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Otolith shape has long been known to be species specific, but recent reports have pointed to its value as an indicator of stock identity. To test this hypothesis, all three pairs of otoliths were sampled from 2349 Atlantic cod (Gadus rnorhua) collected on spawning grounds throughout the northwest Atlantic. Otolith shape was determined with Fourier analysis and combined with measures of otolith area and perimeter. There were highly significant differences in otolith shape among most of the cod samples, but shape also differed among ages, sexes, and yearclasses. The sagittal otoliths (largest pair) provided slightly better stock discrimination than did the lapillar or asteriscal otoliths. The first discriminant function was highly correlated with both fish and otolith growth rate, indicating that stock discrimination improved as the difference in stock-specific growth rate increased. Reevaluation of published studies on other species indicated that growth rate contributes more variation to regional differences in otolith shape than does stock origin. Differences in otolith shape among ages, sexes, and yearclasses were also attributable to growth rate differences. To the extent that growth rates vary more between than within stocks, otolith shape analysis can provide an easily determined measure of stock identityaf.On sait depuis longtemlps que la forrne des otolithes est particuli6re 3 une espece, mais des travaux recents ont souligne sa valeur comrne indicateur de Itidentit$ d'un stock. Pour tester cette hypoth6se, on a pr4levt5 les trois paires d'otolithes chez 2 349 morues franches (Gadus rnsrhua) capturees sur les frayeres dans tout I'Atlantique nord-ouest. La forme des otolithes a 4t4 determinee par une analyse de Fourierl et combinee 2 des mesures de la supedicie et du perimetre des otolithes. On a not4 des differences haketement significative5 dans la forme parmi la plupart des echantillons de morue, mais la forme diffkrait aussi entre les iges, les sexes et les classes annuelles. Les sagitta (la paire la plus grosse) permettaient une discrimination des stocks legerement meilleure qkee les lapillus su les asteriscus. La premiere fonction discriminante 6tait fortement corrklee au taux de croissance du poisson et de l'otolithe, ce qui indique que la discrimination des stocks s'ameliorait 21 mesure qu'augmentait la difference dans le taux de croissance de chaque stock. Le reexamen des travaux publiks sur d'autres especes indique que le taux de croissance fait varier davantage les differences regionales dans la forme des otolithes qkee l'origine du stock. Les differences dans la forrne des otolithes entre les ages, les sexes et les classes annuelles etaient aussi attribuables 2 des differences dans le taux de croissance. Bans la mesure oti les taux de croissance varient davantage entre les stocks qu'h I'interieur d'un stock, l'analyse de la forme des otolithes peut constituer une rnethode facile pour determiner l'identite des stocks.

Various population parameters and physiological, behavioral, morphometric, meristic, calcareous, biochemical, and cytogenetic characters have been used to identify fish stocks. We define a stock as an intraspecific group of randomly mating individuals with temporal or spatial integrity. Each character set and the associated methodology relates to specific aspects of the stock definition. Population parameters are useful primarily for the recognition of putative stocks at the practical fisheries management level. Physiological and, to some degree, behavioral characters are used primarily to study differences in the adaptation of stocks to different environments. Behavioral characters are also important for the recognition of stocks and the study of their spatial and temporal discreteness. Morphological characters, including morphometric measurements, meristic counts, and the shape, size, and type of zonation in calcareous structures provide data that are useful for the precise description of and differentiation among stocks. Although the genetic control of this type of variation is poorly understood, multivariate methods coupled with shape analyses provide techniques that describe intraspecific subdivisions that have been found to correspond to genetic stock structure as determined by other methods. Intraspecific chromosomal variation has, on occasion, been employed for stock identification. This variation has had only limited application to the study of stocks because of complications arising from intraindividual variation and artifactual variation introduced by the methodology. Electrophoresis provides an important method for measuring the genetic discreteness of stocks and for the study of genetic relationships among stocks. Electrophoretic data have recently attained a primary position among the methods used for stock identification.Key words: stock identification, genotype, phenotype, population parameters, marking, physiological, behavioral, morphometric, meristic, calcareous, cytogenetic, and biochemical characters

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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.