Citation:
GLAHN, J. F., M. E. TOBIN, AND B. F. BLACKWELL,
editors. 2000. A science-based initiative to manage double-crested
cormorant damage to southern aquaculture. USDA Animal
and Plant Health Inspection Service, Wildlife Services
National Wildlife Research Center, Fort Collins, CO, APHIS
11-55-010.
Contents
Executive
Summary
Aquaculture has expanded rapidly in the
Southern United States during the past two decades, especially
the cultivation of catfish, crawfish, and bait fish. These
fish usually are cultivated on farms with extensive systems
of large shallow ponds that are highly susceptible to
predation by birds. Double-crested cormorants (Phalacrocorax
auritus), American white pelicans (Pelecanus
erythrorhynchos), wading birds (e.g., Ardea alba,
Ardea herodius), and scaup (Aythya spp.)
are among the birds most frequently implicated. Well-documented
problems associated with cormorant predation on catfish
farms have coincided with the increase of this industry
and the rapid growth of cormorant breeding populations
on northern breeding grounds. From 1995 to 1998, the number
of cormorants spending the winter in the catfish production
region of Mississippi has more than doubled and now exceeds
60,000 birds. Also in 1998, cormorants were discovered
breeding in Mississippi and Arkansas for the first time
in decades. Without human intervention, breeding populations
in the Great Lakes will likely continue to increase, resulting
in more habitat destruction, competition with other colonial
waterbirds, competition with sport fishermen, and depredations
on southern aquaculture farms.
The nature and expansiveness of southern
aquaculture and the continued growth of cormorant populations
limit options for managing depredations on aquaculture
farms. Most efforts rely on devices designed to frighten
them from ponds and roosts, although the effectiveness
of this strategy is limited and, due to expanding habitat
utilization by cormorants, is becoming increasingly difficult
to implement. In the long-term, further research may lead
to the development of barriers, new fish-culturing practices,
or other techniques that may help alleviate problems in
certain situations. However, no such strategies seem promising
at this time and may be limited in the future by the rapid
proliferation of this species. In the short term, lethal
control strategies under the current cormorant depredation
order may need to be implemented to their fullest extent
at aquaculture facilities and may need to be expanded
to roosting sites to reinforce harassment strategies.
Authority should also be pursued to manage southern breeding
colonies at levels compatible with aquaculture to forestall
future depredation problems. However, such localized population
control efforts are unlikely to affect continental or
flyway populations, and problems are likely to grow as
long as the interior population grows. Managers should
consider managing cormorant populations on a flyway basis,
which will require setting biologically and socially acceptable
population goals and evaluating management options for
achieving these goals. Construction of a realistic, deterministic
population model for cormorants would facilitate these
ends. Increased dialog among public agencies and private
organizations concerned about the management and conservation
of cormorants is critical to the development of a realistic
and effective plan for managing the depredations caused
by this species.
Introduction
The U. S. Department of Agriculture, Animal
and Plant Health Inspection Service, Wildlife Services
(USDA-WS) program provides national leadership in managing
conflicts between wildlife and humans. USDA-WS strives
to facilitate interagency discussions, understanding,
cooperation, and planning to enhance professional responses
to public demands for assistance in managing adverse impacts
caused by wildlife (Acord 1995).
Populations of double-crested cormorants
(Phalacrocorax auritus) have irrupted during
the past two decades and are of increasing concern to
commercial aquaculturists in the Southern United States,
commercial and sport fishermen on the Great Lakes and
in the Northeastern United States, and conservationists
worried about habitat destruction and impacts to other
waterbirds. In response to these concerns, this document
was prepared for the Eastern Regional Office of USDA-WS
based, in part, on a previous planning document that was
a collaborative effort of the following current and former
USDA-WS personnel: Keith Andrews, Jerry Belant, Travis
Carpenter, Pete Poulos, David Reinhold, P. G. Ross, and
Charles "Bo" Sloan. It has been reviewed by
USDA-WS State Directors and circulated for review to wildlife
and fisheries administrators in the following states:
Alabama, Arkansas, Connecticut, Florida, Georgia, Kentucky,
Louisiana, Maine, Michigan, Minnesota, Mississippi, New
York, North Carolina, Ohio, Oklahoma, Pennsylvania, Rhode
Island, South Carolina, Tennessee, Texas, Vermont, and
Wisconsin.
The objectives of this document are: 1)
provide an overview of double-crested cormorant conflicts
with southern aquaculture and concerns with cormorants
elsewhere; 2) review the effectiveness and limitations
of current strategies to alleviate conflicts; 3) identify
research needs and management actions: and 4) develop
a systematic plan to meet research needs and set a course
of action.
This document is reflective of contemporary
social and economic values and addresses both prevention
and correction of problems associated with cormorants.
From a research perspective we attempt to define what
information is needed to formulate sound management decisions.
It is intended to facilitate thought, discussion, and
partnerships among wildlife and fishery biologists, aquaculturists,
conservationists, and the public regarding how these conflicts
can be prevented or minimized.
Authorities
Various governmental agencies and private
individuals share responsibility for managing wildlife
damage problems, depending on the type of problem species
involved and where the problems occur. The U.S. Fish and
Wildlife Service (USDI-FWS) is the primary governmental
agency responsible for managing migratory birds and federally
threatened and endangered species. State agencies manage
most other wildlife species and share in the management
of migratory birds. State and federal wildlife management
agencies often share responsibilities with other state
and federal agriculture, land management, and health agencies.
Private organizations and wildlife damage control businesses
may receive authority from governmental agencies to directly
manage specific wildlife problems.
The USDI-FWS has statutory authority for
enforcing the Migratory Bird Treaty Act (16 U.S.C. 703-712)
and thus for managing cormorant populations. However,
the agency exercises this authority in consultation with
other federal, state, and provincial agencies. Under the
Animal Damage Control Act of March 2, 1931, as amended
(7 U.S.C. 426-426c; 46 Stat. 1468), the USDA-WS program
is responsible for protecting American agriculture and
other resources from damage caused by wildlife, including
cormorants.
USDA-WS may implement localized depredation
abatement actions, but must consult with USDI-FWS and
state agencies to implement more far-reaching management
actions.
As part of their authority to manage cormorant
populations, the USDI-FWS previously issued depredation
permits that allowed individual aquaculture producers
to shoot cormorants that were causing or about to cause
damage on their farms. USDA-WS assisted in this process
by certifying that cormorants were in fact causing damage
and that nonlethal means were insufficient to reduce damages.
In March 1998, the USDI-FWS issued a Standing Depredation
Order (50 CFR, Part 21, Section 21.47) that eliminated
the requirement that producers obtain individual permits
and enabled fish farmers in 13 states to shoot double-crested
cormorants that are committing or about to commit damage
at their farms. Some states still require individual permits.
