Phalacrocorax Auritus Research Articles

Tethered Raptor Displaces Roosting Cormorants

Double-crested Cormorant (Phalacrocmax auritus) numbers in the Great Lakes of North America have increased dramatically during the past three decades. Current concerns include negative effects of cormorants on aquatic food webs, destruction of nesting habitat and competition with other avian species and odors due to feces. Control of cormorant nesting and roosting has been variably successful and sometimes involves lethal force and/or loud devices. These approaches may be unsuitable in some settings, such as parks or residential areas. A large group of cormorants (up to 2,000 individuals) has used Farr Island, a small island near houses on the North Shore of Hamilton Harbour at the west end of Lake Ontario, Canada, as a post-breeding roosting area. The concentration of birds has resulted in complaints regarding odors from the island. In an attempt to alleviate the strong odor and to examine options to limit cormorant nest site competition with other species, a tethered Steppe Eagle (Aquila nipalensis) was placed on the roost island for twelve days from 9 to 21 September 2003. The eagle was effective at causing roosting cormorants to avoid the island, apparently causing many to move their roost site a short distance to another island nearby. Complete cormorant displacement lasted at least four days beyond the period of eagle presence on the island. Using a tethered large raptor appears to be an effective non-lethal tool for local displacement of roosting Double-crested Cormorants.

Susceptibility of Juvenile Steelhead to Avian Predation: the Influence of Individual Fish Characteristics and River Conditions

AbstractIdentification of the factors that influence susceptibility to predation can aid in developing management strategies to recover fish populations of conservation concern. Predator–prey relationships can be influenced by numerous factors, including prey condition, prey size, and environmental conditions. We investigated these factors by using juvenile steelhead Oncorhynchus mykiss from the Snake River (Pacific Northwest, USA), a distinct population segment that is listed as threatened under the U.S. Endangered Species Act. During 2007–2009, steelhead smolts (n = 25,909) were captured, examined for external condition characteristics (e.g., body injuries, descaling, external signs of disease, fin damage, and ectoparasite infestations), marked with passive integrated transponder (PIT) tags, and released to continue their out‐migration. Recoveries of PIT tags on a downstream colony of Caspian terns Hydroprogne caspia (n = 913 tags) indicated that steelhead susceptibility to Caspian tern predation increased significantly with decreases in steelhead external condition, decreased water discharge, and decreased water clarity. Susceptibility to Caspian tern predation also increased with increasing steelhead fork length up to 202 mm but then decreased for longer steelhead. Recoveries of PIT tags on a downstream colony of double‐crested cormorants Phalacrocorax auritus (n = 493 tags) indicated that steelhead susceptibility to double‐crested cormorant predation increased significantly with declining external condition of steelhead, and that steelhead of hatchery origin were more susceptible than their wild counterparts. Results indicate that steelhead susceptibility to avian predation is dependent on fish condition and length and is influenced by river conditions and rearing environment.

The double-crested cormorant: symbol of ecological conflict

The double-crested cormorant: symbol of ecological conflict

Economic Impact of Double‐Crested Cormorant, Phalacrocorax auritus, Depredation on Channel Catfish, Ictalurus punctatus, Aquaculture in Mississippi, USA

The Yazoo River Basin of Mississippi, USA, supports the largest concentration of hectares devoted to channel catfish, Ictalurus punctatus, aquaculture production in North America. The Yazoo Basin also supports large numbers of resident, wintering and migrating fish‐eating birds, with the Double‐crested Cormorant, Phalacrocorax auritus, implicated as the most serious depredating species. We used data from aerial surveys of numbers and distribution of cormorants in the Yazoo Basin and on commercial catfish ponds during winters (November–April) 2000–2001 and 2003–2004 to refine estimates of regional economic losses due to cormorant depredation. In both periods, the greatest monthly estimates of cormorant foraging occurred from 1 January to 31 March. Losses in terms of biomass, number, and dollar value were greater for foodfish ponds than fingerling ponds. Monthly weighted estimates of catfish consumed were 1775.3 and 1346.6 m.t. over winters 2000–2001 and 2003–2004, respectively. Total estimated losses for foodfish and fingerling ponds in 2000–2001 were $11.56 and $0.48 million, respectively, and in 2003–2004 were $5.22 and $0.40 million, respectively. Maximum dollar loss occurred during March in 2000–2001 and during February in 2003–2004. In this study, the volatility in variable production costs and nominal sales price, and distribution of cormorants on pond types and regionally were key factors in resulting economic loss estimates.

