We examined stressors that have led to profound ecological changes in the Lake Ontario ecosystem and its fish community since 1970. The most notable changes have been reductions in phosphorus loading, invasion by Dreissena spp., fisheries management through stocking of exotic salmonids and control of sea lamprey (Petromyzon marinus), and fish harvest by anglers and double-crested cormorants (Phalacrocorax auritus). The response to these stressors has led to (i) declines in both algal photosynthesis and epilimnetic zooplankton production, (ii) decreases in alewife (Alosa pseudoharengus) abundance, (iii) declines in native Diporeia and lake whitefish (Coregonus clupeaformis), (iv) behavioral shifts in alewife spatial distribution benefitting native lake trout (Salvelinus namaycush), threespine stickleback (Gasterosteus aculeatus), and emerald shiner (Notropis atherinoides) populations, (v) dramatic increases in water clarity, (vi) predation impacts by cormorants on select fish species, and (vii) lake trout recruitment bottlenecks associated with alewife-induced thiamine deficiency. We expect stressor responses associated with anthropogenic forces like exotic species invasions and global climate warming to continue to impact the Lake Ontario ecosystem in the future and recommend continuous long-term ecological studies to enhance scientific understanding and management of this important resource. 490Résumé : On trouvera ici un examen des facteurs de stress qui ont modifié profondément l'écosystème du lac Ontario et sa communauté de poissons depuis 1970. Les changements les plus importants ont été la réduction de l'apport de phosphore, l'invasion des Dreissena spp., la gestion de la pêche, notamment l'empoissonnement de salmonidés exotiques et le contrôle de la grande lamproie marine (Petromyzon marinus), ainsi que la récolte des poissons par les pêcheurs sportifs et les cormorans à aigrette (Phalacrocorax auritus). La réaction à ces facteurs a eu pour conséquen-ces: (i) le déclin de la photosynthèse des algues et de la production du zooplancton épilimnétique, (ii) la diminution de l'abondance du gaspareau (Alosa pseudoharengus), (iii) la réduction des Diporeia indigènes et des grands corégonesCan.

Ecosystem change often affects the structure of aquatic communities thereby regulating how much and by what pathways energy and critical nutrients flow through food webs. The availability of energy and essential nutrients to top predators such as seabirds that rely on resources near the water's surface will be affected by changes in pelagic prey abundance. Here, we present results from analysis of a 25-year data set documenting dietary change in a predatory seabird from the Laurentian Great Lakes. We reveal significant declines in trophic position and alterations in energy and nutrient flow over time. Temporal changes in seabird diet tracked decreases in pelagic prey fish abundance. As pelagic prey abundance declined, birds consumed less aquatic prey and more terrestrial food. This pattern was consistent across all five large lake ecosystems. Declines in prey fish abundance may have primarily been the result of predation by stocked piscivorous fishes, but other lake-specific factors were likely also important. Natural resource management activities can have unintended consequences for nontarget ecosystem components. Reductions in pelagic prey abundance have reduced the capacity of the Great Lakes to support the energetic requirements of surface-feeding seabirds. In an environment characterized by increasingly limited pelagic fish resources, they are being offered a Hobsonian choice: switch to less nutritious terrestrial prey or go hungry.

The alewife Alosa pseudoharengus, an invader to the Laurentian Great Lakes from the Atlantic Ocean, has been blamed for causing major disruptions of Great Lakes fish communities during the past 50 years. We reviewed the literature and examined long‐term data on fish abundances in the Great Lakes to develop a new synthesis on the negative effects of alewives on Great Lakes fish communities. The results indicated that certain fish populations are substantially more vulnerable to the effects of alewives than others. More specifically, the effects of alewives on other fish populations appeared to follow a continuum—from such fishes as slimy sculpin Cottus cognatus, lake whitefish Coregonus clupeaformis, and bloater Coregonus hoyi, which were relatively unsusceptible—to Atlantic salmon Salmo salar, lake trout Salvelinus namaycush, and emerald shiner Notropis atherinoides, which were highly susceptible. Intermediate species in this continuum included yellow perch Perca flavescens, deepwater sculpin Myoxocephalus thompsonii, and burbot Lota lota. The predominant mechanism by which alewives exerted their negative effect appeared to be predation on the larvae of other fishes. The key factor in the extirpation of Atlantic salmon from Lake Ontario, however, was probably early mortality syndrome induced by a diet rich in alewives. We conclude that the degree of restoration of the native Great Lakes fish community depends in part on the degree of control of the alewife population.

We describe dynamics of energy density and size of Lake Ontario alewife Alosa pseudoharengus and rainbow smelt Osmerus mordax. and we use a bioenergetics model of a common pelagic piscivorc, chinook salmon Oncorhynchus tshawytscha. to demonstrate the effect of these factors on piscivore daily ration during [1978][1979][1980][1981][1982][1983][1984][1985][1986][1987][1988][1989][1990]. The energy density of alewives varied more than twofold between peaks in September (age 1) or October-November (age >2) and the lows in May (age 1) or July-September (age >2). The previously described seasonal pattern of energy density of Lake Michigan alewives was similar except that energy density of older alewives (age >3) was markedly higher in Lake Michigan. During 1978During -1990, the spring energy density of Lake Ontario alewives peaked in 1979 (6,259 J/g wet weight), declined irregularly until 1985, and then remained stable through 1990 (at approximately 4,600 J/g). The initial decline may have been a density-dependent response to a burgeoning alewife population, but the lack of an increase in alewife condition in the late 1980s, when alewife biomass fell, suggests a decline in lake productivity. Energy density of rainbow smelt increased with age in Lake Ontario and condition was invariant during 1978-1990 despite a threefold change in rainbow smelt biomass. Rainbow smelt energy density was lower and fluctuated less seasonally in Lake Ontario than in Lake Michigan.

