Diets of dabbling ducks (Anas: Anatidae) usually have been recorded only with respect to taxonomic composition and not prey size. Interspecific diet overlap thus has been termed large. However, using published taxonomic diet lists for seven species of ducks, measurements of bill morphology from museum specimens, and "handbook" data on the sizes and caloric density of invertebrates, prey size distributions were found to differ among species (P < 0.001). Also, the sizes of prey in each species' diet differed from that found in the environment. The number of lamellae per centimetre of bill negatively correlated with mean prey size (P < 0.001); body size and bill length did not. Based on the similarity of environmental and dietary prey size distributions, species were classified as specialist or generalist foragers. This classification corresponded well with one generated from analyses of foraging behaviour alone. An explanation for the diminuitive sizes of ducks on islands was advanced: if interspecific competition for food selects for niche divergence among coexisting dabbling ducks along a prey size niche dimension, then in the absence of competition from small species on islands, large solitary species have evolved to a smaller size which may be optimal for the distribution of energy among prey size classes in the environment.

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We describe thermal histories for Lake Ontario chinook salmon (Oncorhynchus tshawytscha) as determined from otolith δ 18 O thermometry using computer-controlled micromilling techniques to recover otolith aragonite at subseasonal resolution. We find that during the summer months chinook salmon inhabited epilimnetic waters with temperatures of~19-20°C as far back as the late 1980s. Chinook would approach but rarely exceed their reported upper incipient lethal limit of approximately 22°C, which suggests that these fish were seeking water with temperatures as high as was tolerable while otolith growth occurred. These results contrast with expected midsummer temperatures for this cold-water salmonine. Bioenergetic simulations indicate significant stress imposed upon chinook salmon. We estimate consumption to be up to 20% more and gross conversion efficiency 18% less annually relative to nominal simulations where chinook salmon are modeled nearer their preferred temperature, reinforcing previous inferences that the chinook salmon population may be near the limits of sustainability. We also find a strong negative correlation between δ 18 O and δ 13 C values. Therefore, seasonal and ontogenetic variation in δ 13 C values of chinook salmon otoliths appear to be related to metabolic rate during pelagic residence and may provide an indirect method for evaluating field activity and other aspects of fish life history.Résumé : Nous décrivons l'histoire thermique des saumons quinnat (Oncorhynchus tshawytscha) du lac Ontario d'après la thermométrie δ 18 O des otolithes à l'aide de techniques de moulinage sous contrôle d'ordinateur pour récupérer l'aragonite des otolithes à une échelle de résolution inférieure à une saison. Durant les mois d'été, les saumons quinnat habitent les eaux épilimnétiques de température~19-20 ºC depuis la fin des années 1980. Les saumons quinnat s'approchent du seuil de leur limite thermique létale supérieure connue qui est d'environ 22 ºC, mais la dépassent rarement, ce qui laisse croire que les poissons recherchent des eaux de la plus haute température tolérable durant la pé-riode de croissance des otolithes. Ces résultats tranchent avec les températures attendues en mi-été pour ce salmoniné d'eau froide. Des simulations bioénergétiques indiquent qu'il se produit un important stress chez le saumon. Nous estimons que la consommation est de 20 % supérieure et que l'efficacité brute de conversion est de 18 % inférieure à l'échelle annuelle par rapport à des simulations nominales dans lesquelles les saumons quinnat sont traités plus près de leur température préférée; cela renforce les déductions antérieures voulant que la population de saumons quinnat soit près de ses limites de maintien. Il y a aussi une forte corrélation négative entre les valeurs de δ 18 O et de δ 13 C. Ainsi, les variations saisonnières et ontogéniques des valeurs de δ 13 C des otolithes des saumons quinnat semblent être reliées au taux de métabolisme durant leur temps de résidence pélagique et cela peut fournir une méthode indirecte ...

We examined biomass and abundance of organisms at the nonpiscivorous fish, benthic invertebrate predator and nonpredator, and microcommunity trophic levels at 30 stream sites in southern Ontario to test the food limitation, predation, and habitat limitation hypotheses of regulation of trophic structure. The presence of piscivorous fish at seven sites allowed us to contrast the predictions to the "bottom—up" (food limitation) and "top—down" (predator limitation) hypotheses directly. Also, we tested the size—efficiency hypothesis (size—selective predation) at the nonpiscivorous fish and benthic invertebrate trophic levels. In the presence of piscivores, the biomass of nonpiscivorous fish was lower and the biomass and abundance of benthic invertebrates were higher than in the absence of piscivores. Microcommunity biomass did not differ at sites with vs. without piscivores. Thus, we rejected the bottom—up hypothesis in favor of the top—down hypothesis. The influences of the top trophic level on lower levels decreased with "distance" from the top of the food chain. Also, benthic invertebrate predators were larger in the presence of piscivores, consistent with the size—efficiency hypothesis. The habitat limitation hypothesis was also supported at the benthic invertebrate and nonpiscivorous fish trophic levels. Invertebrate biomass and abundance were positively related to extent of riffles, whereas fish biomass and abundance were negatively related to extent of riffles. We speculate that this complementary distribution may be determined by fish predation on the invertebrates.