Relative demand by double-crested cormorants and anglers for fish production from lakes on Manitoulin Island, Lake Huron

The magnitude of angler harvest (kg·ha−1·yr−1) and cormorant consumption (kg·ha−1·yr−1) were compared for a set of lakes (N=11) on Manitoulin Island. Empirical models relating total phosphorus to total fish production as well as production to body mass were used to scope the possible range of fish production and to partition production among small, medium and large size segments of fish populations, respectively. Medium (66–112g) and large (>200g) size segments were defined as size categories targeted by cormorants (stomach diet analysis) and anglers (creel interviews), respectively. Angling effort and cormorant density were estimated from aerial surveys of the lake set during the open water season and for anglers during the winter ice-fishing season. Results showed that anglers harvested almost all large fish production, assuming the mean total fish production model, and 43% of large fish production under the more optimistic upper 80% prediction limit of total fish production. Cormorant consumption of medium fish production was less (39% using mean regression model; 15% using upper 80% prediction model) than angler consumption of large fish production. Anglers therefore imposed more population stress on their preferred sizes of fish than cormorants imposed on their preferred sizes of fish. Population stress was increased when cormorant consumption of medium size fish was discounted from contributing to large fish production. Angler harvest near (or above) sustainable yield levels will be exacerbated and appear as a fish collapse when cormorants consume fish production destined for fish size segments preferred by anglers.

Gastrointestinal Helminth Parasites of Double-Crested Cormorants (Phalacrocorax auritus) at Four Sites in Saskatchewan, Canada

Abstract We examined the gastrointestinal parasite fauna of adult double-crested cormorants from breeding colonies on 4 very different lakes spanning a major ecotone from prairie to boreal forest in Saskatchewan, Canada. Our objectives were to document regional parasite fauna and identify potential differences based on breeding colony location. Eight species of helminths were identified from the stomach and small intestinal contents of 92 cormorants collected during the summers of 2006 and 2007: 2 nematodes—Syncuaria squamata and Contracaecum cf. rudolphii; 2 trematodes—Ribeiroia ondatrae and Drepanocephalus spathans; and 4 cestodes—Ligula colymbi, Schistocephalus solidus, Paradilepis caballeroi, and Paradilepis sp. Our results include an apparently undescribed species of Paradilepis, new host records for L. colymbi, S. solidus, and R. ondatrae, and new geographic records for S. squamata, D. spathans, and P. caballeroi in cormorants of western Canada. Anisakids, C. cf. rudolphii, were present in total sto...

Colony connectivity of Pacific Coast double‐crested cormorants based on post‐breeding dispersal from the region's largest colony

AbstractTo reduce conflicts with fish resources, other colonial waterbirds, and damage to habitats, double‐crested cormorants (Phalacrocorax auritus) are currently controlled (lethally and non‐lethally) throughout much of their range. Concerns are growing over the Pacific Coast's largest double‐crested cormorant colony at East Sand Island (ESI), Oregon near the mouth of the Columbia River, where cormorants forage on juvenile salmonids, many of which are listed under the United States Endangered Species Act. Management of this colony is currently under consideration and may call for a redistribution of a portion of this colony numbering more than 12,000 breeding pairs in 2009. We investigated regional and site‐specific connectivity of ESI cormorants using satellite‐telemetry to track post‐breeding dispersal. Cormorants dispersed widely west of the Cascade‐Sierra Nevada Mountains from British Columbia, Canada to northern Mexico. Tracking data demonstrated direct connectivity between the double‐crested cormorant colony at ESI and nesting sites throughout the dispersal area. Results of this study indicate that some cormorants from ESI could disperse to prospect for nesting sites throughout much of the western portion of the range of the Western Population; however, regional variation in connectivity with the ESI population, distance from ESI, and site‐specific nesting history will likely result in variable prospecting rates among regions and sub‐regions. Management efforts aimed at redistributing ESI cormorants across western North America (e.g., social attraction or dissuasion techniques) might be best allocated to areas or sites known to be used by tagged cormorants, particularly those sites with an established nesting history. © 2012 The Wildlife Society.