The present cormorant depredation order does not restrict
the number of cormorants that may be shot nor the methods
that may be used to bring birds within shooting range.
Under these provisions, farmers must keep a log of the
numbers of cormorants killed each month and make these
logs available to wildlife enforcement officials.
Southern
Aquaculture
Aquaculture is the intensive commercial
propagation of various fish, crawfish (crayfish) (Procambarus
clarkii, P. acutus), or shrimp (Family Pennaeidae).
Southern aquaculture is devoted primarily to the culture
of catfish (Ictaluridae), bait fish (Cyprinidae),
and crawfish in large (> 2 ha) shallow (< 2 m) ponds
and is located primarily in the states of Alabama, Arkansas,
Louisiana, and Mississippi. Most production of channel
catfish (Ictalurus punctatus) is concentrated in Mississippi,
which has more than 41,000 ha of ponds and is responsible
for 70 percent of the domestic commercial production.
Arkansas ranks second, with over 9,000 ha of catfish ponds.
Alabama and Louisiana rank third and fourth, respectively.
More than 90 percent of all catfish production in the
United States occurs in these four states (USDA 1998).
Arkansas raises approximately 80 percent of all cultured
bait fish in the United States, having almost 12,000 ha
in production (Collins 1995). Almost all crawfish are
produced in Louisiana, which has more than 40,000 ha of
ponds in production (Avery and Lutz 1996).
Although southern aquaculture farms vary
greatly in size, a typical Mississippi catfish farm has
20 ponds, each containing about 6 surface hectares of
water. Because of the size of bait fish and crawfish and
multi-batch cropping systems with catfish, almost all
ponds are vulnerable to predation. Both bait fish and
catfish ponds are stocked at extremely high densities
ranging from 5,000 to 150,000 fish/ha with catfish and
123,000 to almost 500,000 fish/ha with bait fish. Such
crowding make fish highly susceptible to bird predation,
particularly by cormorants.
Southern aquaculture production has seen
phenomenal growth in the past 30 years, due mostly to
the expansion of the catfish industry. In both Mississippi
and Arkansas, the first crops were raised in a few ponds
in the early 1960s. The industry expanded in the delta
region of Mississippi from the mid-1970s through the 1980s,
when the acreage increased almost tenfold. In the early
1990s this growth slowed due to low market prices but
resumed again in 1996 when acreage increased by 4 percent
in both Mississippi and Arkansas (USDA 1996). Continued
increases were observed in 1997 (USDA 1997) in the four
major production states. The additional acreage in recent
times has come from expansion of existing farms in the
delta region of Mississippi and rapid expansion of catfish
farming in areas such as east Mississippi. Crawfish production
has also increased. Between 1960 and 1996, commercial
crawfish acreage in Louisiana increased from 800 ha to
45,000 ha (J. Avery, Louisiana Coop. Ext. Service. Pers.
Commun.). Overall, aquaculture is the fastest growing
agricultural enterprise in the United States (Van Gorder
1992) and by the year 2000, is predicted to account for
$59 billion, or 40 percent of the world's fish production
(Price and Nickum 1995).
Cormorant
Demography and Biology
Historical trends reveal several factors
affecting the fluctuation in cormorant populations. Cormorant
populations were suppressed during the early 1900s due
to egg collecting for human food and nest destruction
by fishermen who considered the cormorant to be a competitor
(Lewis 1929, Hatch 1984, Dolbeer 1990, Chapdelaine and
Bédard 1995). From the 1920s through mid-1940s,
population increases throughout the Great Lakes, New England,
and Canada (Baillie 1947, Fargo and Van Tyne 1927, Hatch
1984, Postupalsky 1978) were probably due to newly-created
reservoirs that killed trees and created new islands for
nesting (Markhan and Brechtel 1978).
Cormorants are highly susceptible to pollutants
bioaccumulated by prey fish species, and pesticide-related
bill deformities continue to occur (Fox et al. 1991, Ludwig
et al. 1996). From the mid-1940s through the early 1970s,
human persecution, human competition for fish resources,
but most importantly widespread use of environmental contaminants
(e.g., organo-chlorine compounds) led to a decline of
cormorant populations (Noble and Elliott 1986, Craven
and Lev 1987, Ludwig et al. 1989, Dolbeer 1990, Weseloh
et al. 1995). During this period, the Great Lakes cormorant
population as a whole suffered a reduction in excess of
80 percent (Postupalsky 1978) due to eggshell thinning
and reproductive failure attributed to pesticide deposition
(e.g., DDT/DDE) (Postupalsky 1978, Weseloh et al. 1983,
Weseloh et al. 1995). In addition, between 1944 and 1952,
the USDI-FWS incorporated cormorant egg-spraying into
a herring gull (Larus argentatus) reduction effort
in New England, primarily targeting cormorant colonies
in Maine (Gross 1952). A cormorant control program was
initiated on Lake Winnipegosis, Manitoba, in 1945 that
reduced the colony from 9,862 to 4,656 nests by 1951 (McLeod
and Bondar 1953).
Protected status was granted to cormorants
in the United States by the Migratory Bird Treaty Act
in 1972. That event, combined with DDT use restrictions
implemented in the mid-1970s, contributed to the resurgence
of cormorant populations (Bishop et al. 1992, Ludwig 1984,
Noble and Elliott 1986, Tyson et al. 1999). A dramatic
increase in food availability (e.g., alewife [Alosa pseudoharengus]),
particularly in the Great Lakes, has also aided this recovery
(Hobson et al. 1989, Price and Weseloh 1986, Weseloh et
al. 1995). An annual increase in cormorant breeding pairs
has been reported for the following areas: southern New
England (20 percent), the Canadian lower Great Lakes (about
40 percent from 1976-1990), and the entire Great Lakes
(29 percent from 1970-1991) (Hatch 1984, Blokpoel and
Tessier 1991, Weseloh et al. 1995). Cormorants also began
to colonize areas south of their traditional range (Post
and Seals 1991). Overall, the cormorant nesting population
in the Great Lakes increased from 89 nesting pairs in
1970 to about 93,000 pairs in 1997 (Tyson et al. 1999).
Between 1986 and 1989, recently established breeding populations
in South Carolina increased 310 percent from 60 nesting
pair to 186 nesting pairs (Post and Seals 1991). Concurrent
with the rapid growth of cormorant populations in North
America, great cormorant (Phalacrocorax carbo)
populations in Europe experienced a similarly dramatic
resurgence largely due to increased protection and restrictions
on persistent pesticides (Veldkamp 1997). The similar
life history and conflicts caused by great cormorants
provide insight into management issues with double-crested
cormorants.
Most double-crested cormorants that affect
southern aquaculture breed in the Northern United States
and Canada (Dolbeer 1991), although flocks of cormorants
have been observed in the delta region of Mississippi
during the summer and small breeding colonies have recently
been documented in Mississippi (Reinhold et al. 1998)
and Arkansas (Thurmond Booth, Wildlife Services, Pers.
