Response of yellow perch (Perca flavescens) in Oneida Lake, New York, to the establishment of zebra mussels (Dreissena polymorpha) C.M. Mayer, A.J. VanDeValk., J.L. Forney, L.G. Rudstam, and E.L. Mills Abstract: We used long-term data on Oneida Lake, New York, to evaluate hypotheses about the effects of introduced zebra mussels (Dreissena polymorpha) on yellow perch (Perca flavescens). We detected no change in survival, diet, or numbers of young-of-the-year (YOY) yellow perch. YOY growth increased in association with zebra mussel introduc- tion and was marginally correlated with zooplankton size, which increased after zebra mussel introduction. Low num- bers of YOY in recent years did not explain their increased growth rate. The percentage of age 3 and older yellow perch that consumed zooplankton and benthos increased after zebra mussel introduction. Water clarity, which has increased since zebra mussel introduction, was inversely related to the percentage of the adult population with empty stomachs and positively related to the percentage that consumed benthos. The percentage of adult yellow perch that consumed zooplankton was positively related to zooplankton size. Despite the increase in percentage of adults consum- ing both types of invertebrate prey, we detected no changes in adult growth associated with zebra mussel introduction. This suggests that the principal effects of zebra mussels on yellow perch in Oneida Lake were not via benthic path- ways but through modifications of water clarity and zooplankton. Thus far, these effects have not been negative for the yellow perch population. Résumé : Nous avons utilisé des données à long terme sur le lac Oneida (NY) pour évaluer les hypothèses concernant les effets de l’introduction des moules zébrées sur la perchaude. Nous n’avons observé aucune modification de la survie des jeunes perchaudes de l’année, de leur régime alimentaire ou de leur nombre. La croissance de ces jeunes a augmenté avec l’introduction des moules zébrées et elle était faiblement corrélée avec la taille des zooplanctontes, laquelle a augmenté après l’introduction des moules zébrées. Le faible nombre de jeunes enregistré ces dernières an- nées n’explique pas l’augmentation de leur taux de croissance. Le pourcentage de perchaudes âgées de 3 ans et plus qui se nourrissent de zooplancton et de benthos a augmenté après l’arrivée des moules zébrées. La plus grande limpi- dité de l’eau observée depuis l’arrivée de ces moules était inversement proportionnelle au pourcentage de la population adulte à l’estomac vide et directement proportionnelle au pourcentage d’adultes qui se nourrissaient d’organismes ben- thiques. Le pourcentage de perchaudes adultes qui consommaient du zooplancton était directement proportionnel à la taille des zooplanctontes. Le pourcentage d’adultes se nourrissant des deux types d’invertébrés a augmenté, mais nous n’avons décelé aucune modification de la croissance des adultes associée à l’introduction des moules. Cette observation porte à croire que les principaux effets des moules zébrées sur la perchaude du lac Oneida n’étaient pas associés aux organismes benthiques, mais qu’ils étaient attribuables aux modifications de la limpidité de l’eau et du zooplancton. Jusqu’à maintenant, ces effets ne sont pas néfastes pour la population de perchaude. [Traduit par la Rédaction] Mayer et al. 754 Introduction A large body of ecological literature addresses the quanti- fication and importance of indirect effects in food webs (Abrams et al. 1996). The results of such indirect, or higher order, interactions are difficult to predict, even when pair- wise species interactions are well understood (Peckarsky and McIntosh 1998). This is especially true when the population of interest is removed from another species by several trophic levels. The potential effect of the introduced zebra mussel (Dreissena polymorpha) on North American fish populations exemplifies such a situation. Since zebra mus- sels were introduced into the Great Lakes in 1986, they have been associated with system-level changes and modifica- tions to virtually all trophic levels, including (i) increased water clarity (Fahnenstiel et al. 1995), (ii) changes to nutri- ent cycles (Mellina et al. 1995), (iii) decreased water column chlorophyll a levels (Caraco et al. 1997), (iv) declines in small zooplankton (Pace et al. 1998), and (v) increased den- sities of benthic invertebrates associated with mussel beds (e.g., Botts et al. 1996; Mayer 1998). However, documented effects of zebra mussel introduction on fish populations in natural water bodies are rare. The available studies of effects of zebra mussel on fish are equivocal as to the magnitude or the direction of effects. In pond enclosures, adult yellow perch (Perca flavescens) Can. J. Fish. Aquat. Sci. 57: 742–754 (2000) © 2000 NRC Canada 742 Received March 29, 1999. Accepted December 22, 1999. J15089 C.M. Mayer, 1 A.J. VanDeValk, J.L. Forney, L.G. Rudstam, and E.L. Mills. Cornell Biological Field Station, 900 Shackelton Point Rd., Bridgeport, NY 13030, U.S.A. 1 Author to whom all correspondence should be sent at the following address: Biological Research Laboratory, Department of Biology, Syracuse University, 130 College Pl., Syracuse, NY 13244-1220, U.S.A. e-mail: cmayer@syr.edu