Pollock and the decline of Steller sea lions: testing the junk-food hypothesis

Abstract

The decline of Steller sea lions (Eumetopias jubatus) in the Gulf of Alaska and the Aleutian Islands may be the result of them eating too much pollock (a gadid fish) instead of a more balanced and diverse diet containing fattier fishes, such as herring or sandlance. We sought to test this junk-food hypothesis by feeding six captive Steller sea lions (ages 0.9-4.5 years) only pollock or herring. All sea lions gained mass while eating herring. However, eating only pollock for short periods (11-23 d) caused the study animals to lose an average of 6.5% of their initial body mass (0.6 kg/d) over an average feeding trial of 16 d (initial mass averaged 125 kg). The animals were allowed to eat as much pollock as they wanted but did not increase their food intake to compensate for the low energy they were receiving. The sea lions showed progressive metabolic depression while losing body mass on a pollock-only diet. The loss of body mass while eating pollock was due to the lower gross energy content of pollock versus herring, the higher cost of digesting pollock, and the increased energy loss from digesting the larger quantity of fish needed to compensate for the lower energy content of pollock. Thus, our sea lions would have had to eat 35-80% more pollock than herring to maintain similar net energy intakes. Results from our captive-feeding studies are consistent with the junk-food hypothesis and have serious implications for Steller sea lions that have been eating primarily pollock in the Gulf of Alaska and the Aleutian Islands. Résumé : Le déclin des Otaries de Steller (Eumetopias jubatus) dans le golfe d'Alaska et les îles Aléoutiennes est peut-être attribuable à la consommation excessive de goberge (un gadidé) au lieu d'un régime équilibré et diversifié contenant des poissons plus gras tels le hareng et le lançon. Nous avons tenté d'éprouver cette hypothèse du régime de mauvaise qualité (junk-food hypothesis) en nourrissant six Otaries de Steller en captivité (âgées de 0,9 à 4,5 ans) uniquement de goberge ou uniquement de hareng. Toutes les otaries ont subi des augmentations de masse à la consommation de hareng. Cependant, la consommation de goberge pour de courtes périodes (11-23 jours) a entraîné des pertes moyennes de 6,5 % de la masse initiale (0,6 kg par jour) au cours d'une période alimentaire d'essai de 16 jours (masse initiale moyenne de 125 kg). Les animaux pouvaient manger autant de goberge qu'ils voulaient, mais ils n'ont pas augmenté leur consommation totale pour compenser la perte d'énergie encourue. Les otaries ont subi une dépression métabolique progressive par perte de masse lorsqu'ils ont adopté le régime constitué uniquement de goberge. La perte de masse au régime de goberge est attribuable au contenu énergétique brut réduit des goberges comparativement à celui des harengs, au coût énergétique supérieur de digestion de la goberge, et à la perte énergétique plus grande encourue lors de la digestion dune quantité de poisson plus grande nécessaire pour compenser le contenu énergétique plus faible de la g...

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