Download 1995 Why is the Double-crested Cormorant a Problem

Why is the Double-crested Cormorant a Problem? Insights from
Cormorant Ecology and Human Sociology
Marine Sciences Research Center
S a t e University of New York at Stony Brook
Stoney Brook, New York 11963, USA
Present address:
A l a s h NaturaI Heritage Program
and Department of Biology
Univenicy of .Alaska
Anchorage. Alaska 99301. USA
Abstract--Comorano Phalacrocoracidae have along his~ocyof conflict w i t h fishermen and more recenrly with
fish-farmen. Cormorant species tend to be oppormnisdc, adaptable, and highly a t m c t e d co concentrated food
sources. .it the population level, &ere is lirde evidence to suggest chat cormomnu serioudy deplete commercial
food sources, although ac small scales, individuals or small numbers of cornorants may cause problems, especially
ac fish h i n g operadons o r haccheries. O n che orher hand, despite a high potendal race oFpopuladon increase,
cormorants are among the more sensitive seabirds to human disturbance during breeding. Conuol mesures would
be very effective at breeding sites, but it is not clear chat such efforn would remove the propomon of the population
likely to be in direct conflict with humans, except at colonies adjacent to fish farms.
Key words.-Double-crested Cormownr disturbance, pest species, Phalnmcurar oun'rirs, p o p ~ ~ l a d odyamics,
n
wildliie management.
Colonial IVacerbirds IS (Special Pubiicadon I):%-32, 1993
This paper mes to set conflicts between
cormorants and fishermen in the framework
of the biology of cormorants and how we humans deal with problem species. It then
makes some recommendadons, based both
on bioloz and on human attitudes, in the
context of ecological problem-solving. It is
not designed to be a review of the licerature,
but rather to highlight why the cormorant
can be a problem, how we can ecaluate h a t
problem, and how we can take adtantage of
the species' biology to reduce the problem.
In wildlife management, only part of the
process is scientific. We can measure populations, life history parameters, diet, energetics and behatior of the wildlife and not
couch on the source of the problem. Most
problems and solutions involve human attitudes and actions which serve as a filter for
the science, so, in dealing with the problem
of the Double-crested Cormorant (Phalacrocorax auntus) competing with fishermen, we
have many scientific facts, but there are differing opinions about the problem and what
can be done about it. Ac the extremes, some
say that there is in fact no biological prob-
lem; that cormorant predation is minor compared to other losses at fish farms. Others,
animal-righa advocates, would claim that
the connorants have a right to fish and that
we have no right to stop them. Finally, there
are those who fish or farm who believe that
their right to make a I i ~ i n gis impaired by
cormorants so they should have free rein to
deal b5th the problem. Most of us fall somewhere in the middle of all this.
Whatever our differing tiewpoine, we accept that there is a conficc. I and other cormorant investigators can talk about the
science, but I think we scientists need to
keep in mind that our science can take us
only so far. On the other hand, I would ask
non-scientists to examine what the science
has to say with an open mind. It may contradict their experience or their impressions,
but that is what exactly what science is designed to do, separating our preconceptions
from the cold reality of hard data. Things are
not as they appear or as we may wish them to
be when dealing with tormoranu (Bayer
1989).
The Perception of Pest Status
Illinois and as of special concern in klichigan (Blokpoel and Scharf 1991). We have
Pest status of animals is inherently a sub- the illogical result that some states are trying
jective and political judgement (Berryman to protect and increase cormorant popula1986). This classification is based on a deter- tions, while the same cormorants are being
mination that the animal is "a health hazard, harassed o r shot when they reach other
causes economic damage or is a general nui- states. Until such time as a national or bi-nasance to one or more persons" (Salmon and tional (United States and Canada) consenLickliter 1984) or prevents "optimization of sus emerges on whether the Doubletrested
the resource under management" (Stark Cormorant is a problem, this piecemeal
1977) o r has a "negative impact on human management can treat local s>mptoms,but it
surcival o r well-being" ( B e q m a n 1986). is unlikely to achieve much of a solution, no
There appear to be few efforts to establish matter how good the scientific advice or the
objective standards of pest status, takng into management expertise.