Commun.). Up to 70 percent of cormorants banded at nesting
colonies from Saskatchewan through the Great Lakes prior
to 1988 were recovered in the lower Mississippi River
Valley (Dolbeer 1991). This breeding area encompasses
most of what is referred to as the "interior population,"
that makes up 61 percent of the total North American breeding
population, recently estimated between 1 and 2 million
birds (Hatch 1995, Tyson et al. 1999). Past band recovery
analyses reveal no apparent "focal point" of
breeding birds that conflict with southern aquaculture
(Dolbeer 1991), but it seems clear that the conflict involves
birds associated with the Mississippi flyway. A more up-to-date
analysis of band recoveries is needed to begin to understand
current movements along this flyway.
Increased winter survival of juveniles due
to a higher forage base provided by catfish may have contributed
to this growth (Duffy 1995, Vermeer and Rankin 1984, Weseloh
and Ewins 1994). A recent study has confirmed that premigratory
cormorants from the delta region of Mississippi are in
better body condition than cormorants from non-aquacultural
areas (Glahn et al. 1999). Cormorant mortality has been
estimated at 50 to 70 percent during the first year after
hatching and 15 to 25 percent annually thereafter (Hickey
1952, Palmer 1962, van de Veen 1973). Price and Weseloh
(1986) suggest a pre-breeding mortality (up to age 3)
of 70 percent in stable populations and 31 percent in
expanding populations; van de Veen (1973) reports survival
to breeding age as 30 percent in the stable western population.
Historically, human activities have been
the primary cause of cormorant population fluctuations,
including the current population resurgence. Most notable
has been the rise and fall of persistent pesticides in
the environment, the protection afforded the species,
and the recent increase of the food base on the wintering
grounds provided by southern aquaculture. Although it
is impossible to accurately predict future trends in cormorant
populations, there is little evidence that populations
will decline markedly without human intervention. Density-dependent
factors could lead to eventual stabilization of the population,
albeit at a high level. This has been demonstrated, in
part, through population modeling of the great cormorant
populations in Europe (Bregenballe et al. 1997). In North
America, density dependent factors might limit the size
of some individual breeding colonies, but available breeding
habitat for further colonization remains abundant (Hatch
and Weseloh 1998). Prey can become depleted for individual
cormorant breeding colonies (Birt et al. 1987; Hatch and
Weseloh 1998), but range-wide reduction in the availability
of prey is unlikely. Although epizootic diseases may help
regulate localized populations, additional research is
needed to clarify the potential for diseases to limit
population growth throughout the entire range. Recent
outbreaks of Newcastle's disease (avian paramyxovirus)
may have slowed the growth of some established breeding
colonies (Hatch 1995); however, no viable cultures were
isolated from exposed birds (M. Avery, NWRC, per. commun.)
even though more than half of the wintering birds examined
in a recent ongoing study had been exposed to this disease
organism.
Cormorant
- Human Conflicts and Values
Conflict
With Southern Aquaculture
The increasing conflict between cormorants
and southern aquaculture has been chronicled through population
trends of wintering cormorants in areas of intensive aquaculture.
With the expansion of Mississippi aquaculture in the 1980s
came a corresponding increase in the number of cormorants
spending the winter in this region (Glahn and Stickley
1995). Prior to 1980, few cormorants remained there for
the winter (Glahn and Stickley 1995). However, during
the 1980s, the number of cormorants recorded during Christmas
bird counts increased dramatically (Glahn and Stickley
1995, Jackson and Jackson 1995). Since 1990, mid-winter
counts of this species doubled from approximately 30,000
birds in 1990, when USDA-WS biologists began conducting
roosts censuses, to 67,000 birds in 1998 (Glahn et al.
2000). These counts have remained approximately at 1998
levels through 2000 (USDA-WS files). Less is known about
wintering cormorants in other aquaculture production areas,
but recent midwinter counts suggest populations of approximately
10,000 birds inhabit the rapidly expanding aquaculture
region of East Mississippi and West Alabama. In the catfish
production region of Arkansas, surveys in February 2000
revealed 50,000 cormorants roosting in several different
roost sites (M. Hoy, USDA-WS, Pers. Commun.). Despite
the value of these counts as indices to potential conflicts,
little is known about overall cormorant populations that
might utilize southern aquaculture production areas over
time. However, banding records indicate approximately
120,000 birds were moving through the lower Mississippi
valley in 1989 (Dolbeer 1990). Considering the increased
breeding populations since that time, this number may
have more than doubled.
Cormorants traditionally arrive on their
wintering grounds in November and depart by mid-April
(Aderman and Hill 1995). However, appreciable numbers
now arrive in September and do not depart until late April
or early May (Reinhold and Sloan in press), thus extending
the period of depredations. These wintering birds congregate
at night in bald cypress (Taxodium distichum)
or tupelo gum (Nyssa aquatica) trees that are
typically over water in oxbow lakes or other naturally
occurring wetlands associated with river drainages (Aderman
and Hill 1995, Glahn et al. 1996). From a dynamic number
of active night roost sites, cormorants travel only a
mean distance of 16 km to forage on catfish ponds (King
et al. 1995). Thus, depredations are temporarily highly
concentrated on ponds in close proximity to active roost
sites, but shifts in roosting activity (King 1996) cause
depredations to be a widespread problem.
The impact of cormorant foraging activity
on the catfish industry has been well documented but their
impact on bait fish and crawfish remains unclear. Most
catfish producers in the Southern United States perceive
cormorants as threats to their livelihood (Stickley and
Andrews 1989, Wywialowski 1999). In a 1996 national survey
of catfish producers, depredations by cormorants were
the most commonly cited wildlife problem. Losses due to
cormorants were cited by 77 percent of Mississippi producers,
66 percent of Arkansas producers, and 50 percent of Alabama
producers. The main problems reported were cormorants
feeding on catfish, injuring catfish, and disturbing feeding
patterns. Losses reported from all depredating species
approximated 4 percent of catfish sales, or a 16 to 33
percent loss of profits (Wywialowski 1999).
Observational studies of cormorants foraging
at catfish ponds were the first concrete evidence of their
potential to impact catfish production. The smaller subspecies
of Florida cormorants were observed feeding at a fingerling
catfish pond at an estimated a consumption rate of 19
fingerlings/bird/day, or approximately 304 g/bird/day
(Schramm et al. 1984). Hodges (1989) only rarely observed
cormorants on catfish ponds but concluded that they pose
the greatest threat to catfish farmers because of their
gregarious behavior and ability to dive for fish. It has
been calculated that 30 cormorants feeding throughout
the day would consume half of the fingerling population
in an 8-ha pond in 167 days (Stickley et al. 1992). In
a recent study, captive cormorants consumed 516 to 608
g, or about 10 catfish, per day (Glahn unpubl. data).
