Double-crested Cormorants (Phalacrocorax auritus) increased dramatically in North America during the 1990s, providing the opportunity to study the effects of an increase of a top predator on an existing predator-prey system. In Oneida Lake, New York, USA, Double-crested Cormorants were first observed nesting in 1984 and had increased to over 360 nesting pairs by 2000. Concomitant with this increase in piscivorous birds was a decrease in the adult walleye (Stizostedion vitreum) and yellow perch (Perca flavescens) populations. Analysis of a 40-yr data series shows higher mortality of subadults (age 1-2 yr perch and age 1-3 yr walleye) for both species in the 1990s compared to the previous three decades. Cormorant diet was investigated from 1995 to 2000 using a combination of cast pellets, regurgitants, and stomach analysis. Walleye and yellow perch were a major portion of the cormorant diet during these years (40-82% by number). The number of subadult walleye and yellow perch consumed by cormorants suggests that the increase in subadult mortality can be explained by predation from cormorants. Mean mortality rates of adult percids attributed to cormorant predation were 1.1% per year for walleye and 7.7% per year for yellow perch. Our analysis suggests that predation by cormorants on subadult percids is a major factor contributing to the decline in both the walleye and the yellow perch populations in Oneida Lake. Other ecosystem changes (zebra mussels, lower nutrient loading, decrease in alternate prey) are not likely explanations because the potential mechanisms involved are not consistent with auxiliary data from the lake and would not affect subadult mortality. The likely impact of bird predation on percid populations in Oneida Lake occurs because cormorants feed on larger fish that are beyond the size range where compensatory mechanisms are important.

Abundance of eight successive year-classes of walleyes (Stizostedion vitreum vitreum) was measured at intervals from hatching into the second year. Each year-class arose from an initial stock of 12–18 billion eggs. Population of pelagic larvae was augmented in some years by the release of hatchery-reared larvae but the effect on year-class size was obscured by mortality after young became demersal. Decrease in number of fingerlings was attributed to predation by older walleyes. Intensity of predation was influenced by abundance of alternate prey and duration of cannibalism by growth of young walleyes. Five year-classes that were monitored through age 4 contributed 12,000–478,000 walleyes to the adult stock.

Species of forage fish i_n stomachs of walleye and their abundance in trawl catches were compared in 1968-71. Young yellow perch were the predominant species in trawls and were consistently selected by walleyes. Consumption of young white perch and walleyes by older walleyes inckeased during periods of low yellow perch abundance which suggested that young yellow perch might act as a buffer controlling intensity of predation. This possibility was assessed by comparison of relative survival of white perch and walleye cohorts between the first and second year of life with indices of yellow perch density between 1959 and 1970. Close correlations between these variables support the conclusion that abundance of young perch governs intensity of predation on other forage size fish and indirectly controls the size of the walleye population by regulating cannibalism. Piscivorous species are of 'fundamental importance in molding species and size composition of fish populations. This has been demonstrated by manipulation of predatorprey combinations in ponds (Swingle 1950) and by introduction of predators into natural waters (Gammon and Hasler 1965). Although these and other studies illustrate the importance of predators in regulating abundance of prey, the interactions between predator and prey in multi-species populations have remained obscure. The object of this study was to determine how changes in density of young (0+) yellow perch (Perca ]lavescens) affect the diet of yearling (I+) and older walleye (Stizostedion vitreum) and the survival of alternate prey. Abundance of forage species was monitored from 1959-1971, and their occurrence in walleye stomachs was compared with their abundance in trawl catches in 1968-71. Young perch were the predominant species eaten in all four years, and young walleyes and white perch (Morone americana) were of secondary importance. Other forage fish of lesser importance are the tessellated darter (Etheostoma olmstedi), trout-perch (Percopsis omiscomaycus), mottled sculpin (Cottus hairall), logperch (Percina caprodes) and =A contribution from Federal Aid in Fish Restoration Project F-17-R, New York. 15

