Conservation and Management

Colony-nesting birds can be readily exploited, and cormorant bones occur widely in middens. In historical times, cormorant eggs were used for making soap (van Tets 1959) and as food for humans and animals, the skins for clothing, and carcasses for bait (Hatch 1995). Widely perceived as competitor by commercial and recreational fishermen (see Conflicts, below) and subject to extensive persecution, particularly by destruction of nests, eggs, and young, but also shooting of adults. Such actions probably account for widespread decline of this species in nineteenth century (see Distribution: historical changes, above). Regional differences in wariness of nesting birds has been thought to reflect differences in this persecution (Bent 1922).

Shooting And Trapping

Effects of shooting seldom have been assessed by any means other than counting carcasses; however, shooting at nesting colonies has substantial additional effects through disturbance and resulting mortality of young (Ewins and Weseloh 1994). In a Quebec control program, shooting of adults at nests led to disproportionate mortality of males (Bédard et al. in press; see Management, below). No recent trapping reported, but native Americans caught sleeping cormorants (probably Double-crested) by hand in the seventeenth century (Hatch 1995).

Pesticides And Other Contaminants/Toxics

Extensive literature includes field, aviary, and lab studies of levels and effects of contaminants on this species (Elliott et al. 1989, Bishop et al. 1992, Powell et al. 1997, Rattner et al. 1999). Cormorants acquire contaminants from the fish they eat. Influence of contaminants increased during 1960s and 1970s, when cormorant populations declined dramatically and various other biological effects (see below) were commonplace. Breeding populations that accumulated the greatest burdens and showed the most severe impacts were in the Great Lakes (Weseloh et al. 1995), Gulf of St. Lawrence (Pearce et al. 1989), and Pacific Coast of s. California (Gress et al. 1973). In Great Lakes, the species was extirpated from Lakes Michigan and Superior, probably because of chemical contamination, and in 1970–1972, contaminant levels (µg/g wet weight) in eggs were: for DDE, 18.56; for PCBs (Aroclor 1260), 27.25; for dieldrin, 0.47; for hexachlorobenzene, 0.18 (Postupalsky 1978, Ryckman et al. 1998). Cormorants breeding in Maine and on Canadian prairies had lower levels (Kury 1969, Vermeer and Reynolds 1970). Cormorants wintering in Houston Ship Channel, TX, accumulated polychlorinated styrenes from Nov to Feb (King et al. 1987).

Effects of contaminants on cormorants have been studied most intensively on Great Lakes; they include eggshell-thinning (Anderson and Hickey 1972, Postupalsky 1978), elevated embryonic mortality (Gilbertson et al. 1991), reproductive failure and population declines (Weseloh et al. 1983, 1995; see also Gress et al. 1973 for California), increased adult mortality (Greichus and Hannon 1973), increased embryonic abnormalities and crossed bills (Fig. 9; Fox et al. 1991, Yamashita et al. 1993, Ludwig et al. 1996), egg mortality (Tillitt et al. 1992), brain asymmetry (Henshel et al. 1997), and induction of cytochrome P–450 1A1 (Sanderson et al. 1994). Crossed bills also observed in Massachusetts (JJH).

Contaminant levels began to decrease in 1970s and have continued to do so through 1990s, with improvement in most associated biological parameters. By 1995 in Great Lakes, most contaminant levels in cormorant eggs had declined by 83–94% and were as follows (for same sites as above in early 1970s): DDE, 2.80; PCBs (Aroclor 1260), 4.74; dieldrin, 0.06; hexachlorobenzene, 0.01 (Ryckman et al. 1998). However, contaminant levels in Great Lakes cormorants remain much higher than in most other areas of North America (cf. Somers et al. 1993, Sanderson et al. 1994), and biological effects persisted into 1990s (Fox et al. 1991). Prevalence of deformities and of hatching failure are higher at more contaminated sites (bill deformities to 52/10,000 in Lake Michigan). Various polyhalogenated aromatic hydrocarbons (including DDE, PCB, and dioxins) are suspected as causes, but links are not yet firmly established. Populations in Washington State, which showed low contaminant levels but continued low productivity, had been disturbed by humans (Henny et al. 1989). Dioxin equivalents in eggs from Great Lakes ranged from 155 to 382 pg/g wet weight and showed a 31.3 mean biomagnification factor from forage fish to cormorant eggs (Jones et al. 1994, Williams et al. 1995).

