Cormorant Diet Research Articles

Spring Prey Use by Double-Crested Cormorants on the Penobscot River, Maine, USA

-We analyzed 2 sets of data for Double-crested Cormorant (Phalacrocorax auritus) stomach contents (including esophageal contents) thatwere collected from April throughJune of 1986-1988 (N = 580) and 1992-1993 (N 200) on the Penobscot River, Maine. Our objectives were to examine temporal and spatial variation in the spring diet and estimate the importance of Atlantic salmon (Salmo salar) smolts to the cormorant diet. We analyzed stomach contents relative to samples from 3 river sections: 5 mainstem dams collectively, above the head of tide, and free-flowing areas above and below the head of tide. Between years composition of taxa lists were compared (P = 0.05) relative to time and river section. We estimated taxon importance for data collected during 1992-1993 by ranking taxa according to 3 statistics: frequency of occurrence, mean percent volume, and numerical abundance. Data from 1986-88 were analyzed by frequency of occurrence only. Across the 3 river sections, the number of prey species recovered from cormorant stomachs increased from 15 in late April to at least 31 through May. Cormorants collected above the head of tide consumed 12 fish species (including freshwater, anadromous, and catadromous types), whereas birds collected below the head of tide consumed 28 freshwater and seasonally-available estuarine, marine benthic, and pelagic species. Salmon smolts were not recovered from stomachs collected in April, rare in stomach samples during the first week of June, and absent from the diet thereafter. In contrast, smolts were among the 5 most frequently occurring (1986-88) and highest ranking (1992-1993) prey taxa across the 3 river sections through May. Received 26 August 1996, accepted 5January 1997.

Sexual differences in the diet of king cormorants at Macquarie Island

Sexual differences in the diet of king cormorants,Phalacrocorax albiventer, were investigated during the 1993/1994 austral summer at Macquarie Island. The major food items, identified by otoliths in regurgitations, were demersal fish; fish mass consumed could be estimated using a wet mass-otolith length relationship. Two fish species,Paranotothenia magellanica andHarpagifer georgianus, constituted 98% of the wet mass (male and female cormorants combined). Estimated individual fish mass ofParanotothenia magellanica (19.6±11.6 g) was greater than that ofH. georgianus (2.8±1.3 g). Total wet mass of food and number of fish in regurgitations did not differ statistically between the sexes of cormorants. However, males tended to feed on larger fish than did females.

Pellet Analysis in the Assessment of Great Cormorant Phalacrocorax carbo Diet: Reducing Biases from Otolith Wear When Reconstructing Fish Length

-Fish otoliths recovered from pellets of piscivorous birds are often used to reconstruct size and mass of the ingested fish, but erosion of otoliths by gastric fluids usually results in underestimations. We tested the hypothesis that using only otoliths would greatly improve fish length estimations. In so doing, we compared size-frequency distributions of grayling (Thymallus thymallus) reconstructed from and eroded otoliths in pellets of Great Cormorants (Phalacrocorax carbo) with size-frequency distributions from a small sample of stomach contents and with samples from various types of fisheries, all from the Rhine River in Switzerland and Germany. Distributions reconstructed from eroded and otoliths were significantly different. Distributions from worn otoliths failed to delineate different cohorts and produced strong underestimates of fish lengths, thus not identifying larger grayling at all. Distributions reconstructed from otoliths classified as uneroded represented the whole range of fish sizes eaten and showed the different grayling cohorts, similar to the catch samples. Compared to these, however, average length-at-age in cohort 2 was slightly underestimated (ca. 6-9% in 32 cm long grayling). It is concluded that, at least in fish with relatively large and robust otoliths, careful exclusion of otoliths showing signs of erosion can remove much of the bias in indiscriminately reconstructed size-frequency distributions, thus greatly improving conclusions about the age or size composition of fish in the cormorant diet. Received 6 March 1996, accepted 2 September 1996.