These findings are consistent with previous bioenergetic
projections for these birds (Glahn and Brugger 1995) and
based on replacement costs indicate that one cormorant
subsisting exclusively on catfish would remove about $1
worth of fingerlings per day.
Food habits studies have also documented
the prevalence of catfish in the diet of cormorants on
their winter range. A 3-year study found that approximately
half of the cormorant diet (wt/wt) in the delta region
of Mississippi was composed of channel catfish (Glahn
et al. 1995). Most of the remaining diet was gizzard shad
(Dorosoma cepedianum). Catfish were most often
consumed during the spring months in areas with the highest
concentration of fish farms. Catfish consumed in this
study averaged 16 cm in length, equivalent to the average
size fingerlings stocked by producers.
For the winters of 1989-90 and 1990-91,
a bioenergetics model estimated cormorant-related production
losses on catfish farms in the delta region of Mississippi
at 18 to 20 million fingerlings per winter, or approximately
4 percent of the available fingerling-class during the
November to April study periods (Glahn and Brugger 1995).
The annual cost of replacing these fingerlings was estimated
at approximately $2 million. Cormorant populations in
the delta region of Mississippi have more than doubled
since this study, and the annual impact of cormorants
on the catfish industry in the Mississippi delta may now
exceed $5 million (Glahn et al. 2000). Based on the estimated
value of these fish at harvest, actual production losses
might be 10 times greater, but more study was needed to
assess whether losses due to predation are additive or
compensatory as related to density-dependent growth and
mortality of catfish. To partially address these questions
one study (QA-634, in progress) is examining catfish production
at harvest with and without cormorant predation simulating
average cormorant numbers seen foraging by Stickley et
al. (1992) on catfish ponds (i.e., 30 cormorants foraging
on a 6 ha pond for 100 days). Also to simulate field conditions,
an ample supply of buffer prey was provided along with
catfish fingerlings stocked at 12,355 fish/ha (5,000 fish/ac).
Preliminary results suggest that despite the buffer prey
reducing depredation on catfish by an estimated 33 percent,
catfish population declines due to cormorant predation
at harvest ranged from 26 to 33 percent where catastrophic
disease problems did not occur. Because of density-dependant
compensatory growth, actual biomass production loss ranged
from 19 to 21 percent. Considering this 20 percent loss
in production, losses at a commercial pond scale would
be $10,500 or 5 times the value of the fingerlings lost.
Because of small profit margins in the catfish industry,
some agricultural economists suggest that a 20 percent
loss in production would result in a 100 percent loss
in profits (C. Engle University of Arkansas, Pers. Commun.).
Other
Cormorant Conflicts
Controversy surrounding cormorants has polarized
people (Shetterly 1986) whose views range from those who
wish to declare cormorants a nuisance species in need
of control (Arkansas Senate Bill 345 [1993], Arkansas
Senate Concurrent Resolution 12 [1995], Bayer 1989, Oklahoma
Senate Bill 362 [1991]) to those who feel that cormorant
populations are causing no problems and have the right
to recover to the fullest extent (Duffy 1995). Historically,
animosity towards cormorants has been based on their perceived
impact on fisheries (Lewis 1929, Mendall 1934, 1936) and
has generated extended periods of intense persecution
by commercial fishing interests (Baillie 1947, Craven
and Lev 1987, Ludwig 1984, Omand 1947, Postupalsky 1978),
at times leading to sanctioned efforts to reduce cormorant
populations (Hatch 1995). Both sanctioned and unsanctioned
reduction efforts have occurred at breeding colonies during
the most recent build up of populations (Ewins and Weseloh
1994, USDI-FWS 1998). These efforts have largely been
justified by impacts on fisheries but have also been spurred
by a growing concern about cormorant impacts on unique
insular habitats and on other colonial nesting birds (Bèdard
et al. 1995). These latter concerns have recently lead
natural resource managers in both the United States (Garland
et al. 1998) and Canada (St. Martin and Loftus 2000) to
express concern and either call for action or definitive
studies to defend a control program. Consistent with these
views, conservationists developing the North American
Colonial Waterbird Conservation Plan have referred to
cormorants as "pests." Below, we briefly review
the literature concerning these other cormorant conflicts.
Research to clarify the impact of cormorants
on sport or commercial fisheries has yielded mixed results.
Most studies, principally in the Great Lakes region, indicate
that cormorants feed primarily on abundant small forage
fish in these ecosystems, namely alewife and shad (Lewis
1929, Mendall 1936, Omand 1947, Craven and Lev 1987, Ludwig
et al. 1989, Weseloh et al. 1995). More recent and detailed
studies on Lake Erie (Bur et al. 1999) and Lake Ontario
(Ross and Johnson 1999) concluded that cormorants usually
did not have a significant impact on either game fish
or their forage base. However, cormorants on Lake Champlain
consumed primarily yellow perch, a preferred sport fish
in Vermont (Garland et al. 1998).
Cormorants can have a direct impact on fisheries
during stocking, such as when trout or salmon are released
into rivers (Blackwell et al. 1997, Derby and Lovvorn
1997, Meister and Gramlich 1967, Ross and Johnson 1999).
Cormorants can also reduce sports fisheries where forage
prey such as alewife are less numerous. During a series
of intensive investigations on the Eastern Basin of Lake
Ontario (Schneider et al. 1998), cormorants shifted their
diet to smallmouth bass when alewife and other prey species
populations were low. Although smallmouth composed only
a small percentage of the cormorant diet, specific age
classes important to this fishery (primarily 3- to 5-year
old fish) may have been significantly reduced.
The resurgence in the interior cormorant
population has spurred growing concern that migrating
cormorants might adversely affect sport and commercial
fisheries along the Upper Mississippi River, but limited
data suggest that cormorants foraging in such areas take
mainly gizzard shad (Kirsch 1995). Little scientific information
is available on cormorant food habits on natural waters
(e.g., rivers, lakes and reservoirs) in the Southern United
States. Results of a study in Texas (Campo et al. 1993)
indicated that most cormorants fishing in natural waters
took mainly shad and sunfishes. Nonetheless, the researchers
acknowledged that cormorants could have an impact on sport
fish in some locations. Another study (Glahn et al. 1998)
reported that cormorants foraging at lakes during winter
in Mississippi and Alabama took mostly shad and sunfish,
but recommended more in-depth studies of cormorant impact
in southern waters.
Based on a review of the literature and
a survey of state agencies, Trapp et al. (1999) concluded
that cormorants have only a minor impact on sport fish
populations except in highly localized situations. This
view may summarize the situation best as of now, but further
research is needed to clarify this issue in areas where
cormorants are concentrated.