Little work has been done with metals in cormorants, and no effects have been identified in the wild. Mercury is most often reported: mean levels in eggs 0.11–0.83 µg/g wet weight (Heinz et al. 1985, Noble and Elliott 1986, Henny et al. 1989). In New Brunswick, total mercury concentrations in tissues of Double-crested Cormorants were highest of 9 seabird species (Braune 1987). For other locations, tissues, and elements, see Mora and Anderson 1995 (Baja California), Larson et al. 1995 (Lake Michigan), and Sepulveda et al. 1998 (Florida).

Very susceptible to oiling, like other surface-swimming and diving seabirds, but cormorants are only a small fraction of birds reported killed in oiling accidents and no major kills reported (Clapp et al. 1982).

Ingestion Of Plastics, Lead, Etc

Not reported in pellets (see Food habits: diet, above, for sources).

Collisions

Occasionally reported at power lines.

Fishing

In marine areas, known to be caught on hooks (especially live-baited hooks) and in gill-nets, lobster traps, and trawls, but no data on incidence are available for the inshore waters frequented by most individuals. Similar mortality occurs in freshwater areas. In many populations significant fishery-related mortality has been through direct killing by fishermen (see Conflicts, below).

Alteration Of Habitat

Clearing of forested wetlands reduced available inland habitat for nesting and foraging, especially in the South, but extensive new foraging habitat has been created by dams, and additional food sources by recent aquacultural ventures (Jackson and Jackson 1995). Increases in food are not limited to such new feeding areas but include large changes in fish communities following introductions of new species and other changes (see Christie et al. 1987). The value of impoundments to cormorants is greatly enhanced by intentional and systematic stocking with sport and forage fish to promote recreational fisheries. Impoundments without islands or trees are unsuitable for breeding. Degradation of nesting sites results chiefly from the birds’ own actions; they pluck twigs for nest material, and their feces kill trees in a few years (Lemmon et al. 1994).

Disturbance At Nest And Roost Sites

Very susceptible to disturbance at mixed ground colonies; where nesting with gulls, cormorants are first to leave and last to return, providing ample opportunity for the gulls to eat regurgitated fish, the cormorants’ eggs, and newly hatched young, which they do readily, and often in that order (Kury and Gochfeld 1975, JJH). Hasty departures caused by sudden disturbances lead to eggs being tossed from nests because incubating parent holds eggs on its feet. However, the species is well adapted to such losses and relays readily and rapidly. The synergistic effects of disturbance by Bald Eagles and humans in enabling predation by crows on Mandarte I., British Columbia, were described by Verbeek (1982). Predation by Fish Crows (Corvus ossifragus) is facilitated by disturbance; may restrict coastal nesting in the Carolinas and favor nesting inland (Post 1988). Frequent human visits caused gull predation and nest abandonment, and discouraged settlement by late-nesting cormorants in Quebec (Ellison and Cleary 1978). Disturbance by people or Bald Eagles may lead to slow shrinking of a colony, or abrupt shift to new site.

Young cormorants are very sensitive to disturbance, especially during the first 2 wk of life, before they can thermoregulate (see Breeding: parental care, above). Flushing of adults from their nests at this time may lead to significant mortality of young because of exposure to sun (Weseloh et al. 1995). Small chicks may die in 11 min (mean 22.7 min ± 11.6, n = 38) from such exposure, at deep body temperature 45.7°C ± 1.7, but shorter times are sufficient to cause larger chicks to move and thus fall from nests. When exposed to cold, nestlings became comatose at 16–19°C but recovered upon warming; cooling to 11.5°C was fatal (Van Scheik 1985). In ground colonies, young older than about 21 d leave their nests and may enter the water; they are thought to return to nests later, but effects of such disturbance have not been examined systematically.

Nocturnal visits to arboreal roosts in Mississippi for purpose of capturing and radio-tagging appeared to have no effect on use of those sites by the birds (King et al. 1995). However, extended harassment at nocturnal roosts with pyrotechnics reduced both numbers of cormorants using roost and those feeding at nearby catfish farms (Mott et al. in press): use of pyrotechnics is now a recognized control method.

Direct Human/Research Impacts

See above.

Cormorants have long been viewed with antipathy, and there is a long but poorly documented history of persecution (Duffy 1995). Conclusions of early studies (e.g., Taverner 1915, Lewis 1929, Mendall 1936) were that cormorants have only small effects on openwater fisheries, and recent work has shown that measuring their impact is difficult and interpretations are disputed (see below). Increases in numbers since 1980 have led to enhanced conflicts on 3 main fronts: not only (1) perceived competition with fishermen in marine and freshwater areas, but also (2) depredations at commercial aquaculture facilities (primarily catfish) and (3) alteration of vegetation and nest trees, resulting in lowering of property values and impacts on other colonial waterbirds. Less attention has been given to complaints of fouling vessels or buildings, of eutrophication or lowering water quality, and of possible transmission of fish diseases or parasites.

