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.

Handling time for prey of different sizes and morphological types was studied in the largemouth bass (Micropterus salmoides). Prey ranged in size from about 1/10 to 2/3 the length of the bass. Handling time increased rapidly with prey of increasing size. For prey of equivalent length, crayfish were the most time-consuming morphological type to handle and swallow, followed by bluegill (Lepomis macrochirus), yellow perch (Perca flavescens), brook stickleback (Culaea inconstans), bluntnose minnow (Pimephales notatus), and finally bullfrog tadpoles (Rana catesbeiana). The ratio of prey length to bass length that minimized handling time per unit weight of prey consumed, for prey types common in the diet of largemouth bass, was 0.22 for tadpoles, 0.24 for yellow perch and crayfish, and 0.29 for bluegill. Comparison of the experimentally derived optimum prey size based on handling time with that consumed by three natural populations showed that bass commonly chose prey of similar or smaller size than the optimal predicted. The reasons for this are discussed.

No abstract

Cercopagis pengoi, a waterflea native to the Ponto-Caspian region, was discovered during 1998 in Lake Ontario. Individuals were found throughout the lake during summer snagged on sportfishing lines. The population included parthenogenetic (92%) and sexual (2%) females and males (6%). Cercopagis has a very long caudal appendage that is more than five times the body length and terminates in a distinctive loop. Females and males from Lake Ontario were significantly smaller than individuals from the Neva Estuary, Baltic Sea. In Eurasia, C. pengoi occurs in relatively warm fresh and brackish waters (0-14‰) at population densities usually <3000 individuals·m-3; mean and maximum population densities in Lake Ontario were 170 and 322 individuals·m-3, respectively. The presence of females with resting eggs indicates that Cercopagis will likely establish in Lake Ontario. As with other recently introduced invertebrates, Cercopagis likely was transported to the Great Lakes in ballast water from eastern Europe. The rapid influx of Ponto-Caspian species into the Great Lakes warrants further study including identification of source populations, mechanisms of dispersal, impacts on recipient ecosystems, and efficacy of ballast water exchange programs.

An experimental study was carried out on prey handling time (from initial seizure of prey to completion of swallowing movements) in the grass pickerel (Esox americanus vermiculatus), using bluegill (Lepomis macrochirus) as prey. Esox differs from the largemouth bass (Micropterus salmoides), which was studied previously, in having a more specialized dentition. It was found that for individuals of the same mouth width (20 mm), handling time was 40–80% less in Esox for bluegill prey of total length 20–50 mm. Further, "optimal" and maximum size of prey consumed were 56 and 46% larger, respectively, for the esocid. The results are also related to body weight and age in Esox and Micropterus and the ecological significance of the findings is discussed.