Systemwide Evaluation of Avian Predation on Juvenile Salmonids from the Columbia River Based on Recoveries of Passive Integrated Transponder Tags

AbstractWe recovered passive integrated transponder (PIT) tags from nine piscivorous waterbird colonies in the Columbia River basin to evaluate avian predation on Endangered Species Act (ESA)‐listed salmonid Oncorhynchus spp. populations during 2007–2010. Avian predation rates were calculated based on the percentage of PIT‐tagged juvenile salmonids that were detected as passing hydroelectric dams and subsequently were consumed and deposited by birds on their nesting colonies. Caspian terns Hydroprogne caspia (hereafter, “terns”) and double‐crested cormorants Phalacrocorax auritus (hereafter, “cormorants”) nesting on East Sand Island in the Columbia River estuary consumed the highest proportions of available PIT‐tagged salmonids, with minimum predation rates ranging from 2.5% for Willamette River spring Chinook salmon O. tshawytscha to 16.0% for Snake River steelhead O. mykiss. Estimated predation rates by terns, cormorants, gulls of two species (California gull Larus californicus and ring‐billed gull L. delawarensis), and American white pelicans Pelecanus erythrorhynchos nesting near the confluence of the Snake and Columbia rivers were also substantial; minimum predation rates ranged from 1.4% for Snake River fall Chinook salmon to 13.2% for upper Columbia River steelhead. Predation on ESA‐listed salmonids by gulls and American white pelicans were minor (<2.0% per ESA‐listed salmonid population) relative to predation by terns and cormorants. Cumulative impacts were greater for Snake River and upper Columbia River salmonids than for salmonids originating closer to the estuary because upriver salmonids must migrate past more bird colonies to reach the ocean. Predation rates adjusted for colony size (per capita rates) were significantly higher for terns and cormorants nesting at inland colonies (upstream of Bonneville Dam) than for those nesting in the estuary, suggesting that inland colonies have a greater reliance on salmonids as a food source. Management actions to increase salmonid survival by reducing avian predation in the estuary could be offset if birds that disperse from the estuary relocate to inland nesting sites on or near the Columbia River.

Double-Crested Cormorant (Phalacrocorax auritus) Nesting Effects on Understory Composition and Diversity on Island Ecosystems in Lake Erie

The context for this study is the management concerns over the severity and extent of the impact of cormorants on island flora in the recent past on Lake Erie islands. Accordingly, this study sought to quantify the nesting colonies' influence on coarse woody litter and how nest densities and litter depth may influence the herbaceous layer, the seed bank composition and viability across the extent of three Lake Erie islands. The data for this study were collected from 2004 to 2008 on East Sister Island and Middle Island using two main strategies. First, herbaceous layer surveys, cormorant nest counts, soil seed bank cores, and litter depth measurements were executed using a plotless-point quarter method to test island-wide impacts from nesting activities (data were also collected on a third island, West Sister Island as a reference for the other two islands). Secondly, a sub-sample of the entire plot set was examined in particularly high nesting density areas for two islands (Middle Island and East Sister Island). Kruskal-Wallis tests indicated that there are subtle changes in the herbaceous diversity (total, native and exotic) and seed bank composition across the islands. The sub sample set of the plots demonstrated that Phalacrocorax auritus nest density does influence litter depth, herbaceous species abundance and diversity. Cormorant nesting pressures are restricted to areas of high nesting pressures and competition. However, there remains a risk to the interior herbaceous layer of the island if the effects of nesting pressures at the edges advance inward from this perimeter.