account the positive and negacive effects of T h e Science of Pest Species
species in communities at different times
and places, or of differing view ~\ithinthe Species' characteristics
human population. A species is a pest when
Seabird biologists and managers are
someone says it is, n,hether that someone is a more accustomed to dealing with problems
homeo~\ner,a fish farmer or a scientisi If of rarity in seabirds than of pest species. The
the one person feels strongl~enough, they l o ~ intrinsic
c
rate of population increase and
do something about the problem, ranging vulnerabilit). to nest disturbance and predafrom the lead such as puuing up a scarecrow tion of many species of seabirds have led to
or obtaining a nuisance permit, to the illegal major population decreases (cf. Crosall et al.
such as harassing animals or attacking breed- 1984; Crosall 1991; Xetdeship el al. 1991), so
ing sites. If enough people agree that there that 13 percent of all species in che seabird
is a problem, reach a consensus on a solu- families are threatened or endangered and,
tion, and have the political power, then may- in some families, as many as 3040% (DuQ
be something gets done at a larger scde, 1992). Even among c o m o r a n o , 11% of the
such as a government program (MVagnerand species are at risk ( D u e 1992). Biologists
Seal 1992).
are thus cautious about adding to this probProblem species involve perceptions and lem through population manipulations that
perceptions change (cf. bellert 1985). For might backfire; afterall, some of today's pest
example, \valves were once viewed as pests in species were endangered less than a century
Yellowstone Sational Park, so they were ex- ago. For esample, Double-crested Cormoterminated. Now we are re-introducing rants and Great Black-backed Gulls (Larur
wolves at great cost back into the same park. marinus) were almost estirpated in the UnitThe wolf has not changed at all; public atti- ed States at the turn of this century; now
tude has (McNaught 1987). Wolves still kill both species are common and cause problivestock o n occasion, but the public has lems (Buckley and Buckley 1981). The spereached a consensus that wolves in the Yel- cies have not evolved, human behavior has
lowstone ecosystem are more important changed and we have ceased hunting, reduced pollutants, and restricted disturbance
. than the costs of wolf predation to livestock
and the opposition of many livestock owners. at nest sites.
W'hile definition of a pest species deFor less charismatic species, such as cormorants, n o such consensus esisa. As this pends on human decisions and economics,
collection of papers illustrates, there are there are several biological criteria that may
many areas where cormorants are consid- point to species than can obtain population
ered pests, taking free-ranging or farmed levels and esploit suitable environments in
fish, yet at the same time the Double-crested sufficient abundance 50 be considered pests
Cormorant has been listed as endangered in by humans.
Population.-Population size or density
alone are poor indicators of pest status. For
example, the Double-crested Cormorant
population in the Great Lakes is now estimate d at approximately 27,000 pairs in an area
of 244,000 krn? (Hatch 1995), or a densicy of
o n e bird for every 4.5 km2. The latest estimates for the Xorth American breeding population are 330,000 pairs (Hatch 1995) in an
area of 19 million km', or a density of one
bird per 23 km'.
P~nrlation growth.-While
population
size in itself may not identi@ problem species, population trends may be more useful.
Double-crested Cormorants have shown astounding rates of population increase, such
as 20% per year in southern New England
(Hatch 1984). 40.4% per year in the Canadian lower Great Lakes between 1976 and 1990
(Blokpoel and Tessier 1991), 41% per year
during 19'73-1981 for the entire Great Lakes
(Lud!~ig 1985), and 56% per year in Lake
Ontario during 197G1982 (Price and Weseloh 1986). In contrast, the Guanay Cormorant (Phnlncrocorm bougainuill~l i ~ i n gin the
highly productive waters of chr Peru~ianup\>.ellinghad a mean annual rate of increase
of only 18% (Duffi 1983).
Population dynamics are determined by
the relative contributions of reproduction,
m o r d i y , and migration. Reproduction includes clutch size, breeding success, and age
at first breeding. Cormorants have relatively
large clutches compared to ocher seabirds
(DuQ 1980) and have asynchronous hatching that facilitates survival OF different numbers of young, based on food supply
(Williams and Burger 1979). Price and Weseloh (1986) report reproductive success for
Lake Ontario cormorants OF 1.7-3.2 young
per nest and they suggest a mean of 2.8
young per nest is possible for this population. Drent et a1 (1964) reported 2.4 young
per nest over three years at blandarte island;
rates of 2.1-2.4 were reported by PiIon et al.
(1983a) for the Magdalen Islands, Quebec.
