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

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 receiv- ing. 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 hypothe- sis and have serious implications for Steller sea lions that have been eating primarily pollock in the Gulf of Alaska and the Aleutian Islands. Resume : Le declin des Otaries de Steller (Eumetopias jubatus) dans le golfe d'Alaska et les iles Aleoutiennes est peut-etre attribuable a la consommation excessive de goberge (un gadide) au lieu d'un regime equilibre et diversifie contenant des poissons plus gras tels le hareng et le lancon. Nous avons tente d'eprouver cette hypothese du regime de mauvaise qualite (junk-food hypothesis) en nourrissant six Otaries de Steller en captivite (âgees de 0,9 a 4,5 ans) uni- quement de goberge ou uniquement de hareng. Toutes les otaries ont subi des augmentations de masse a la consomma- tion de hareng. Cependant, la consommation de goberge pour de courtes periodes (11-23 jours) a entraine des pertes moyennes de 6,5 % de la masse initiale (0,6 kg par jour) au cours d'une periode alimentaire d'essai de 16 jours