Towards optimized population control efficiency in space and time: A modelling framework adapted to a colonial waterbird

The double-crested cormorant is a native North American waterbird that recently underwent a dramatic population expansion. Population control efforts in the USA and Canada attempt to mitigate cormorant damages to natural resources and aquaculture. However, there is currently no coordination among the various stakeholders involved in management activities as well as no attempt to optimize population control efficiency. In this paper, we present for the first time a spatially explicit stage-structured metapopulation model parameterized for the cormorant. We developed simulation tools to get insights into the efficiency gain that can be expected from a better planning of management activities in both space and time. A case study is presented, in which we randomized where (on which colonies) and when (which years) a pre-determined amount of management activities would occur on 4 or 8 of 16 active colonies arranged on a 4×4 or 2×8 spatial grid over a period of 8 years, including 2 or 4 management years. We calculated two indices measuring the location of management activities, namely the average date of management years and the average degree of peripherality of colonies undertaking management, together with two indices measuring the resulting correlation of management activities, i.e., the degree of clustering of management activities both in space and time. Different spatio-temporal configurations of management activities generally yielded different metapopulation trajectories. Room for improving management efficiency increased with the intensity of management activities. However, the greatest efficiency gains are to be expected when colonies are far from carrying capacity, while the majority of management operations are undertaken when colonies are near or at carrying capacity. Locations of management activities in space and time appeared more important than resulting spatio-temporal correlations to explain the dispersion of metapopulation trajectories. When colonies were far from their individual carrying capacity, management was more efficient when applied earlier (a consequence of delayed reproductive maturity) and to more central colonies (due to greater immigration). The situation was more complex when colonies were closer to or at carrying capacity. Our modelling framework is flexible enough to allow more complex scenarios to be investigated in the future.

Genetic Sequence Data Identifies the Cercaria of Drepanocephalus spathans (Digenea: Echinostomatidae), A Parasite of the Double-Crested Cormorant (Phalacrocorax auritus), with Notes on its Pathology in Juvenile Channel Catfish (Ictalurus punctatus)

An unidentified xiphidio-type cercaria, previously thought inconsequential to catfish health, was found to be released from marsh rams-horn snails (Planorbella trivolvis) inhabiting ponds on a commercial catfish operation in the Mississippi Delta. A preliminary challenge of cohabiting channel catfish ( Ictalurus punctatus ) with snails actively shedding the unidentified cercariae resulted in death of some fish. A second cohabitation trial yielded similar results, as did a third challenge of 250 cercariae/fish. Histopathology revealed developing metacercariae concentrated in the cranial region, especially within the branchial chamber, with several metacercariae at the base of the branchial arches within, or adjacent to, blood vessels, possibly the proximate cause of death. Genetic sequence analysis of the 18S small subunit ribosomal DNA (ssDNA), 28S large subunit rDNA (lsDNA), and cytochrome oxidase (Cox1) genes all matched the cercariae to Drepanocephalus spathans (Digenea: Echinostomatidae), a parasite of the double-crested cormorant (Phalacrocorax auritus), a piscivorous bird endemic on most catfish farms. This is the first commentary regarding pathology of D. spathans in juvenile channel catfish as well as the first report of the marsh rams-horn snail as an intermediate host in the D. spathans life cycle. The data presented here suggest this parasite could have limiting effects on catfish production, further supporting the need for adequate snail control programs to reduce trematode prevalence on commercial catfish operations.

Connecting Breeding and Wintering Habitats of Migratory Piscivorous Birds: Implications for Tracking Contaminants (Hg) Using Multiple Stable Isotopes

Contaminant concentrations in migratory species are complicated by differential accumulation and elimination among geographically separated biomes. Double-crested cormorants ( Phalacrocorax auritus ) are used as monitors of Hg in North America; however, migration from northern breeding colonies to southern marine, freshwater, and aquaculture systems exposes individuals to spatiotemporal variations in contaminant uptake. We used stable isotopes (δ(34)S, δ(13)C, δ(15)N, δ(2)H) in primary feathers and a combined Bayesian assignment and isotopic threshold model to identify feather origins and the potential winter use of aquaculture (δ(34)S < 10‰, δ(13)C > -14‰), freshwater (δ(34)S < 10‰, δ(13)C < -20‰), and marine habitats (δ(34)S > 10‰). This approach allowed us to contrast body burden Hg derived from the breeding and wintering grounds, as well as from marine and freshwater habitats. We found feathers grown on Lake Winnipeg had greater Hg concentrations (mean = 4.26 ± 1.47 μg/g fresh weight; n = 20) than winter-grown feathers (3.19 ± 1.64 μg/g; n = 19), but Hg in winter-grown feathers was not related to any specific habitat. Isotopic assays of tissues of migratory birds allowed the source and degree of contaminant exposure to be identified throughout the annual cycle.