These races appear high relative to ocher cormorants (Johnsgard 1993) and are certainly
high compared to most seabirds (Lack
1967).
y
T h e age of first breeding for Doublecrested Cormorants is unknown for the
Great Lakes, but Van de Veen (1973) found
that over 20% of breeders of a slowly increasing (8% per year) Pacific coast population
were only one to nvo years of age. That the
species can breed this early is icself remarkable, compared to most seabirds which may
wait three years or more (Lack 1967). If the
Great Lakes population has a much higher
percentage of young birds breeding than in
the Pacific population, this would also help
explain the rate of increase (Price and Weseloh 1986).
Turning to mortality, Price and MreseIoh
(1986) re\ien.ed data that suggesc chat stable
populations of Double-crested Corrnorancs
suffer a 70% pre-breeding mortality and
15% annual adult mortali?.. In a Pncific population increasing at 8% per year, \Bn de
Veen (1973) reported m o r t a l i ~during the
first n\-o years (here assumed to be the same
as prebreeding) to be 64.5% and adult mortality to be 15.1%. To fit Great Lakes esdmates of nest producri\ic>: and population
increase, Price and M'eseloh (1986) suggest a
prebreeding mortalin- of only 31% and an
adult morcalicy of 10%. It is in~erestingto
speculate that the high race of immature surbival may be linked to food on the xbintering
grounds, such as at fish Farms.
T h e last aspect, immigration, ma); be important locally. Price and J\'eseloh (1936) argue that annual population increases up to
56% on Lake Ontario could be produced by
the breeding coIonies themselves, but chat in
three of h e i r nine study years, immigration
was responsible for 10571, 3176, and 33% of
the population increases of up to 171%.
However, it remains unclear where such migrants come from, as there seem to be few
colonies chat are decreasing in the United
Scates and Canada (Vermeer and Ranking
1984).There may however be a pool of nonbreeding birds that recruit into colonies during excepuonal years. '
Taking these aspects together, at least in
the Great Lakes, Double-crested Cormorant
reproduction seems to have more in common with rabbits than with most seabirds.
'
The species is undergoing a remarkable increase in population. This suggests that
problems between cormorants and humans
will only increase.
Foraging.-Problem species have relarively wide habitat and food tolerances, and the
abiliq to adapt to locally abundant food resources when these become alailable. At first
glance, cormorants do not appear to fill
these criteria. Cormoranu as a family are relatively limited in how they can forage. Aithough other cormorant species have been
reported feeding by plunge-dising ( D m et
al. 1986) and feeding on uaw-ler offal (Blaber and Wassenberg 1989), most cormorants
catch their prey by underwater diving in pursuit of live prey (e.g., Ainley et al. 1981).
Cormorants may locate a suitable feeding
area from the air, while flying, using the presence of fish ('C'ogt 1942, Barlow and Bock
19S4), prior knowledge, or other foraging
birds as clues (Duffi 1987). Palmer (1962)
reported a foraging range of 8-16 km for
Double-crested Cormorants. Others, tracked
by airplane at n o breeding colonies in Wisconsin, had masimum foraging ranges of
11.6 krn and 40 h but mean distances of
only 2.0 !an and 2.6 km (Custer and Bunk
1992). 011 the other hand, in rhe Farallon Islands off California, Xinley et al. (1990)
found that breeding Double-crested Cormorants travelled 30-80 km to feed in shallow
waters over smooth substrates inshore, because suitable nesting places $\.ere not available closer to the fishing grounds. Such a
long commute uould require that the food
resource be consistently available and abundant and, in fact, Double-crested Cormorant
nesting success was more consistent than
that of other species that fed closer to the
Farallones, on less predictable prey (Boekelheide et al. 1990).
Pennycuick's (1989) anaiysis of wing
shape indicated that the species forages most
efficiendy oniy at short distances from the
colony. Ainley (1977:678) has argued that
these flighc constraints restrict d i ~ i n gseabirds such as breeding cormorants to areas
14th "reliable food sources relatively close to
the breeding site" and to "regions of high biologicsl productibity and standing stocks of
organisms". Fish farms fit these requiremenu superbly.
Once a foraging site is chosen, cormrants search for prey u n d e n a t e r and lunge
at them with their beaks ( O w e 1967). M W e
most foraging seems to take place in shallow
waters, Double-crested Cormorants have
been reported foraging in water 13-22 m
(Palmer 1962, ECnopf and Kennedy 1981,
Johnsgard 1993). As cormorants are visual
hunters ( O w e 1967), their hunting efficiency may be reduced at lower visibilities.
