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Humans and colonial piscivorous birds are often perceived to be in conflict over shared aquatic habitats and fisheries resources in inland lakes. We examined angler perception of birds and the relative abundance of American white pelicans (Pelecanus erythrorhynchos), double-crested cormorants (Phalacrocorax auritus), western grebes (Aechmophorus occidentalis), and boats on two lakes in Saskatchewan, Canada. Anglers perceived cormorants to be the biggest threat to fisheries (60%), compared to pelicans (47%), and western grebes (34%). The density of these birds and boats varied significantly between sections of the two study lakes. Boat density was higher in developed sections with shoreline communities (range 0-7/km2) compared to those surrounded by agricultural land or native prairie (0-1/km2). In contrast, cormorant and pelican densities were highest in areas with an undeveloped shoreline (0-22/km2), and were reduced to near zero in developed sections. Western grebes did not follow the same pattern as the other two species; grebe density was generally more uniform within lakes (0-23/km2 in all sections). Boat density was a negative predictor of pelican and cormorant density on one lake, but was a positive predictor for grebes on both lakes. Our results indicate that pelicans and cormorants avoid sections of lakes that have higher levels of human development, potentially altering the location of their foraging sites on the scale of kilometres. In contrast, western grebes were abundant in all areas of the two lakes and did not appear to avoid human development or activity. We conclude that angler perceptions are not congruent with levels of habitat use overlap with birds. In addition, western grebe responses to human activities appear counterintuitive, making interpretations difficult in a conservation context; further study is required.

Habitat conversion is among the most important causes of environmental change worldwide, yet relatively little is known about its potential influence on trophic interactions. We investigated the effects of agricultural land use on carbon and nitrogen stable isotope values, trophic status, population density, and body condition of deer mice ( Peromyscus maniculatus (Wagner, 1845)) in a grassland ecosystem. Muscle δ15N (cropland = 7.6‰ ± 1.3‰; hay fields = 7.9‰ ± 1.3‰; native prairie = 7.2‰ ± 2.1‰) from deer mice did not vary with land use despite baseline soil and vegetation δ15N differences. Enrichment of deer mice over vegetation (Δδ15N) was, on average, a full trophic level (~2.5‰) higher on native prairie (6.4‰ ± 1.6‰) than on cropland (3.9‰ ± 2.3‰), and intermediate in hay fields (5.9‰ ± 2.0‰). Relative density of deer mice was more than twofold higher in crop and hay fields compared with native prairie, but body condition did not vary with land use. Our results suggest that agricultural activity caused a shift in the trophic level and relative abundance of a generalist grassland omnivore. Soil and vegetation δ15N reflected anthropogenic N inputs to agricultural fields but were not useful as general markers of habitat use in this study.

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 ( Phalacro- corax auritus; hereafter cormorants) from three different lakes using stable carbon and nitrogen isotopes (5l3C and 615N). The 815N 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 513C 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 isotopie 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.

Increasing populations of Double-crested Cormorants (Phalacrocorax auritus; hereafter cormorants) are of concern regarding the displacement of other species from breeding colonies, but little is known about such interactions. Agonistic interactions between ground-nesting cormorants and American White Pelicans (Pelecanus erythrorhynchos; hereafter pelicans) were investigated and compared to those involving cormorants and Herring Gulls (Larus argentatus). Also, a survey of cormorant eggs in nests of other ground-nesting species was conducted. Of 158 agonistic interactions between cormorants and pelicans, 83% were threat displays, 9% attempts to steal nest material and 8% involved some form of physical contact. Pelicans defended their nests less frequently and engaged in fewer physical contacts (8%) in colonies mixed with cormorants compared to areas with only pelicans (33%). Herring Gulls engaged in more physical contact (26%) with cormorants in a mixed colony than when nesting only with other gul...

Cormorants are desirable subjects for food web studies using stable isotopes (C and N) because of global fisheries conflicts, but no validated lipid-normalization procedures are currently available for any cormorant species. Accordingly, the effects of chloroform-methanol and petroleum-ether lipid extractions and three published lipid-normalization models on stable C and N isotope signatures in Double-crested Cormorant (Phalacrocorax auritus) muscle and liver tissues were investigated. The presence of lipids in cormorant muscle and liver decreased δ13C values by approximately 1–2‰, more so than has been reported in other birds. Cormorants showed large variation in the relationship between the C:N ratio of bulk tissue and the change in δ13C values after lipid extraction, violating a major assumption of published lipid-normalization models. Despite this violation, two of the three tested models performed reasonably well for correcting δ13C values. The circumstances under which these models might f...

Fisheries for yellow perch Perca flavescens are economically important in North America, but in many lakes this species does not attain sizes desirable to anglers (total length [TL] > 250 mm). We investigated factors potentially contributing to the lack of angling-sized yellow perch in Dore Lake, Saskatchewan. This large boreal lake has a long history of exploitation and is subject to current controversy regarding the impact of double-crested cormorants Phalacrocorax auritus on its fisheries. We found that yellow perch in Dore Lake attained greater maximum ages (mean ± SD = 7.4 ± 1.0 years) but smaller maximum sizes (215 ± 7 mm TL) than other yellow perch populations. Thus, the lack of angling-sized yellow perch in Dore Lake is probably the result of slow growth rates rather than short life spans. Double-crested cormorants preyed heavily on yellow perch, but predation was highly skewed toward much smaller sizes (

Colonial birds must derive benefits from living in conspecific groups and with other species. One possible benefit is that they follow previously successful individuals to foraging sites (information center hypothesis). To test for evidence of intra- and interspecific information transfer, we assessed the group size and composition of flights of double- crested cormorants (Phalacrocorax auritus) and American white pelicans (Pelecanus erythrorhynchos) at two mixed colonies. Flights by solo individuals were the most common outbound type, suggesting that the birds do not immediately follow others. However, the majority of the total number of outbound birds traveled in groups. In comparison, groups in- bound to colonies and flying between feeding locations were large. Regardless of flight direction, groups were almost al- ways largest during the late chick-rearing period. This suggests that groups may have some function, although likely not to share information about foraging-sites. Mixed-species groups in general were rare, so it is unlikely that these birds commonly locate prey by following other species in flight.

Mercury and selenium concentrations were measured in double-crested cormorants (Phalacrocorax auritus), piscivorous fish, and common prey items in five lakes in two ecoregions in Saskatchewan, Canada. Hg and Se concentrations in cormorants were within the natural ranges of birds living in un-impacted sites. Site explained a significant proportion of the variation in total Hg (THg) and methylmercury (MeHg) concentrations in both cormorant breast muscle and livers. Birds nesting on more northern lakes in the Boreal Plain ecoregion (THg range 0.11-1.06 and 0.26-9.27 μg g(-1) wet weight, for breast and liver respectively) had lower THg concentrations compared to those from lakes in the Prairie ecoregion (THg range 0.60-4.26 μg g(-1) ww and 1.59-25.11 μg g(-1), for breast and liver respectively). Concentrations of MeHg in livers was also lower in birds from northern sites (0.06-1.15 μg g(-1) ww) compared to those from prairie sites (0.22-4.06 μg g(-1) ww). We documented a wide range of %MeHg in livers (4.5-52 %), indicative of detoxifying MeHg via demethylation to inorganic Hg. Our data suggest that the threshold value where demethylation rates increase substantially appears to be ~10 μg g(-1) ww MeHg, similar to thresholds in other wildlife. Molar ratios of Hg:Se suggests that some birds from highly saline Reed Lake in the prairie region had insufficient Se available to bind to Hg, thereby removing Se binding as a mitigative strategy for high Hg levels for these birds.