Fish Assemblage Shifts and Population Dynamics of Smallmouth Bass in the Beaver Archipelago, Northern Lake Michigan: A Comparison between Historical and Recent Time Periods amidst Ecosystem Changes

AbstractThe ecological and economic importance of Great Lakes nearshore areas and the paucity of information on nearshore Lake Michigan fish assemblages prompted us to document changes that occurred from a historical time period (1969–1972, 1975, 1977, and 1984) to a recent period (2005–2008) in a nearshore northern Lake Michigan (Beaver Archipelago) fish assemblage, with an emphasis on smallmouth bass Micropterus dolomieu. From historical to recent periods, the Beaver Archipelago fish assemblage shifted from predominantly brown bullheads Ameiurus nebulosus to predominantly smallmouth bass. Relative abundance of brown bullheads and white suckers Catostomus commersonii declined from historical to recent time periods, as did overall species richness. The relative abundance, recruitment variability, and mortality rates of smallmouth bass have not significantly changed since the historical time period, whereas both condition (ages 5–7) and growth (ages 2–7) of this species have significantly increased. Our results suggest that the smallmouth bass population in the Beaver Archipelago area has not been negatively affected by recent ecological changes (i.e., declining primary productivity, increasing benthic invertebrate densities, increasing numbers of double‐crested cormorants Phalacrocorax auritus, and increasing introductions of nonnative species). The smallmouth bass is currently the dominant nearshore species and remains a critical component of the nearshore fish assemblage in northern Lake Michigan.

Bone Laminarity in the Avian Forelimb Skeleton and Its Relationship to Flight Mode: Testing Functional Interpretations

Wing bone histology in three species of birds was characterized in order to test hypotheses related to the relationship between skeletal microstructure and inferred wing loading during flight. Data on the degree of laminarity (the proportion of circular vascular canals) and the occurrence of secondary osteons were obtained from three species that utilize different primary flight modes: the Double-crested cormorant, a continuous flapper; the Brown pelican, a static soarer; and the Laysan albatross, a dynamic soarer. Laminarity indices were calculated for four quadrants for each of the three main wing elements. Ulnae and carpometacarpi were predicted to exhibit quadrant specific patterns of laminarity due to hypothesized differences in locally applied loads related to the attachment of flight feathers. However, few differences among the quadrants were identified. No significant differences were identified among the three elements, which is notable as different bones are likely experiencing different loading conditions. These results do not support the concept of bone functional adaptation in the primary structure of the wing elements. Significant differences in laminarity were found among the three primary flight modes. The dynamic soaring birds exhibited significantly lower laminarity than the flapping and static soaring birds. These results support the proposed hypothesis that laminarity is an adaptation for resisting torsional loading. This may be explained by overall wing shape: whereas dynamic soaring birds have long slender wings, flappers and static soaring birds have broader wings with a larger wing chord that would necessarily impart a higher torsional moment on the feather-bearing bones.

Double‐crested cormorant distribution on catfish aquaculture in the Yazoo River Basin of Mississippi

AbstractEstimating the catfish aquaculture production losses that can be attributed to double‐crested cormorants (Phalacrocorax auritus) has proved problematic because knowledge of the distribution of cormorants on catfish aquaculture is lacking. We evaluated use versus availability of various production pond types and landscape‐scale factors affecting the distribution of cormorants on channel catfish (Ictalurus punctatus) aquaculture facilities in Mississippi, USA. Cormorant distribution on aquaculture pond types indicated selection against brood‐fish ponds, neutral selection on fingerling ponds, and selection for food‐fish ponds (n = 29, Chesson's α = 0.19, 0.36, and 0.45, respectively). Modeled and validated correct classification rate (CCR) of general linear mixed models of cormorant occupancy of clusters of catfish ponds indicated seasonality of use and roost distance from aquaculture ponds was predictive (CCR = 81% and 71%, respectively). Modeled and validated ordinal models of levels of use (low, moderate, high) were less predictive (CCR = 67% and 59%, respectively). However, predictability within use levels for the validation data set was mixed, ranging from 0.19 to 0.86. Odds ratios indicate both higher risk of occupancy and levels of use over the period February–April relative to October. Management efforts targeted during this time frame will have the greatest impact in reducing depredation losses. The majority of cormorants occurred on food‐fish ponds. Consequently estimates of economic loss should be revisited and refined based on distributional information provided in this study. Published 2012. This article is a U.S. Government work and is in the public domain in the USA.

