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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. I I I 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. I I I I i 1 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. ~ I 1 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 C o r m o n n t Pages 191217 in Seabirds of the Farallike Bald Eagles and Brown Pelicans (Bayer lon Ishnds: ecology, dynamics and structure of an 1989) or it might mgger boycotts of aquaculupwelling-sytern communig (D. G. Anley and R J. ture products, as happened w i t h tuna and Boekelheide, Eds.). Stanford Cnivenig Press, Scanford. California. Japanese whaling. Finally, we should keep in Buckley, P. A and F. G. Buckley. 1984. Seabirds of the mind that our grandchildren might find north and middle Atlantic coast of the United Scaces: themselves reintroducing cormorants into cheir suu and consenation. Pages 101-133 in Scants and conservation of the world's seabirds (J. P. Mississippi sometime in &e future, much as Cro.dl, P. G.H. Evans and R tV. Schreiber. Eds.). 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