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.

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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.

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 6 SD ¼ 7.4 6 1.0 years) but smaller maximum sizes (215 6 7 mm TL) than other yellow perch populations. Thus, the lack of anglingsized 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 (,100 mm TL) than those preferred by anglers. Stomach contents and stable nitrogen isotopes confirmed that yellow perch in Dore Lake undergo expected ontogenetic diet shifts and become predominantly piscivorous at around age 3. Analysis with stable isotopes also revealed long-term dietary differences between age-2 and age-3 yellow perch captured in littoral versus offshore habitats, suggesting some degree of intrapopulation variability in resource and habitat use among yellow perch in Dore Lake.