On the other hand, ald~oughthe foraging methods of Double-cresced Cormorants
are limited to surface d i ~ i n gtheir
,
choices of
foraging areas and prey are very broad and
they appear very adaptable to local conditions (Ludvig et al. 1989). For example, Ainley el al. (1981) reported this species
foraging on mid-water schooling fish in the
eastern Pacific, while Robertson (1974)
found them feeding on inshore benthic species on Mandarte Island, British Columbia.
They fed on "small, shailow water, bottom
species" in Lake Superior (Craven and Lev
1987), ~ihileelse~\.herein the Great Lakes
Ludvig et al. (1989) found them feeding
both on benthic species and on shallow-mter schooling fish, when the later moved inshore to spaI\.n.
Prey size ranges from 3 cm to 30 cm, basically anything cormorants can catch and
swallow (Xitchell 1977, Pilon et al. 19S3b).
Socially, their foraging is also very adaptable.
Double-crested Cormorants are s o l i v
feeders on the Adantic coast (pers. observ.)
and Great Lakes (Craven and Lev 19S7), but
social feeding is common in the Pacific (Bartholornew 1912).
Cormorants spend a great deal of time
searching for prey (flight to foraging site,
then di~ing)relative to actual feeding which
may take less than a second for small fish. In
terms of foraging theory (MacZrthur and Pianka 1966),such a predator should take tirtually any edible prey it encounters and can
eat, rather then being selective, because handling and consuming the prey costs little in '
terms of time and energy reIative to the energy or time espended before encountering
another prey. In practical terms this means
cormorants will probably feed on almost anything edible they encounter while dicing.
Effects of Abundant Species
We can examine competition benveen
fisheries and avian piscivores through a series of measurments that are simple in theory, no matter how complex their logistics.
If we know the mass or weighc of a bird, we
can calculate its energy consumpdon based
on allometric equadons or direct obsenations of consumption. For example,
Schramm et al. (1987) used allometric equations to determine that at a mass of 1,800 g,
the Doubleirested Cormorant required a
food consumption of 205-247 g d" (1 1.513.7% of body weight) in Florida. Based on
fieId obsenations that cormorants ate 19
channel catfish (Ictal~tnisptrnctatrls) per day
(length 7-16 cm), Schramm et al. (1984) suggested an inditidual consumption of 304 g dl
(16.9% of body weight) at fish ponds in south
Florida.
If we h o w the number of birds feeding
ac the site, we can calculate total consumption by multiplying daily food needs in , p m s
of fish per cormorant by the number ofindi~ i d u acormorana
l
present. For example, the
13 cormorana resident at the same Florida
pond consumed 246 fish per day or approximately 3.9 kg d-' (Schramm et al. 1984).
These figures make sense only in relation
to the number of fish present and to the
gro~rthof the fish. For example, the same
pond was stocked in Xug~utx\ith 75,000 fish
of 3-8 cm. Consumption by 13 cormorants
was 246 fish per day which works out to 7,380
per month or 89,790 per year, o r 20% more
fish rhan the initial stocking. At first this
looks terrible, but we need to take inco consideration notjust standing stock (the number of fish at any one time), but the
production or growth of the fish (Duffy and
Schneider 1994). Basically, catfish grow, but
the mass of food (304 g b'bira') of food consumed per cormorant per day remains the
same, so the number of fish needed to satisfy
the cormorant's daily food requirements
1vouId drop conrinuoisly U'e also need to
consider size of fish in models of consump
tion by cormorants with losses from other
sources (Parkhurst et al. 1987). Glahn and
Scickley (1995) estimate that cormorants
consume 4% of catfish fingerlings each year.
Is this the largest source of mortality? If not,
then perhaps k n d i n g and research should
be focused elsewhere to achieve greatest return per research dollar.
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Point and Non-point Sources of Problem
Birds
From an ecological point of vie\\; it is often helpful to think of problems, such as pollution, as originating either from point or
non-point sources (Odum 1989). A point
source for nitrogen entering a bay might be
a sewage ourfall; non-point sources would be
hundreds of lawns being ferdlized and the
subsequent surface and grounduater runoff
at a thousand different locations. 111 a point
r problem at the
source it is easy to go a f ~ e the
source; for a non-point source, i[ may be easier to protect against the problem than to
solve it at its m>riad sources.