The impact of nesting cormorants on habitat
and other colonial waterbirds is well documented in localized
areas. Cormorants strip leaves, break branches, and deposit
guano that ultimately kills the trees. The resulting habitat
destruction is highly visible (Weseloh and Ewins 1994,
Bédard et al. 1995, Jarvie et al. 1999, Shieldcastle
and Martin 1999). Habitat destruction, combined with competition
for nest sites, impacts black-crowned night herons (Nycticorax
nycticorax) and other heron and egret species ( Jarvie
et al. 1999, Shieldcastle and Martin 1999). Although these
impacts seem localized at present, they are clearly density
dependent and likely to be an increasing problem as cormorant
populations continue to expand.
Benefits
Associated With Cormorants
The double-crested cormorant is a native
species that is of intrinsic as well as esthetic value
to humans. Cormorants are potential indicator species
for environmental contaminants (Noble and Elliott 1986,
Fox et al. 1991, Ludwig et al. 1995). Some aquaculture
producers believe that cormorants are beneficial when
they feed on undesirable fish such as gizzard shad (Dorosoma
cepedianum) and sunfish (Lepomis sp.) that infest
commercial ponds. Cormorants can increase species diversity
on natural waters and reservoirs and may stabilize the
relationship between predatory fish and their prey. Bird
watchers enjoy viewing cormorants in their natural setting
(Bédard et al. 1995, Mendall 1936, Vermeer 1970).
Most aquaculturists recognize that some depredation is
natural and to some degree is a cost of business (Thompson
et al. 1995), but the threshold for acceptable depredation
losses may have long since been exceeded.
Alleviating
Depredations on Southern Aquaculture
Alleviating problems caused by or related
to the presence of wildlife is integral to the field of
wildlife management (Berryman 1992, Leopold 1933, The
Wildlife Society 1992). Responsible wildlife managers
balance the needs of humans and wildlife, foster tolerance
toward wildlife, and advocate cost-effective and environmentally
acceptable remedial solutions that reduce the implementation
of environmentally or legally unacceptable actions by
those experiencing the problem.
The utility of any damage management strategy
depends on the costs of deploying the strategy relative
to the anticipated reduction in damage. Environmental,
biological, social, physical, and legal considerations
also influence the selection and application of management
strategies (Owens and Slate 1991, Slate et al. 1992, USDA
1994). Because each damage situation is unique, appropriate
management actions must be determined on an individual
basis.
Numerous factors determine which methods
are most environmentally sound, socially acceptable, and
cost-effective. Is the population of the problem wildlife
rare or abundant? Is it stable, increasing, or decreasing?
What are the behavioral traits of the wildlife? Is the
proposed management strategy legal and feasible? What
are the potential impacts on other wildlife species? Are
weather or local conditions likely to influence effectiveness?
Is the method likely to affect soil, water, or air quality?
What are public perceptions toward the method? Are resource
managers and the public likely to accept the human and
nontarget risks associated with the method?
Alleviating wildlife damage entails employing
one or a combination of three strategies: 1) managing
the resource being impacted; 2) physically separating
the wildlife from the resource; or 3) managing the wildlife
responsible for, or associated with, the damage (USDA
1994). Below we describe these strategies in more detail
relative to catfish production and discuss research needs.
Resource
Management
Managing a resource to reduce wildlife conflicts
usually involves modifying cultural practices (e.g., animal
husbandry or crop selection), altering the habitat to
reduce its attractiveness to wildlife, or adjusting human
behavior. In the case of aquaculture production, the objective
would be to reduce the vulnerability of fish to predation
by cormorants.
Pond size and location
- Smaller ponds would facilitate the installment and maintenance
of bird exclusion structures, as well as improved management
of fish diseases and water quality. However, production
is typically reduced and levee maintenance costs are typically
increased when smaller ponds are used. Pond construction
costs, a major determinant of economic success in the
industry, also increase as pond size decreases (Tucker
and Robinson 1990). Changing pond depth probably would
have no effect on cormorant foraging efficiency as the
birds dive to depths >20 m (Palmer 1962, Knopf and
Kennedy 1981). Locating fingerling ponds or other ponds
that are especially susceptible to predation near areas
with human activity (e.g., warehouses, processing plants)
or where they are easily accessible facilitates harassing
birds and reduces susceptibility to predation (Mott and
Boyd 1995).
Cultural practices
- Catfish ponds are managed either as multiple-cropping
systems, which contain two or more size classes of fish
and are selectively harvested over a period of years,
or single-batch systems, which contain only one year class
of fish and are completely harvested before restocking.
All multi-crop ponds contain a fish size class that is
vulnerable to predation by birds and must be protected.
The vulnerability of single-crop ponds to predation varies
depending on the size of fish. Thus, farms with the single-crop
system usually have fewer ponds with vulnerable fish.
The multi-cropping system was adopted by most catfish
farmers to meet the needs of processors but has a number
of disadvantages when compared to the single-batch system
(Tucker and Robinson 1990). Although the industry is large
enough now to assure year-round supplies of fish using
single-batch culture (Tucker and Robinson 1990), most
farmers continue to use the multi-batch culture so they
can harvest fish throughout the year and improve their
cash flow.
Other modifications in fish stocking regimens
have the potential to limit predation by cormorants. Barlow
and Bock (1984), Brugger (1995), Glahn et al. (1995),
and Mott and Boyd (1995) suggest that modifying stocking
rates, size-class of fish stocked, or stocking times can
reduce resource losses to birds. Reduced stocking rates
might reduce the foraging success of cormorants and, in
turn, reduce the attractiveness of catfish ponds (Mott
and Boyd 1995). Conversely, the current industry trend
is to use higher stocking rates that might compensate
producers for production losses due to predation and other
causes. Research is needed to determine optimal stocking
rates with respect to bird predation that maintains acceptable
profit margins. Delaying transfer of fingerlings into
food-fish ponds would shorten the period when producers
would need to protect these fish and allow more concentrated
bird-control efforts at fewer fingerling ponds. By delaying
restocking from late winter until mid-April, producers
would miss the peak period of cormorant depredation (Glahn
et al. 1995). However, delaying stocking is not consistent
with the multi-batch cropping system and may increase
the risk of stress-related mortality from disease due
to water temperature changes. Also, because these fish
are not expected to grow during the winter (Tucker and
Robinson 1990), fall and winter stocking might not be
cost-effective because it increases the period of exposure
to cormorant predation in food-fish ponds.
Other cultural practices to reduce cormorant
predation might include the use of buffer prey and pond
water dyes to reduce the visibility of fish to the cormorant
(Mott and Boyd 1995). In studying the predation by cormorants
on catfish ponds, Stickley et al. (1992) noticed that
cormorants fed heavily on shad, which are more easily
manipulated than catfish for swallowing. However, preliminary
results of controlled captive cormorant studies suggest
that over time, cormorants had no real preference for
a more readily manipulated buffer prey (i.e. golden shiners)
despite this prey having some desired effect in reducing
overall production losses (Glahn unpubl. data). Even if
preferred buffer prey could be identified, use of buffer
prey to reduce damage on catfish remains controversial
because of the possibility that more abundant prey will
attract more cormorants to these ponds (Mott and Boyd
1995). Along similar lines, some authors (Mott and Boyd
1995, Erwin 1995) suggest the use of developing alternative
foraging sites stocked with preferred buffer prey. However,
even if such ponds could be developed, they would quickly
be depleted by increasingly large cormorant populations
exploiting these areas in southern aquaculture regions.
