 States
(Arizona and Louisiana) thought the Service should be willing to issue
additional site-specific depredation permits; 2 (Arkansas and Virginia)
advocated proactive population control; 2 (Minnesota and Missouri)
suggested additional outreach and education efforts; 1 (Maine) advocated
local control to protect newly stocked hatchery fish; and 1 (Pennsylvania)
recommended issuing limited take permits to aquaculture facilities.
Addressing the issue of population control, Indiana provided the following
in the history of terrestrial game management, predatory
control was often attempted to produce more game for hunter harvest; in
all but a few, restricted, circumstances this did not prove effective.
10. Has your agency developed any educational or informational
materials on DCCOs and the problems they cause?--None of the 11
States responding to this question had developed any educational or
informational materials on DCCOs. DCCO "fact sheets" developed
by the Canadian Wildlife Service (Weseloh
and Collier 1996) and the Service (U.S.
Fish and Wildlife Service 1995) have been posted on the Internet.
PROVISIONAL STRATEGIES FOR REDUCING DCCO-HUMAN CONFLICTS
In light of growing concerns (whether perceived or real) about DCCOs,
the Service is currently formulating a policy to address DCCO-human
conflicts. This policy will be formalized following input received at this
symposium and at a meeting with State agencies and other cooperators in
the northeastern U.S. in January 1998, and finalization of the proposed
depredation order for DCCOs at aquaculture facilities (U.S. Fish and
Wildlife Service 1997). The range of activities that one could consider in
a strategy for addressing DCCO management problems are provided for
discussion and illustrative purposes. They could be applied and adapted
depending on the circumstances.
We believe that every effort should be made to base bird management
decisions on biological data. Therefore, we strongly support monitoring
and research that will allow science-based decisions about DCCO control.
We believe that it is imperative for DCCO management activities to be done
in close cooperation and coordination with the U.S. Department of
Agriculture's (USDA's) Wildlife Services (formerly Animal Damage Control)
program, State and Provincial wildlife agencies, and the Canadian Wildlife
Service. We also recognize that education and outreach are essential
components of an integrated DCCO management program.
Sport Fisheries Impacts
Based on a review of the best available science, we recognize that
DCCOs generally have only minor direct impacts on sport fish populations,
being just one of a myriad of biotic and abiotic regulatory factors,
including water quality, aquatic habitat, natural predation, and angler
take. Therefore, we do not believe that a large-scale reduction of DCCO
populations to benefit sport fish populations is biologically warranted.
However, we also recognize that there may be highly localized situations
in which DCCOs can have significant impacts on sport fish populations.
These are generally situations in which sport fish are concentrated in
extremely high densities, often by human activities (e.g., massive
releases of hatchery-reared fingerlings, intensively managed put-and-take
fisheries, and temporary congregations of fish at nearshore spawning
sites). The Service currently does not issue DCCO depredation permits to
benefit sport fish populations in public waters, but is exploring
potential options that could be used to deal on a case-by-case basis with
localized DCCO predation when it has been proven to be a significant
problem. Two possible options include (1) modification of release
practices for hatchery-reared fish to reduce their vulnerability to DCCO
predation, and (2) harassment of depredating birds. Federal law does not
prohibit (but State and local laws might) the harassment of depredating
birds, provided the activity does not cause the death of birds or eggs.
We recognize that DCCOs can have severe economic impacts on private
aquaculture producers. These impacts have been best documented in the
catfish industry in the mid-south, where losses due to DCCO depredations
have been variously estimated at 3-7% of the catfish standing crop each
year. The Service has issued depredation permits since the late 1980s to
aquaculturists who are able to demonstrate that they are suffering
economic losses and that nonlethal techniques have proven ineffective
(Trapp et al. 1995, Coon et al. 1996). The Service works closely with the
Wildlife Services program of the USDA's Animal and Plant Health Inspection
Service, which is responsible for documenting economic impacts and for
developing nonlethal alternatives. A proposal to establish a DCCO
depredation order (U.S. Fish and Wildlife Service 1997) to help alleviate
depredations on aquaculture stocks was implemented in March 1998 (U.S.
Fish and Wildlife Service 1998). This order allows DCCOs to be taken in
certain States without a permit when causing depredation problems at
commercial aquaculture facilities. We believe that the aquaculture
industry shares responsibility for alleviating this problem. The industry
should be aggressively promoting the design of new facilities (and the
retrofitting of old ones where economically feasible) that exclude or
repel DCCOs and other fish-eating birds from the facilities. We anticipate
that USDA's Wildlife Services program will continue to research nonlethal
means for reducing aquaculture depredation problems.
Impacts on Other Colonial Waterbirds
We recognize that DCCOs can affect other colonial waterbirds at mixed
breeding colonies, both directly (by physical displacement) and indirectly
(by altering the vegetation). Only if there was convincing evidence that a
Federal- or State-listed endangered or threatened bird species, or a
regionally significant population, was being harmed by the actions of the
DCCOs could we recommend issuing a permit for DCCO control. Such control
could be difficult in mixed colonies because of the potential disturbance
to other species, and could be most effective if done before the comorants
We recognize that DCCOs can drastically alter the vegetation of nesting
islands and roost sites, affecting the ecological balance of a site and/or
lowering property, recreational, or aesthetic values. Our recommended
action could depend on whether the affected vegetation was located on
private or public land. On private land, we could recommend issuing a
depredation permit if the landowner could prove that the actions of the
DCCOs were lowering their property values or reducing their use of the
property for aesthetic or recreational purposes. On public land, we could
recommend issuing a depredation permit only if there was convincing
evidence that Federal- or State-listed endangered or threatened plant
species, or rare or declining plant communities, were being harmed by the
actions of the DCCOs. Harassment of birds is a potential option for
dealing with this problem. Provided the activity does not cause the death
of birds or eggs, Federal law does not prohibit the harassment of
depredating birds (but State and local laws might).
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order for the Double-crested Cormorant. Fed. Reg. 63: 10550-10561. <http://www.access.gpo.gov/su_docs/aces/aces140.html>.
Click on Federal Register, select Volume 1998, enter "cormorant"
as a Search Term, click on Submit, scroll to Establishment of a
Depredation Order, and click on preferred option. Accessed 11 June 1998.
Vermeer, K. 1969. Some aspects of the breeding chronology of
Double-crested Cormorants at Lake Newell, Alberta, in 1968. Murrelet 50:
Weseloh, D. V., and B. Collier. 1996. The rise of the Double-crested
Cormorant on the Great Lakes: winning the war against contaminants.
Environment Canada. <http://www.cciw.ca//glimr/data/cormorant-fact-sheet/intro.html>.
Table 1. Comparison of observed and expected
distribution of instances in which members of various fish families were
found to be minor (< 10%) or major (> 10%) components in diet of
DCCOs based on samples collected at 25 sites in 13 States and Provinces
(see Methods section for references).
as minor or major
components of diet
|13 other families
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