The response of a Daphnia pulex population to fluctuations in abundance of young (age‐0) yellow perch Perca flavescens was examined in Oneida Lake, New York, 1975–1979. Comparison of yellow perch biomass and Daphnia pulex abundance suggested daphnid populations could tolerate predation by 10 kg of young fish˙hectare−1 but reproduction could not compensate for predation when biomass of young exceeded 20–40 kg˙hectare−1. Consumption by young yellow perch exceeded Daphnia pulex production in 1975 and 1977, when daphnids disappeared, but maximum daily consumption was less than 27% of production in 1976 when daphnids were abundant through late summer. Since 1968, D. pulex populations have persisted through late summer in years when young yellow perch were scarce and disappeared when young were abundant. The decisive role of young yellow perch in the regulation of Daphnia pulex abundance is of importance to the broader fish community in Oneida Lake.

Predator–prey dynamics and density dependence are fundamental issues in ecology. We use a detailed, individual‐based model of walleye and yellow perch to investigate the effects of alternative prey and compensatory responses on predator and prey population dynamics. Our analyses focus on the numerical and developmental responses of the predator, rather than the traditional emphasis on functional responses. The extensive database for Oneida Lake, New York, USA was used to configure the model and ensure its realism. The model follows the daily growth, mortality, and spawning of individuals of each species through their lifetime. Three ecologically distinct periods in the history of Oneida Lake were simulated: baseline, high mayfly densities, and high forage fish densities. Mayflies and forage fish act as alternative prey for walleye. For model corroboration, the three periods were simulated sequentially as they occurred in Oneida Lake. Model predictions of abundances, size at age, and growth and survival rates compared favorably with Oneida Lake data. Three hypotheses suggested by the data were evaluated: alternative prey stabilizes yellow perch and walleye populations; alternative prey increases yellow perch and walleye recruitment; and density‐dependent growth and survival compensate for changes in young‐of‐the‐year mortality. Model simulations were performed under increased mayfly densities, increased forage fish densities, and increased egg mortality rates. Predicted recruitment and population stability depended on the magnitude of increased walleye prey and differed between mayflies and forage fish. Compensation was driven by density‐dependent growth, resulting in younger age at maturation and increased fecundity. We compare our results using a detailed, size‐structured model capable of numerical and developmental responses of predators to results from classical predator–prey theory. Weaknesses in the current version of the individual‐based model and knowledge gaps that require additional empirical data collection are also discussed.

No abstract

A study to determine the energy content of young (age‐0) yellow perch Perca flavescens and their prey, measure metabolism, and relate energy supply and intake to fish growth was conducted in Oneida Lake, New York during the summers of 1975–1977. Growth rates in these years were similar in early summer when young yellow perch were 20 to 50 mm total length and fed almost exclusively on Daphnia pulex but varied widely in late summer when the diet was more diverse. In early summer, food intake averaged 23% of body energy content, young yellow perch assimilated about 68% of the daily ration, and gross efficiency averaged 27%. Comparison of observed growth and energy available for growth calculated from an energy budget indicated estimates of food intake and respiration were reasonable. In years when the D. pulex population collapsed in late summer, young yellow perch shifted to alternate prey and growth declined. Variation in first‐year growth, mediated by D. pulex abundance, may determine the contribution of yellow perch cohorts to the adult stock.

The sequence of events that led to a decline in numbers of American eel (Anguilla rostrata) and esocids and an increase in numbers of walleye (Stizostedion vitreum vitreum) in Oneida Lake from 1900–74 are reviewed. Demise of eel and esocids was followed by a rise in numbers of walleye, but little change in mean size of individual walleye. Reduced interspecific competition apparently enhanced walleye recruitment, but the anticipated increase in growth of walleye was probably inhibited by intensified intraspecific competition. This interpretation was supported by evidence that recruitment and growth of walleye in 1957–74 were regulated by prey abundance. Transition from a diverse predator population to one dominated by walleye was orderly and yellow perch (Perca flavescens) remained the most abundant prey. Key words: Percidae, competition, walleye biomass, Stizostedion, population dynamics, community ecology