Open Water Fish

Compelling evidence that cormorants seriously damage fisheries is rare. Diet alone is not a measure of impact. Frequently it can be shown that they eat too few fish of commercial or sporting interest to be a serious problem, but even where they do eat substantial numbers of such fish, it is difficult to demonstrate damage indisputably (Draulans 1987, Duffy and Schneider 1994; see Demography and populations: population regulation, above). To show that cormorant predation is additive rather than compensatory to other losses requires studies that are difficult and expensive; such studies have thus not been performed. In some ecosystems, increases in cormorants, feeding primarily on forage fish, may be a consequence of diminished stocks of predatory fish (the target of the fishery) rather than a cause of such declines (although such changes may also arise from changes at the other end of the food chain).

On Lake Ontario, annual consumption of fish by cormorants was calculated as 2.88, 1.64, and 0.90 × 106 kg, respectively, for 1991, 1993, and 1994 (Weseloh and Casselman 1992, Ross and Johnson 1995). Total forage base of fish was estimated as 418 × 106 kg, so cormorants consumed <1% of available fish, compared to 13.3% taken by salmonids (Weseloh and Casselman 1992). In Lake Erie, cormorants consumed <2% of biomass of forage fish taken by walleyes, the major commercial species, and ate fewer fish than did Red-breasted Mergansers (Mergus serrator), Herring Gulls, or Ring-billed Gulls (Madenjian and Gabrey 1995). Cormorants have only a small impact on yellow perch in Lake Huron (Belyea et al. in press). These and other studies conclude that cormorants usually have only small impacts on fish populations (see also Food habits: diet, above). However, such estimates may be at an inappropriate scale—lakewide rather than local; impacts may differ in small bays, for example (especially for sedentary species; Birt et al. 1987). Although smallmouth bass constitute <2% of total prey taken (see Food habits: diet, above), this amount may represent 21–35% of all bass available in Eastern Basin of Lake Ontario and would thus be a substantial impact on that fishery (Schneider et al. 1998).

In marine areas, perceived conflicts have been most severe in vicinity of rivers in e. Canada and New England where restoration of Atlantic salmon is ongoing (Krohn et al. 1995), and at other salmon rivers in the Northwest (Bayer 1989), but cormorants have been blamed for declines of other species also.

Cormorants respond rapidly to concentrations of fish or to fish made vulnerable by human activities. For example, on Penobscot River, ME, after release of hatchery-reared salmon smolts, cormorants forage disproportionately at the dams, where the naive fish are disoriented and easily caught (Blackwell and Krohn 1997). Spawning fish are similarly concentrated and vulnerable, which accounts for some of the seasonal patterns (see Food habits, above).

Aquaculture

By concentrating desirable prey in places where they can be easily caught, fish farms may attract many cormorants. In Florida, cormorants exploited shallow ponds teeming with easy-to-capture prey by becoming resident in the area and breeding nearby (Schramm et al. 1984). At catfish farms and other aquacultural ventures, depredations by cormorants are unevenly distributed for reasons that are not well understood. Although net production losses from these depredations have not been measured, wintering cormorants may take 4% of the standing crop of stocking-size catfish in the Delta region of central Mississippi (Glahn and Brugger 1995). Comprehensive studies that address all costs and benefits are needed (Jackson and Jackson 1995). Injuries to fish in pound-nets described by Craven and Lev (1987).

Terrestrial Habitats

Accumulation of cormorant droppings, which contribute excessive ammonium nitrogen, and stripping leaves for nest material may kill trees within 3–10 yr (Lemmon et al. 1994). Such damage may be perceived as a problem if these trees are rare species, or aesthetically valued. Adverse impacts on other breeding birds may result from alterations to vegetation (e.g., Black-crowned Night-Herons [Nycticorax nycticorax] and Common Eiders [Somateria mollissima] in St. Lawrence River; Bédard et al. in press), or occupation of limited nesting space (e.g., Common Terns [Sterna hirundo], Oneida Lake, NY). Positive impacts may occur to species that nest in open areas.

Conservation Status

From being a Species of Concern in 1970 because of low numbers in many jurisdictions (e.g., Meier 1981), numerical increases have led to widespread perception of Double-crested Cormorant as a pest (Hatch 1995, Nettleship and Duffy 1995; see also Demography and populations, above). The species was not included in the Migratory Bird Convention (in 1916) between U.S. and Canada, and consequently in Canada cormorants continue to be under jurisdiction of provinces (Keith 1995). The species has been protected under federal law in U.S. since 1972, when cormorants and other birds were added to list for U.S.–Mexican Convention (Trapp et al. 1995).

