No abstract

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

and ha. A. Miller. 1993. Accumulation of PCBs by lake trout (Salvebinus namaycush): an individual-based model approach. Can. ). Fish. Aquat. Sci. 50: 97-109.Po explain the variation in growth and in concentration of polychlorinated biphenyls (PCBs) among individual fish, an individual-based model (IBM) was applied to the Bake trout (SaBvelinus namaycush) population in Lake Michigan. The lBM accurately represented the variation in growth exhibited by the different age classes of lake trout. Uncertainty analysis of the lBM revealed that mean PCB concentration for the lake trout population was most sensitive to PCB concentration in their prey. The variability in PCB concentration among lake trout individuals was not adequately explained by the IBM, unless variation in prey fish PCBs was included in the model. To accomplish this, the simulated lake trout population was divided into subsets subjected to different levels of PCB concentration in the prey fish. Thus, model results indicated that variability in prey fish PCB concentration was an important component of the variation in PCB concentration observed among individual lake trout comprising the Lake Michigan population.Les auteurs ont utilise un modele de type individuel afin d'expliquer Bes &arts de la croissance et de la concentration des biphenyles pealychlor4s (BPC) entre les individams d'une population de touladi (Salvelincss namayccssh) du lac Michigan. Le msd6le a permis de representer adequatement les &arts de croissance notes entre les diverses classes d'ige des touladis. Une analyse d'incertitude du modele a rnontre que la concentration de BPC des pssies etait le facteur influant le plus sur la concentration de BPC de la population de touladi. La variabilite de cette concentration entre les touladis nf6tait pas expliquee de f a~o n ad4qu$te par le modGle rnsins que I'on ne tienne cornpte aussi de la variation de cette concentration chez les proies. A cette fin, la population sirnulke de tokeladi a et6 divis$e en sous-ensembles en fonctiow des diverses teneurs en BPC des proies. Le modkle indiquait donc que la variabilite de la concentration des BPC des proies etait uw important facteur de la variation de la concentration des BPC notee chez les individams de la population de tsuladi du lac Michigan.

Major changes in fish populations occurred in Lake Michigan between the early 1970s and 1984. The abundance of lake trout (Salvelinus namaycush) and several nonnative species of salmonines increased greatly as a result of intensive stocking. The exotic alewife (Alosa pseudoharengus), which had proliferated to extremely high levels of abundance in the mid-1960s, declined, particularly in the early 1980s. We believe that the sharp decline in alewives in the 1980s was caused primarily by poor recruitment during the colder than normal years of 1976–82. Several of Lake Michigan's endemic species of fish appeared to be adversely affected by alewives: bloater (Coregonus hoyi), lake herring (C. artedii), emerald shiner (Notropis atherinoides), yellow perch (Perca flavescens), and deepwater sculpin (Myoxocephalus thompsoni), and possibly spoonhead sculpin (Cottus ricei). All declined when alewives were abundant, and those that did not become rare, i.e. the bloater, perch, and deepwater sculpin recovered when alewives declined. We present evidence suggesting that the mechanism by which alewives affect native species is not by competition for food, as has often been hypothesized, and discuss the possibility that it is predation on early life stages. Despite the decreased availability of alewives in the early 1980s, salmonines continued to eat mainly alewives. The highly abundant alternate prey species were eaten only sparingly, but alewives still may have been abundant enough to meet the forage requirements of salmonines. Two new exotics, the pink salmon (Oncorhynchus gorbuscha) and threespine stickleback (Gasterosteus aculeatus), increased in abundance in the 1980s, and could become detrimental (particularly the salmon) to other species.

1985. Lake Michigan'scapacity to support lake trout (Salvelinus narnaycush) and other salrnonines: an estimate based on the status of prey populations in the 1970s. Can. J . Fish. Aquat. Sci. 42: 449-454.We used a mass balance equation relating total mortality of age II and older alewives (Abosa psewdoharengers) to their removals by predatory dish and other sources of mortality as the basis for estimating that the forage base in Lake Michigan could support an additional 13 000 to 21 000 t of Bake trout (Sabvelinus namaycush) or a total lake trout biomass between 15 080 and 23 000 t. This biomass estimate is projected with biomasses of other trout and salmon held at 1979 levels. Major premises of this approach are that (1) the sustained availability of age Bi and older alewives to salrnonines will ultimately limit the expansion of salmonine stocks, (2) the alewife population was oscillating without trend during 1973-80, and (3) additional limited predation on alewives would be compensated by a reduction in natural mortality caused by physiological stress and disease.Uwe 6quation de I'kquilibre des masses liant la mortalitetotale de gaspareaux (Aiosapseepdohasewgus) ag6s de deux ans ole plus a Beur prelevernent par des poissons predateurs ou d'autres facteurs de mortalite a kte utilisee cornme base dans I'estirnation suivante : le stock en poissons fourrages presents dans ie lac Michigan pourrait soutenir de 13 000 2 21 000 t supplementaires de tcduladi (Salvebinus narnaycush) ou une biornasse totale d e 45 008 a 23 000 t. Cette estimation a et4 determinee a partir des biomasses d'autres especes de truites et de saumons maintewues aux niveaux de 1979. Cette approche repose sur les principes suivants : (I) la disponibilite soutenue de gaspareaux 2ges de deux ans csu plus cornme proies pour les salmonid6s limitera en din de compte B'expansion des stocks desalmonides, (2) la population de gaspareaux a oscill6 sans tewdance de 1973 21 1980 et (3) une predation Iimitee accrue du gaspareau serait compewsee par une baisse de la rnsrtalite natureile due au stress physiologique et aux maladies.