We compared predatory demand by pelagic planktivorous prey fish with invertebrate production in Lake Michigan during 1987 and in Lake Ontario during 1990. Predation by the planktivores in Lake Ontario was nearly fourfold higher than in Lake Michigan Qapprox. 87 g wet ~e i g h t -r n -~-~e a r -' ) .Predation rates on Mysis were comparable in Lakes Michigan and Ontario (approx. 21 g-m-2-year-'), while predation on Diporeia was markedly higher in Lake Michigan than in Lake Ontario (21.3 vs. 8.5 g wet ~e i g h t . m -~.~e a r -' ) .In Lake Ontario, predatory demand on zooplankton exceeded our best estimate of production by a factor of 1.7. Similarly, predation estimates on Mysis in Lake Ontario were 1.2-2.0 times the estimated rate of production, depending on the production model used. Lake Michigan planktivores consumed approximately 55% of available zooplankton production in 1987, indicating that competition for prey resources, if operating, was not as intense as that in Lake Ontario in 1990. It is unclear how to resolve the paradox that predation could markedly exceed available prey production in some cases. There could be sources of error in the estimates of both the supply and demand sides of these trophic relationships.RCsnmC : Nous avons compare B la production d'invertCbrCs la demande predatrice des poissons pelagiques planctivores servant eux-rnemes de proies, en 1987 dans le lac Michigan et en 1990 dans le lac Ontario. Dans ce dernier lac, la prkdation par les planctivores Ctait prks de quatre fois plus ClevCe que dans le lac Michigan (environ 87 g poids frai~-rn-~.an-'). Les taux de predation 5 17&gard des Mysis 6taient comparables dans les deux lacs (environ 21 g.ms2.an-'1, tandis que la prddation i lqCgard des Diporeia etait nettement plus haute dans le lac Michigan que dans le lac Ontario (21,3 contre $ 3 g poids frai~-m-~-an-l). Dans le lac Ontario, la demande des prkdateurs i l'kgard du zooplancton dkpassait d'un facteur de 1,7 notre meilleure estimation de la production.Be meme, les estimations de la prCdation a l'kgard des Mysis dans le lac Ontario correspondait B 1,2-2,0 fois le taux estimC de production, selon le modkle de production retenu. Les planctivores du lac Michigan ont consommC environ 55% de la production disponible de zooplancton en 1987, ce qui indique que la concurrence 8 l'Cgard des ressources em proies, si elle existait, n'itait pas

Diets and length–weight relationships of Lake Ontario alewife (Alosa pseudoharengus) in 1972 differed from those in 1988; the large cladoceran Bythotrephes cederstroemi colonized the lake during the mid-1980's. Micro-crustacean zooplankton were the dominant prey of alewife during April–October in 1972 and 1988. Although Bythotrephes was not found in 1988 net samples, it replaced other zooplankters in the alewife's diet. Typically, tailspines were the only part of Bythotrephes in alewife stomachs; their frequency was high in April–May, diminished rapidly in summer and was very low by fall. In spring 1988, alewife [Formula: see text] were in better condition than in spring 1972 and this may have been due to larger fish feeding more heavily on Bythotrephes. Variation in diet among widely separated sampling sites was due to differences in alewife abundance, stability of thermal structure, progress of zooplankton community development and distance to the mouth of the Niagara River (through which Bythotrephes probably enter the lake in summer and fall). In the Great Lakes, inter- and intralake differences in diet clearly exist, and these must be incorporated into models of alewife planktivory to gain an accurate understanding of energy flow between trophic levels.

The offshore fish community of Lake Ontario is presently dominated by intensively managed, nonnative species: Alewife Alosa pseudoharengus and rainbow smelt Osmerus mordax at the planktivore level and stocked salmonines at the piscivore level. Salmonine stocking rates per unit area of Lake Ontario are the highest in the Great Lakes, and fishery managers are concerned about the sustainability of the fishery under present stocking policies, particularly with the recent collapse of the Lake Michigan fishery for chinook salmon Oncorhynchus tshawytscha. In this paper, we describe and present the results of a simulation model that integrates predator demand estimates derived from bioenergetics, prey and predator population dynamics, and a predation model based on the multiple‐species functional response, Model reconstructions of historical alewife biomass trends and salmonine diets corresponded reasonably well with existing data for the period 1978–1992. The simulations suggest that current predator demand does not exceed the threshold beyond which alewife biomass cannot be sustained, but they indicate that the sustainability of the prey fish community is extremely sensitive to fluctuations in overwinter survival of alewife; an additional mortality of 25% in a single winter would be sufficient to cause the collapse of the alewife population. The model includes a number of assumptions and simplifications with a limited empirical basis; better estimates of salmonine survival rates, an evaluation of the importance of spatial and temporal interactions among predators and prey, and incorporation of the effects of recently observed declines in system productivity at lower trophic levels would significantly increase confidence in the modelˈs projections.

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