This chapter has been reprinted with written permission from the journal Transactions of the American Fisheries Society and should be cited as:
DeBruyne, R. L., J. T. H. Coleman, J. R. Jackson, L. G. Rudstam, and A. J. VanDeValk. 2013. Analysis of prey selection by double-crested cormorants: a 15-year diet study in Oneida Lake, New York. Transactions of the American Fisheries Society 142: 430-446.
ABSTRACT
Piscivorous birds, in particular cormorants Phalacrocorax spp., have been reported to cause declines in some fish populations in both Europe and North America, but not in others. This difference may be due to prey selection by cormorants that is further dependent on the composition of the fish assemblage present. We present 15 years of diet data collected Oneida Lake, NY, where we have previously documented negative effects of double-crested cormorants Phalacrocorax auritus on two fish populations valued by anglers: walleye Sander vitreus and yellow perch Perca flavescens. The Oneida Lake fish community changed
through the study period and this change was reflected in cormorant diet samples. Diet samples were variable based on season and year with emerald shiner Notropis atherinoides, gizzard shad Dorosoma cepedianum, Lepomis spp., logperch Percina caprodes, walleye, and yellow perch having the highest overall relative importance. In years when age-0 gizzard shad were abundant they dominated double-crested cormorant diets in the fall after shad reached a length of 45mm. Consumption of emerald shiner and gizzard shad was positively related to each species’ abundance, but no significant correlation between availability and consumption was found for walleye, white perch Morone americana, or yellow perch. Double-crested cormorants may be displaying prey switching behavior and selecting for
smaller, soft-rayed prey species. We conclude that variation in fish recruitment influences double-crested cormorant food selection habits and highlight the importance of continued monitoring with changing fish communities to reliably assess potential impacts of cormorants on a fish community over time.
INTRODUCTION
Predator-prey interactions are central to the dynamics of food webs and ecosystems. Diet studies reveal the structure of the food web and how species interact within a community (Polis 1991; Wooten 1997; Harvey et al. 2003). Diet studies, coupled with estimates of abundance of available prey species, can indicate if and how predators select prey;
information essential for understanding the dynamics of predator-prey systems with multiple potential prey species (Wilbur and Fauth 1990; Hansson et al. 2007). Diet studies conducted over a brief time period may not reveal the full range of predator behavior, especially when prey composition varies over time. Piscivorous waterbirds, such as double-crested
cormorants Phalacrocorax auritus (hereafter cormorants), are top predators in many aquatic systems and several studies have demonstrated that they can impact prey populations (e.g., Lantry et al. 2002; Burnett et al. 2002; Steinmetz et al. 2003; Rudstam et al. 2004; Fielder 2008). However, impacts to prey populations have not been detected in many systems (e.g., Craven and Lev 1987; Ludwig et al. 1989; Neuman et al. 1997; Diana et al. 2006; Dalton et al. 2009). The degree of potential cormorant impacts may depend on a number of conditions, including the quantity of cormorants foraging pressure, the composition of the prey
assemblage, and patterns of prey selection exhibited by cormorants. Fish community
structure and prey selection will therefore affect the degree of impact of cormorant predation on sport fisheries. Demonstrating preference for specific prey species, or prey groups, may aid in predicting cormorant impacts on local fish communities and clarify our understanding of cormorant foraging behavior.
Variability in cormorant diets among locations and across time has led to their
characterization as generalist predator consuming prey in the same frequency as encountered within the environment (Lewis 1929; Ludwig et al. 1989; Neuman et al. 1997; Johnson et al. 2010). However, variability in diet composition alone does not verify that cormorants are not selective predators. Detailed studies of cormorant feeding selectivity are rare because they require comparison of diets with quantitative estimates of prey abundance which is seldom available (but see Rudstam et al. 2004; Diana et al. 2006; Dalton et al. 2009). In addition, prey switching (predators disproportionately consuming alternative prey when original prey becomes rare relative to the alternative prey) may be investigated by comparing cormorant diets in years with contrasting prey fish abundance (Murdoch 1969). Prey switching can stabilize predator-prey interactions by reducing the magnitude of prey abundance fluctuations and be more complete if different prey require different foraging tactics (Murdoch 1969; Oaten and Murdoch 1975). For example, cormorants may show differential preference between feeding on open-water schooling fish like gizzard shad Dorosoma cepedianum or emerald shiners Notropis atherinoides, compared to benthic fish species such as yellow perch Perca flavescens and round goby Neogobius melanostomus. Long-term studies of cormorant diets in conjunction with information on fish community structure are rare but highly valuable for detailed characterization of the foraging behavior of this controversial piscivore. One location with long-term quantitative information on both cormorant diets and fish community composition with abundance estimates is Oneida Lake, New York.
The objectives of this study were to examine double-crested cormorant diet
composition and evaluate the extent to which diets reflect changes in the fish community. We investigated whether cormorant diets and prey selection varied by season (spring-summer compared to late-summer-fall), year, and in response to changes in the fish community using 15 years of cormorant diet and fish abundance data from Oneida Lake, New York (1995- 2009). We report the overall composition of cormorant diets, statistical evaluations of diet
patterns, and prey selectivity of cormorants. Because cormorants have been implicated in the decline of walleye Sander vitreus and yellow perch populations, the two main species sought by anglers in Oneida Lake (Rudstam et al. 2004; VanDeValk et al., in press), we were particularly interested in how cormorant consumption of these species was affected by their abundance and the presence of alternative prey species such as gizzard shad and emerald shiner.