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 (delta C-13 and delta N-15). The delta N-15 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 delta C-13 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.