Sexual differences in the diet of king cormorants at Macquarie Island

Sexual differences in the diet of king cormorants at Macquarie Island

Cormorants Phalacrocorax carbo as fish predators: An appraisal of their conservation and management in Great Britain

The breeding population of cormorants Phalacrocorax carbo has expanded from 6 400 in 1969–1970 to 7 200 pairs in 1985–1987 and has probably benefited from protection under the Wildlife and Countryside Act 1981. The available studies of cormorant diet show that they exploit a range of fish species according to locality and season but often concentrate on locally dominant species. No study in Britain (or Ireland) has accurately quantified cormorant losses to fisheries, and the significance of depredation by them therefore remains unclear. A wide variety of methods to protect individual sites against cormorants is available. Most remain untested in British situations, and many may be impractical at some large waters where cormorant predation is seen to be a problem. Shooting to reinforce scaring is regarded as a favourable option by the UK Government and is exercised mainly on Scottish river fisheries. Unlicensed control is probably significant in some localities.

Diet of cormorants, Phalacrocorax carbo L., from two fisheries in north‐west England

Diet of cormorants, <i>Phalacrocorax carbo</i> L., from two fisheries in north‐west England

Diets of shags Phalacrocorax aristotelis and cormorants P carbo in Norway and possible implications for gadoid stock recruitment

The diets of shags and cormorants were studied in Norway through analyses of regurgitated pellets. Although this method has many limitations, indications were that both species rely heavily on small gadoids (Gadidae) and sand eels (Ammodytidae) for food throughout their range, but also eat other fish species when available. There was considerable dietary overlap between species, despite a tendency for cormorants to eat larger fish and more benthic items than shags. Predation by shags and cormorants could be a factor limiting the recruitment of cod and saithe into what are now severely reduced, but commercially important stocks in the Norwegian and Barents Seas.

Food Habits and Organochlorine Contaminants in the Diet of Olivaceous Cormorants in Galveston Bay, Texas

Food Habits and Organochlorine Contaminants in the Diet of Olivaceous Cormorants in Galveston Bay, Texas

Population, Diet and Reproductive Success of Double-Crested Cormorants Breeding on Lake Winnipegosis, Manitoba, in 1987

Population, Diet and Reproductive Success of Double-Crested Cormorants Breeding on Lake Winnipegosis, Manitoba, in 1987

Spatial and Temporal Patterns of Diet in the Cape Cormorant off Southern Africa

Diets of Cape Cormorant (Phalacrocorux cupensis) were sampled from 198 1 to 1985, between Mercury (25”43’S) and Dyer (3441’S) islands of the Benguela upwelling ecosystem off Namibia and South Africa. Over 99% of identifiable prey items were pelagic fish: cape anchovy (Engraulis japonicus capensis), 46%; pelagic goby (Suflogobius bibarbatus), 34%; and maasbanker (Truchurus truchurus), 14%. Daily, monthly, and regional variation in diet was three to four times that of annual variation, suggesting that environmental variability in the Benguela ecosystem is greatest at time intervals shorter than 1 year and that caution is necessary when inferring diets of Cape Cormorants from limited sampling.

Feeding Ecology of Marine Cormorants in Southwestern North America

The diets of Pelagic, Brandt's and Double-crested cormorants (Phalacrocorax pelagicus, P. penicillatus and P. auritus) were compared. Data were based on 1,695 pellets and 71 chick regurgitations analyzed by us and information on 34 stomach samples published in the literature. A total of 19 sites between Kodiak, Alaska and central Mexico is represented. The three cormorants often fed in the same areas at the same time using the same technique. They exploited different microhabitats as defined by prey behavior; the prey species overlapped substantially between Brandt's and Pelagic cormorants, but those of the Double-crested Cormorant were quite different. Double-crested Cormorants fed on schooling fish usually occurring well above flat bottoms; Pelagic Cormorants fed on solitary prey on or concealed in rocky substrates; and Brandt's Cormorants fed on prey on or just above the bottom in rocky areas and in areas of flat sand or mud. The latter species fed over flat bottoms more in the northern part of their range than in the southern part where they fed almost exclusively in or near rocky habitat. Double-crested and Pelagic cormorants showed no geographic shift in their feeding habits.