Perfluoroalkyl compounds (PFCs) in wildlife from an urban estuary

Previous research has documented the bioaccumulation of perfluoroalkyl compounds (PFCs) in apex predators in remote locations but few studies have evaluated urban estuaries. To assess the importance of PFCs in San Francisco Bay, two apex predators in the San Francisco Bay, double-crested cormorants (Phalacrocorax auritus) and Pacific harbor seals (Phoca vitulina richardii), were sampled. Prey fish (Atherinops affinis and Menidia audens) were also evaluated to better understand potential sources of PFCs to the foodweb. Perfluorooctane sulfonate (PFOS) was the primary PFC detected in cormorant eggs, small fish and harbor seal serum. PFOS detected in San Francisco Bay seal serum was typically an order of magnitude higher than those at the reference site. PFOS concentrations were highest in seals and cormorant eggs from the highly urbanized southern portion of the Bay. PFOS in eggs from the southern part of the Bay remained relatively constant between 2006 and 2009 despite the phase-out of perfluorosulfonyl-based compounds nationally. In addition, these levels exceed the avian predicted no effects concentration of 1.0 μg mL(-1). Concentrations of the remaining PFCs measured were substantially lower than those of PFOS.

Effects of egg oiling on ground-nesting double-crested cormorants at a colony in Lake Ontario: an examination of nest-attendance behaviour

Context We assessed the effects of egg oiling on ground-nesting double-crested cormorants (Phalacrocorax auritus) in the context of an emerging management strategy for the largest known cormorant colony on the lower Great Lakes. We designed the present study to answer specific questions in response to concerns raised by stakeholders and members of the public regarding this management technique. Aims The aim of the present study was to examine the behavioural response of adult cormorants to egg oiling. Prior work on this issue has focussed on population-response questions rather than the behavioural level. Consequently, detailed observations on how cormorants respond to egg oiling are lacking. Methods Using instantaneous and focal observations to measure behaviours, we compared Treatment nests (n = 24, 23) to Control (n = 24) and Sham (handled, but not treated; n = 24) nests. We observed nest attendance, incubation and mate-presence behaviour, and divided observations into pre-chick and entire-season categories for analysis. Key results Our study determined that egg oiling does not cause immediate nest desertion by adult birds; Treatment birds incubated their nests as long as did Sham and Control birds. We found no difference among the three groups in the proportion of time a mate was present during incubation for Control and Sham nests in the pre-chick period. We found that the total seasonal duration of nest attendance by Treatment birds was shorter than that for the birds in the other groups. Conclusions Both incubation and mate-presence data suggest that egg oiling did not measurably affect the behaviour of adult cormorants in the pre-chick period. Our study also suggests that Treatment birds attended their nest long enough to preclude re-nesting within the breeding season, although this may not apply for regions with longer nesting seasons. Implications Our study indicates that egg oiling, administered judiciously, may be an appropriate management technique for ground-nesting cormorants, although management targets must be clearly articulated.

Energetic considerations for managing double-crested cormorants on Lake Champlain

We studied foraging distribution, activity budgets, fish consumption, and energetics of double-crested cormorants (Phalacrocorax auritus) at two breeding colonies on Lake Champlain. Our objective was to determine if fish consumption and distribution of predation changed with movements of cormorants associated with efforts to reduce numbers of cormorants on one of the colonies. Wildlife managers reduced populations of cormorants on Young Island, Vermont by oiling their eggs, which resulted in dispersal of breeding cormorants 35km south to Four Brothers Islands, New York. We found that as cormorants shifted from Young Island to the colony on Four Brothers Islands, energy demands, foraging distribution, and total fish consumption increased. Birds on Four Brothers Islands foraged a greater distance from the colony compared to birds on Young Island. Additionally, consumption of yellow perch (Perca flavescens) shifted to rainbow smelt (Osmerus mordax) when more birds bred on Four Brothers Islands. The dispersal of cormorants from Young Island to Four Brothers Islands reduced predation on yellow perch but increased overall fish consumption. Our estimates of fish consumption ranged from 322,000–425,000kg of fish per year at Young Island to 899,000–1,086,000kg of fish per year at Four Brothers Islands. Results from this study demonstrate secondary impacts of management on Young Island to unmanaged areas.