Although pond dyes have never been evaluated, the natural
turbidity (Secchi disk readings <40 cm) of most catfish
ponds would probably limit the utility of dyes for reducing
the ability of cormorants to pursue and capture prey.
Exclusion
Techniques
Exclusion, the physical separation of wildlife
from the resource, usually entails erecting fences, nets,
or other barriers. Although total separation might not
be practical, various barrier techniques may serve to
limit cormorant access to ponds or the fish in these ponds
(Littauer et al. 1997).
Complete enclosure
- Supported netting is the only completely effective method
of excluding cormorants from ponds but is physically and
economically impractical for large (>5 ha) catfish
and bait fish ponds. Littauer et al. (1997) estimated
that it would cost approximately $1 million to enclose
a 40-ha farm. Furthermore, the levees on most catfish
farms are not wide enough to accommodate support systems
and vehicle access. Complete exclusion might be cost-effective
and prudent for high-value fish like trout or ornamental
fish in smaller ponds and raceways; however, complete
exclusion of extensive (>50,000 ha) aquaculture production
areas is impractical and may negatively impact other waterbirds
that currently use these wetlands.
Partial enclosure
- Plastic or wire grids suspended over ponds can deter
cormorant flocks from landing or taking off but do not
exclude cormorants from highly attractive aquaculture
ponds (Barlow and Bock 1984, Moerbeek et al. 1987). Although
some research continues with the use of overhead wires
in Arkansas, May and Bodenchuk (1992) concluded that an
overhead wire grid structure over a 3.7-ha pond was impractical
under current catfish culture practices. Materials ($404
per ha; $163/ac) and labor (15.5 person-days) costs were
lower than full net-coverage, but the structural durability
and functional design were not adequate for protecting
large ponds. As with complete enclosure, benefit to cost
ratio for using many partial enclosures may not justify
their use. However, Keller (1999a) found that overhead
wires, in conjunction with harassment efforts, might be
cost-effective for protecting smaller (0.2 to 2.5 ha)
ponds from great cormorants in Germany where the state
of Bavaria subsidized 40 percent of the costs. Overhead
wires can exclude and injure other nontarget waterbirds
(Mott and Boyd 1995), but marking wires with flagging
material can minimize this risk.
Floating and underwater grids
- Floating ropes can hamper cormorants from landing and
taking off from ponds, depending on the prevailing wind
direction. Mott et al. (1995) partially protected 2 catfish
ponds from cormorant flocks (total of 10.6 ha; 26.17 ac)
for 3 and 8 weeks, respectively. The ropes cost $97/ha
($39/ac) and took 57 hours to install and subsequently
disassemble on the 2 ponds. Although floating ropes have
practical advantages over overhead wires, they are similarly
limited in effectiveness and would be less likely to deter
cormorants in situations where few alternatives existed.
Underwater barriers or baffle systems could theoretically
interfere with cormorants' pursuit of fish (Barlow and
Bock 1984), but studies using submerged nets as fish refugia
to deter cormorant predation suggested no significant
effects on predation rates (Gottfried 1998). Because of
repeated harvesting with the multi-batch cropping system,
most underwater barriers would have to be repeatedly removed
and reinstalled, adding to overall costs of such systems.
Localized
Cormorant Management
Management of cormorants at or near the
site of damage currently includes harassment techniques
and lethal removal of birds. These techniques do not presently
appear to reduce regional populations (Glahn et al. 1996,
Glahn et al. 2000, Mott et al. 1998).
Cormorant Harassment
At Aquaculture Facilities
- Scare devices consist of auditory or visual stimuli
intended to move or deter cormorants from a target site
(Booth 1994, Draulans 1987, Littauer 1990a, Littauer 1990b,
Mott and Boyd 1995). This category of harassment includes
human activities, vehicles (truck, all-terrain vehicle,
boat), propane exploders, pyrotechnics (exploding or whistling
projectiles), cormorant distress calls, alarm units, sirens,
and effigies (stationary or inflatable). Single devices
or a combination of methods has thus far proven to alleviate
depredation only temporarily (Draulans 1987, Moerbeek
et al. 1987, Mott and Boyd 1995, Rodgers 1994). The typical
practice is to patrol pond levees in a vehicle throughout
daylight hours and shoot pyrotechnics or shotguns at birds.
Human effigies have been used to augment this strategy
(Stickley unpub. data). Altogether, 245 fish producers
surveyed in the delta region of Mississippi by Stickley
and Andrews (1989) claimed annual expenditures of $2.1
million to harass all species of fish-eating birds. The
cost-effectiveness of these efforts has not been determined.
The expansive size of most catfish farms limits the effectiveness
of harassment techniques (Reinhold and Sloan 1999). Because
simultaneous harassment of all ponds is difficult on large
farms, birds simply move to other ponds, resulting in
no net decrease in predation. Even when birds can be dispersed
from farms, they often return as soon as harassment ceases
or simply move to other farms (Reinhold and Sloan 1999).
Winter Roost Harassment
- Coordinated and simultaneous harassment of cormorants
can disperse them from night roosts and reduce damage
at nearby catfish farms (Mott et al. 1992, Hess 1994).
Electronic noise generators, amplified recordings of cormorant
alarm calls, propane gas exploders, pyrotechnics, and
firecrackers can be used. During a 3-year study, Mott
et al. (1998) evaluated a coordinated roost dispersal
program of cormorants at all known roost sites in the
delta region of Mississippi and observed fewer cormorants
at catfish ponds near harassed roosts. Harassed cormorants
temporarily shifted their roosting activity from the intensively
farmed east-central delta to sites along the Mississippi
River, where they foraged primarily on shad.
Despite the partial success of this program,
several factors limit its usefulness as a long-term solution
to aquaculture predation problems. Roost harassment must
be conducted on a regular basis to have a sustained effect
(Mott et al. 1998) because harassed cormorants usually
establish new roosts at the nearest suitable location,
then return to previous roosting sites when harassment
ceases. In 1993, when the roost dispersal program was
initiated in the delta region of Mississippi, there were
48 known cormorant roosts in this region. That number
increased to 75 by 1999 (Glahn et al. 2000). If this number
continues to increase, there eventually may not be enough
producers to simultaneously harass cormorants at occupied
roosts in the delta region of Mississippi.
