Sympatric populations of dwarf lake whitefish (Coregonus clupeaformis) (DLW) and normal lake whitefish (NLW) commonly occur in north temperate and subarctic lakes. DLW have a much lower growth, mature earlier, and have a shorter life span than NLW. Furthermore, they are usually not found when cisco (Coregonus artedi) are present, possibly due to competitive exclusion. In this study, we compared the energy budget of DLW, NLW, and cisco using food consumption rates estimated with mass balance models of chemical tracers (i.e., mercury and radiocesium). These chemicals are globally distributed and can be readily detected in fish and their prey. Our analysis showed that the energy budget of DLW and cisco was similar. DLW and cisco consumed on average 40–50% more food than NLW. The conversion efficiency of DLW and cisco was two to three times lower than that of NLW. These results suggest that DLW and cisco allocated a larger fraction of their energy budget to metabolism than NLW. Our analysis also suggests that the earlier maturation and shorter life span of DLW and cisco may be due to their higher metabolic rates.

We present a simple method for estimating food consumption rates of fish in the field based on a mercury (Hg) mass balance model. This method requires the determination of fish age, size, and growth and Hg concentration in fish and their food. The model was validated using data obtained from a previously published laboratory experiment. A field test of the model showed that food consumption rates determined with the Hg mass balance model differed from independent estimates obtained with the 137 Cs method by only 0.6-16.1%. The model was applied to fish from various lakes in Quebec and Ontario. Food consumption rates estimated with the Hg mass balance model varied significantly both among species and among populations. Furthermore, female fish tended to eat 30-40% more food than males, probably to meet the larger energy requirement associated with egg production. A sensitivity analysis indicated that the Hg mass balance model was mostly responsive to variables that can be easily measured in the field, such as fish size and Hg concentration in fish and their food. By providing a low-effort approach to quantifying food consumption rates of fish in the field, this method may help to refine our understanding of the environmental factors that influence the quantity of food consumed by fish. 428 Résumé : Nous présentons une méthode simple dérivée d'un bilan massique du mercure (Hg) pour estimer les taux de consommation des poissons en milieu naturel. Cette méthode nécessite une estimation de l'âge, la masse et la croissance des poissons, ainsi que de la teneur en Hg des poissons et de leurs proies et a été validée à partir d'expériences réalisées en laboratoire. Une évaluation effectuée in situ a démontré que les taux de consommation obtenus à partir de ce bilan massique ne différaient que par 0,6-16,1% des valeurs obtenues de façon indépendante à l'aide du 137 Cs. Ce modèle a été utilisé pour estimer les taux de consommation de poissons provenants de divers lacs du Québec et de l'Ontario. Ces estimations différaient significativement entre les espèces et les populations. De plus, les femelles consommaient 30-40% plus de nourriture que les mâles, probablement pour combler les coûts de production de gonades plus élevés. Une analyse de sensibilité a indiqué que le bilan massique du Hg était principalement influencé par les variables qui peuvent être facilement mesurées en milieu naturel, telles la masse des poissons et la teneur en Hg des poissons et des proies. Comme cette méthode requiert peu d'efforts en milieu naturel, elle pourrait nous aider à mieux comprendre les facteurs environnementaux qui influencent la quantité de nourriture consommée par les poissons. Trudel et al.

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We present a simple method for estimating food consumption rates of fish in the field based on a mercury (Hg) mass balance model. This method requires the determination of fish age, size, and growth and Hg concentration in fish and their food. The model was validated using data obtained from a previously published laboratory experiment. A field test of the model showed that food consumption rates determined with the Hg mass balance model differed from independent estimates obtained with the 137Cs method by only 0.6-16.1%. The model was applied to fish from various lakes in Quebec and Ontario. Food consumption rates estimated with the Hg mass balance model varied significantly both among species and among populations. Furthermore, female fish tended to eat 30-40% more food than males, probably to meet the larger energy requirement associated with egg production. A sensitivity analysis indicated that the Hg mass balance model was mostly responsive to variables that can be easily measured in the field, such as fish size and Hg concentration in fish and their food. By providing a low-effort approach to quantifying food consumption rates of fish in the field, this method may help to refine our understanding of the environmental factors that influence the quantity of food consumed by fish.