Measures Proposed And Taken

Shooting and destruction of nests, eggs, and young have long occurred (Lewis 1929, Bayer 1989). In some areas, such activities still continue on large scale (notably in Manitoba, where flamethrowers have been used in colonies; see Sheppard 1994/1995). In response to complaints from fishermen, large-scale egg-oiling projects, intended to reduce numbers, were implemented in New England (1944–1952; Krohn et al. 1995) and the Great Lakes (1948–1963; Ontario Ministry of Natural Resources unpubl.). In New England, eggs were sprayed with emulsions of oil and formalin. On West Coast, shooting and destruction of eggs and nests have been the main methods of reducing numbers (Bayer 1989).

In St. Lawrence River, alterations to forested island habitats by cormorant feces led to a 5-yr management plan, based on a population model, intended to reduce numbers from 17,000 to 10,000 nesting pairs (Bédard et al. 1995a). In ground colonies, eggs were sprayed with nontoxic mineral oil, and in tree colonies adults were shot, starting in 1989. Based on evidence for damage to the smallmouth bass fishery in e. Lake Ontario, control measures of oiling eggs and shooting adults are proposed for 1999–2000 by New York State Department of Environmental Conservation.

In recent years, U.S. Fish and Wildlife Service [USFWS] has issued increasing numbers of depredation permits for control of fish-eating birds at aquaculture facilities (Trapp 1998). In 1993–1994, 251 permits were issued, and 8,239 cormorants were reported taken (57% of all birds reported under such permits). In Mar 1998, USFWS established a depredation order that allows people engaged in commercial aquaculture to shoot cormorants without a federal permit at freshwater aquaculture premises or state-operated hatcheries (Trapp 1998). This order applies to 13 states where cormorant depredations have been recognized as potentially significant: primarily southern states with substantial production of channel catfish for human consumption or bait fish (Minnesota). This shooting may occur only in conjunction with a certified nonlethal harassment program, and the order requires that records be maintained of all cormorants killed. In other states, depredation permits are issued case by case.

To manage fish-eating birds at aquaculture ponds in s. U.S., Brugger (1995) suggested 5 options: frightening devices, aerial barriers, altering of aquacultural practices, changing of cormorant behavior, and reduction in cormorant numbers by shooting. Cormorants habituate to frightening devices, and nets and other barriers are deemed economically impractical at present, especially where ponds are large. Methods currently used near Mississippi catfish farms include harassment at feeding ponds with vehicles and other frightening devices (reinforced by shooting under permit), and harassment at nocturnal roosts using pyrotechnics (Mott et al. in press). Laser guns are a new technology that shows promise for use at roosts. Cormorants soon become wary and difficult to kill, but lethal control is thought to be essential for continued effectiveness of harassment. Modification of underwater habitat to impede predation by cormorants has received little attention.

Nonlethal methods for reducing impact of cormorants on newly stocked fish include delaying releases (e.g., until cormorants are nesting), releasing prey unpredictably, or at night, and making them more difficult to catch by altering environment or behavior of the fish.

To reduce the impact of human disturbance on cormorant colonies, Kury and Gochfeld (1975) recommended planning visits toward end of nesting period (when young are less vulnerable) and regulating the number and duration of visits because most predation by gulls occurs when human intruders leave colony rather than when they are present. In some studies, biologists checked nests and banded chicks after dark to avoid predation by gulls (Ainley and Boekelheide 1990, K. Stromberg pers. comm.). For timing visits to colonies, avoiding midday sun is an important general rule.

Effectiveness Of Measures

In New England, >180,000 eggs were sprayed (1944–1952), and an unknown number of nests were destroyed, but the programs were deemed ineffective at reducing the population because the birds re-layed or moved to new nest sites (Krohn et al. 1995); in retrospect, however, these efforts may have stabilized numbers (Drury 1973–1974). A less extensive program on Canadian Great Lakes (1948–1963) yielded similar results and conclusions (DVW). During the culling program in Quebec, 25,095 nests were sprayed and 7,917 adults shot. This program reduced population from >17,000 nests in 1989 to <10,000 nests in 1993, faster than predicted by the model (Bédard et al. in press). A possible basis for this faster drop in numbers is that males were more vulnerable to shooting. In Delta region of central Mississippi, where several strategies were attempted (Mott and Boyd 1995), modification of cormorant behavior through harassment at nocturnal roosts, making them relocate to other areas, appeared to have the greatest impact on reducing the number of cormorants visiting catfish ponds nearby (J. F. Glahn pers. comm.).