Another problem encountered with the roost
dispersal program is that hunters increasingly express
concerns about unintended effects of harassment on waterfowl
(Mott et al. 1998). Although the use of low-power laser
devices for dispersing cormorants may help alleviate these
concerns (Glahn et al. unpub. data), an increasing number
of hunting clubs and refuges are restricting cormorant
harassment. Cormorant populations have increased at these
sites to 15,000 birds (D. Reinhold, USDA-WS, pers. commun.),
negating efforts to move cormorants out of the protected
area. Repeated harassment with non-lethal frightening
devices may become less effective as birds become accustomed
to them. A recent study (Glahn in press) indicates that
shooting cormorants in roosts is as effective as dispersing
them with pyrotechnics and may not result in habituation.
However, lethal control at roosts is not authorized under
current regulations.
Lethal Control
Shooting a limited number of cormorants
reinforces non-lethal harassment (EIFAC 1988, Hess 1994,
Littauer 1990b, Mastrangelo et al. 1995, Rodgers 1988
and 1994, Tucker and Robinson 1990, USDA [Vol.2, J-12;
Vol. 3, P-32] 1994). This strategy can be part of an integrated
damage management plan implemented at farms when non-lethal
methods are ineffective (Mastrangelo et al. 1995). The
USDI-FWS depredation order for the double-crested cormorant
(50 CFR Part 21) allows for localized control of cormorant
populations at aquaculture facilities (USDI-FWS 1998).
However, most fish farmers can shoot only a small percentage
of the cormorants feeding in their ponds (Hess 1994).
Similarly, if shooting in roosts were restricted to daylight
hours, only a small percentage of roosting birds would
be killed (Glahn in press). Hess (1994) investigated the
feasibility and potential impact of allowing farmers to
shoot an unlimited number of cormorants on their farms.
The producers at two facilities (1,920 ha) in the delta
region of Mississippi spent more than 3,000 person-hours
trying to shoot cormorants, yet killed only 290 birds,
as the birds quickly learned to avoid the hunters. Glahn
(in press) observed the same response when attempting
to shoot cormorants in roosts, resulting in less than
5 percent of the roosting population killed before the
roost dispersed. Waterfowl exhibit similar behavior during
the hunting season (Owens 1977). The cormorant depredation
order allows fish farmers to implement strategies similar
to those used by waterfowl hunters, such as using decoys,
blinds, and camouflage clothing (USDI-FWS 1998). Employing
such tactics might enable farmers to reduce the number
of birds on their farms significantly without affecting
continental or flyway populations. Under the old permit
system, catfish farmers reported taking an average of
only 42 cormorants per year, and the total number reported
shot in any one year never exceeded 68 percent of the
authorized take (Mastrangelo et al. 1995). Assuming an
average take of 42 cormorants per catfish farm, the USDI-FWS
predicted a total annual take of 92,400 cormorants under
the depredation order (USDI-FWS 1998). However, Glahn
et al. (2000) found that the take under the depredation
order might exceed the reported take under the former
depredation permit system and recommended more extensive
monitoring of the number of cormorants taken under the
depredation order. However, even doubling the take would
represent only a small percentage of the annual recruitment,
conservatively estimated at 612,000 cormorants per year
(USDI-FWS 1998).
Flyway Management
Localized management efforts help to reduce
cormorant depredations on southern aquaculture farms but
have little effect on the flyway population (Belant et
al. In press, Glahn et al. 2000, Mott et al. 1998). The
objective of flyway management would be to help alleviate
localized conflicts by managing the "interior"
population of cormorants. Van Eerden et al. (1995) noted
that local conflicts between human interests and great
cormorants (P. carbo) in Europe seemed impossible to mitigate
without managing the entire continental population. For
instance, large-scale sanctioned shooting of great cormorants
in Germany resulted in no reduction of the observed wintering
population (Keller 1999b). This seems to parallel the
lack of population decline of birds wintering in the delta
region of Mississippi despite the large take of cormorants
under the depredation order (Glahn et al. 2000). To some
degree, the same difficulty has been observed in attempting
to control breeding populations with egg oiling (Gross
1952, McLeod and Bondar 1953). However, combining egg
oiling and culling has been effective in controlling populations
on a localized basis (Bedard et al. 1999). Thus, effective
manage ment of cormorants for reducing depredation to
southern aquaculture will likely require more intensive
control (culling) on the wintering grounds, control on
the breeding grounds, or a combination of both. In addition,
preventing new breeding colonies from being established
might be an effective way of controlling populations (Bregenballe
et al. (1997). A recently initiated satellite telemetry
study (QA- 742) will provide insight on where control
may be best implemented but not the extent of control
needed. For instance, if cormorants wintering in southern
aquaculture regions have a strong fidelity for these foraging
areas but come from a wide breeding area, then control
might be best implemented on the wintering area, i.e.
culling at winter roosts. Conversely, control on the breeding
grounds may be important if most cormorants come from
a rather narrow breeding range. However, science-based
management of cormorants will require a thorough knowledge
of population and density-dependant parameters that can
be incorporated into a population dynamics model for determining
the type and extent of control needed to reduce cormorant
conflicts.
Most wildlife management plans include species-specific
population goals, plans for meeting those goals (e.g.,
hunting regulations, habitat management plans), guidelines
for gathering information on important variables (e.g.,
population changes, natality, and mortality rates), and
criteria for evaluating the effectiveness of the plan
(Williams and Nichols 1990, Nichols et al. 1995). As with
waterfowl and other species, management of cormorants
should be based on the ecology of the species on both
the breeding and wintering grounds as well as on biologically
realistic and socially acceptable population goals.
Cormorant Life History - Double-crested
cormorants are considered seabirds but are well adapted
to life away from maritime regions. Like other Pelecaniformes,
cormorants are relatively long-lived (Johnsgard 1993),
an important consideration when deciding how to manage
this species. Populations of long-lived species tend to
be regulated by the mortality of juveniles, rather than
by that of adults (Hickey 1952, Johnsgard 1993). Because
cormorants are long-lived and mortality is highest among
juveniles, management relying on egg-oiling and other
reproductive control measures will likely be less effective
and take longer to achieve the desired result (see also
Bédard et al. 1995, 1999; Dolbeer 1998). Unlike
most seabirds, cormorants have a relatively high annual
reproductive rate of two to three fledglings per year
(Price and Weseloh 1986, Weseloh and Ewins 1994, Weseloh
et al. 1995). Birds usually reach sexual maturity and
breed 3 years after hatching, although some breed during
their second year (Palmer 1962, Weseloh and Ewins 1994,
Weseloh et al. 1995). In addition, cormorants have several
adaptations that help them avoid density-dependent mortality
and maintain their population at or near the environmental
carrying capacity (Johnsgard 1993). Based on great cormorant
population modeling, these density dependant factors would
tend to buffer the impact of management actions, but most
management actions alone or in combination would lead
to stabilization below the environmental carrying capacity
(Bregnballe et al. 1997). In contrast to information available
to generate population models for the great cormorant
in Europe, very little information is available concerning
the demographics of the double-crested cormorant (Erwin
1995, Bédard et al. 1995, 1999).