In terms of fish farms or ocher problem
areas, is there one roost o r colony within efficient foraging range (2-3 km?) rhat is the
source of the birds? Or are there many roosts
or small colonies? Or are there hundreds of
migrating birds that scop to feed in passage?
In the firsc case, the point source, perhaps
the roost or colony, can be relocated by remocing nesting or roosting trees during the
non-breeding season o r by judicious harassment as the colony or roost members first arrive? In the laxer case, protecting the fish
farm would appear more practical.
Knowing something of the ecology of
cormorants in general and of the Doublecrested Cormorant in particular may give us
management options for reducing the problems associated with fishery operations. The
list that follows includes'some such options,
but does not address either their effectiveness or their drawbacks. The cures could be
worse than the problems, so their use should
be considered tvith care.
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Nesting
Nesting Doublecrested Cormorants are
extremely sensitive to disturbance and resulting predation by Fish Crows (Grant 1970,
Post 19%) and gulls (Kury and Gochfeld
1973, Siegel-Causey and Hunt l98l), whether the disturbance results from vandalism
(Weseloh and Sturger 1985, Ludwig et a1
1989), investigator disturbance (Ellison and
Cleary 1978), or nuisance abatement programs (DesGranges and Reed 1981). HOW
ever, such disturbance does not always
m s l a t e into population reductions (DesGranges and Reed 1981).
Foraging
Cormorant foraging is less vulnerable to
disturbance than is nesting. Cormorants can
always feed elsewhere until the disturbance
disappears. However, there may be a number
of ways to make foraging more difficult. Barlow and Bock (1984) suggest that lower
stocking densities of fish make it difficult for
cormorants to locate fish from the air, so that
fewer birds land, consume fewer fish, with
fish productitity possibly ending up greater
than with a higher stocking rate.
Because of their heals bodies and small
wings, cormorants need a clear runway for
takeoffs. Restricting runway space for takeoff
might discourage birds, perhaps with some
small-scale equivalent of the banage balloons that the British used in the Second
World War to discourage low-level bombers.
For another species of cormorant, Barlow
and Rock (1981) suggested steep sides to fish
ponds so that the cormorants cannot walk
easily in and out of the water.
Barlow and Bock (1981) noted that crustaceans could be used to dilute the prey cormorants encountered in Australian fish
ponds. If a noncommercial prey is available
as a sacrifice, then predation on the commercial species would be reduced. Similarly,
Morrisey (1976) suggested that reducing xisibility in farm ponds can restrict foraging efficiency.
Finally, if cormorants can not travel long
distances efficiently to forage, then it may be
feasible to discourage or remove nesting col-
onies or roosts only within a certain ~adiusof
fish farms.
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D~sccrss!os
Cormorants are the most visible piscivores and there is a long standing tendency
to blame them for mortality of commercially
important fish (Bayer 1989). On the other
hand, we have only begun to do the hard science and to measure how much cormorants
actually eat, much less to consider the role of
cormorants in the ecosystems that are fish
ponds. We hear litde about how cormorant
predation compares to other losses, or
whether by reducing the stocking level, cormorants might actually cause the remaining
fish to grow faster, because of increased rations for the sunivors. M'e do not know
whether cormorants pick on weaker or sick
fish, reducing the risk of disease in fish farms
or whether cormorants are vectors for fish
diseases. Finally, we have only an initial idea
of whether harassment p r o p m s pay for
themselves in terms of increased sunital or
production at the local pond level
(Parkhurst et al. 1987, Stickley et al. 1992).
M-ould such programs really pay for themselves at a regional or national level?
Doublecrested Cormorants are undoubtedly a problem for fish farmers, but
these problems are local. Solutions to these
problems should also be local, at least on the
basis of present knowledge. Some of the sclutions suggested above could reduce predation, but they would not eliminate i r At a
larger scale, since we know that wintering
cormorants that cause problems come From
many diierent breeding colonies (Dolbeer
l g g l ) , we u.ould need to 'down-size' all current North American nestin,= cormorant
populations through a massive harassment
program at breeding sites. We wiped out
Doublecrested Cormorants in the United
States once; we can do it again. This would
kill hundreds of thousands of birds, eveh
though only a few tens of thousands (3%)
are actually the problem (Glahn and Stickley
1993). This is unacceptable. Harassment
might a l s ~have unintended consequences,
such as disturbance to endangered species
I
DUFFY
CORLIORLUT
ECOLOGY.L\IDHL'SL~U
SOCIOLOGY
31
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