Cormorant–fisheries conflicts: Stable isotopes reveal a consistent niche for avian piscivores in diverse food webs

Global conflict between piscivorous cormorants (Phalacrocorax spp.) and fish harvesters is one of the most widespread wildlife management issues in history. Despite the persistent belief that these birds adversely affect economically important fish populations, relatively little is known about cormorant trophic ecology and habitat use. We examined the diet and trophic position of breeding populations of Double-crested Cormorants (Phalacrocorax auritus; hereafter cormorants) from three different lakes using stable carbon and nitrogen isotopes (δ13C and δ15N). The δ15N values revealed that cormorants generally occupied top-predator positions in all food webs examined; their trophic position (5.0 ± 0.5) was consistent within and between years in all lakes studied. The δ13C values showed that cormorants occupied very similar dietary niches in three different freshwater food webs. Mean centroid distance, a measure of diversity, varied significantly among food webs (range 1.5–2.9), but not among cormorants from different lakes (range 0.8–1.2). In all three lakes, cormorants relied heavily on non-benthic or pelagic prey. Stable isotopes mixing models demonstrated that cisco (Coregonus artedii), yellow perch (Perca flavescens), and ninespine stickleback (Pungitius pungitius) were most important for cormorant diet independent of the local food web structure. The isotopic values of cormorants and pelagic predatory fish were sometimes similar, suggesting that dietary overlap is possible. Based on our study, we suggest that cormorants may have more specific and uniform dietary niche requirements than previously considered. Consequently, the potential for adverse effects on food webs will heavily depend on whether economically valuable species fill the prey niches consumed by cormorants. For example, cormorants should have little economic impact where food webs are diverse in abundant prey species and niches. Alternately, food webs with less diversity of prey species and niches may be more affected by cormorant predation. Managers should avoid simply assuming that cormorants will have negative impacts on fisheries and should instead consider the structure of the food web as well as the niches occupied by cormorants and fish species of economic interest.

A Comparison of the Teratogenicity of Methylmercury and Selenomethionine Injected Into Bird Eggs

Methylmercury chloride and seleno-L-methionine were injected separately or in combinations into the fertile eggs of mallards (Anas platyrhynchos), chickens (Gallus gallus), and double-crested cormorants (Phalacrocorax auritus), and the incidence and types of teratogenic effects were recorded. For all three species, selenomethionine alone caused more deformities than did methylmercury alone. When mallard eggs were injected with the lowest dose of selenium (Se) alone (0.1 μg/g), 28 of 44 embryos and hatchlings were deformed, whereas when eggs were injected with the lowest dose of mercury (Hg) alone (0.2 μg/g), only 1 of 56 embryos or hatchlings was deformed. Mallard embryos seemed to be more sensitive to the teratogenic effects of Se than chicken embryos: 0 of 15 chicken embryos or hatchlings from eggs injected with 0.1 μg/g Se exhibited deformities. Sample sizes were small with double-crested cormorant eggs, but they also seemed to be less sensitive to the teratogenic effects of Se than mallard eggs. There were no obvious differences among species regarding Hg-induced deformities. Overall, few interactions were apparent between methylmercury and selenomethionine with respect to the types of deformities observed. However, the deformities spina bifida and craniorachischisis were observed only when Hg and Se were injected in combination. One paradoxical finding was that some doses of methylmercury seemed to counteract the negative effect selenomethionine had on hatching of eggs while at the same time enhancing the negative effect selenomethionine had on creating deformities. When either methylmercury or selenomethionine is injected into avian eggs, deformities start to occur at much lower concentrations than when the Hg or Se is deposited naturally in the egg by the mother.