Setting Population Goals - Wildlife managers
must have a clearly defined population goal and guidelines
by which to meet that goal based on their understanding
of the population status and dynamics of a species (even
if limited) in order to justify and defend lethal or reproductive
control measures (Dolbeer 1998). Precedence for such action
is seen in the sustained increases in populations of herring
(Larus argentatus) and lesser black-backed gulls
(L. fuscus) which prompted wildlife managers
in Britain to cull populations to reduce habitat damage
and impacts on other colonial waterbirds (Duncan 1978,
Wanless and Langslow 1983, Wanless et al. 1996). Also,
biologists successfully implemented a 5-year plan in 1989
to reduce numbers in a breeding colony of cormorants whose
excreta was destroying unique insular habitats in the
St. Lawrence Estuary (Bédard et al. 1995, 1999).
In both Britain and the St. Lawrence Estuary, decisions
to remove birds and the population goals established were
assessed relative to reducing species-specific damage,
while maintaining biodiversity within the ecosystem.
Despite the marked increase in cormorant
numbers in the United States and Canada since 1970 and
a population now estimated at more than 1 million birds
(Tyson et al. 1999), natural limits on further increase
and peak population size cannot be accurately predicted.
Nonetheless, such population increases on the breeding
grounds will likely result in increased depredations at
aquaculture facilities and other conflicts in the Southern
United States, given recent band recovery analysis and
satellite telemetry linking cormorants wintering in Louisiana,
Mississippi, Alabama, and Arkansas with breeding populations
in the Great Lakes. Although there are increasing concerns
about the unchecked growth of cormorant populations, there
is no consensus on the biological and social carrying
capacities for cormorants on either the breeding or the
wintering grounds. Although population goals in southern
aquaculture regions are difficult to ascertain, studies
that determine the economic threshold of cormorant predation
may be an important step in helping define socially acceptable
goals. In addition, projections of the extent of control
needed from population modeling, coupled with a knowledge
of the logistics for accomplishing these control efforts,
may provide insight into setting biologically realistic
population goals.
Evaluating Management Options - Public sentiment
has and will continue to spark debate about the lethal
management of wildlife populations. Increasing public
awareness and input on natural resource management issues
demands that wildlife managers investigate thoroughly
all options and stand ready to communicate and defend
their decisions. The path of least resistance may be to
base strategies on natural population regulatory mechanisms
(e.g., limitations on breeding space, fluctuations in
the prey base, or disease). However, such expediency will
likely maintain the status quo and may even lead to increased
in conflicts between cormorant and human activities for
an extended period of time. For example, Bregnballe et
al. (1997) predicted from population modeling that rapidly
increasing great cormorant populations in 1995 would further
increase from 108,700 birds to approximately 600,000 before
stabilizing 14 years later and that most of this growth
would occur during the last 5 years before stabilization.
A variety of management scenarios can be evaluated and
management decisions justified (given current knowledge
of the species' population dynamics) by using population
models that project the effects of various options relative
to a range of realistic population variables (Bregnballe
et al. 1997, see Crouse et al. 1987, Williams and Nichols
1990, Bédard et al. 1995, Nichols et al. 1995,
Wanless et al. 1996, Schmutz et al. 1997). For example,
Bregnballe et al. (1997) used modeling to explore the
efficiency of various control options for great cormorant
populations and found that all strategies alone or in
combination lead to stabilization of the population at
lower levels. However, culling adults and preventing formation
of new colonies were the most efficient means of stabilizing
the population. With better demographic information, population
modeling will help guide the planning process with double-crested
cormorants.
Models that have already been used successfully
to plan the culling of cormorant colonies in the St. Lawrence
Estuary (Bédard et al. 1995, 1999), to evaluate
the growth rate of the cormorant population on Lake Ontario
(Price and Weseloh 1986), and to evaluate the effectiveness
of lethal versus reproductive control measures (Dolbeer
1998) provide a basis for developing future plans to manage
cormorant populations. Even with the limited demographic
data available for cormorants, initial age-classified
matrix models (Caswell 1989, McDonald and Caswell 1993)
can be developed that will aid assessment of cormorant
population growth thus far, and guide future management
decisions.
Assessing Results - Population models are
most useful for developing testable hypotheses; confirmation
of model predictions must still be obtained from the field.
Censuses of managed populations should be conducted during
and after control programs are implemented to judge effectiveness
and prevent excessive losses (Duncan 1978, Wanless and
Langslow 1983, Bédard et al. 1995, 1999, Wanless
et al. 1996). Estimates of population indices following
a management program provide the data necessary to refine
management models important in predicting future population
trends (see Wanless et al. 1996). For example, further
monitoring of population trends and annual take under
the depredation order could provide insight into culling
adults as a regional management technique that might,
if expanded, accommodate flyway-based objectives. Ultimately,
the results of management programs should be assessed
from the standpoint of resource economics (Werner in press)
and reducing the costs or enhancing the effectiveness
of other control strategies (Mott et al. 1998).
Conclusions
Southern aquaculture, primarily devoted
to the cultivation of catfish, crawfish, and bait fish,
involves the extensive use of large shallow ponds that
are highly susceptible to predation by cormorants. Previous
research has clearly documented the resulting impact on
the catfish industry. This problem has been exacerbated
in recent years by the doubling of wintering cormorant
populations in catfish production areas of Mississippi
from 1995 to 1999. Future increases in cormorant wintering
populations combined with growth of southern breeding
populations will likely intensify the problem. Current
damage abatement measures are of limited effectiveness
and are becoming increasingly difficult to implement.
Continued research is needed to evaluate cultural and
barrier strategies to reduce depredations at aquaculture
farms. Methods should also be explored for improving the
implementation of the current standing depredation order
for reducing local populations on aquaculture farms as
well as expanding the depredation order for reinforcing
harassment strategies at roosts. Authority should be sought
to implement control of localized populations at southern
breeding colonies, as needed, to forestall future depredation
problems and population growth.
Further, managers should continue to explore
the factors ultimately affecting the growth of the cormorant
population within the Mississippi flyway and management
options necessary to meet preset population goals. We
suggest that these investigations include the development
of a goal-oriented population model to guide management
decisions. Given suitable population models to guide its
implementation, we suggest that cormorant populations
be managed on a flyway basis, as part of an integrated
strategy to reduce their conflicts to southern aquaculture,
as well as elsewhere.
Without human intervention, cormorant populations
are likely to continue to grow and create increasing problems
with southern aquaculture facilities, fisheries, and other
colonial nesting species. Dialogue among public agencies
and private organizations concerned about the conservation
and management of cormorants would facilitate the development
of realistic population goals that would ensure the continued
well being of this species while mitigating the negative
impacts associated with unbridled population growth. |