Prey consumption of Steller sea lions (Eumetopias jubatus) off Alaska: How much prey do they require?

The effects of seasonal and regional differences in diet composition on the food requirements of Steller sea lions (Eumetopias jubatus) were esti­ mated by using a bioenergetic model. The model considered differences in the energy density of the prey, and dif­ ferences in digestive efficiency and the heat increment of feeding of different diets. The model predicted that Steller sea lions in southeast Alaska required 45–60% more food per day in early spring (March) than after the breed­ ing season in late summer (August) because of seasonal changes in the energy density of the diets (along with seasonal changes in energy require­ ments). The southeast Alaska popula­ tion, at 23,000 (±1660 SD) animals (all ages), consumed an estimated 140,000 (±27,800) t of prey in 1998. In contrast, we estimated that the 51,000 (±3680) animals making up the western Alaska population in the Gulf of Alaska and Aleutian Islands consumed just over twice this amount (303,000 [±57,500] t). In terms of biomass removed in 1998 from Alaskan waters, we estimated that Steller sea lions accounted for about 5% of the natural mortality of gadids (pollock and cod) and up to 75% of the natural mortality of hexagram­ mids (adult Atka mackerel). These two groups of species were consumed in higher amounts than any other. The predicted average daily food require­ ment per individual ranged from 16 (±2.8) to 20 (±3.6) kg (all ages com­ bined). Per capita food requirements differed by as much as 24% between regions of Alaska depending on the rel­ ative amounts of low–energy-density prey (e.g. gadids) versus high–energy­ density prey (e.g. forage fish and salmon) consumed. Estimated require­ ments were highest in regions where Steller sea lions consumed higher proportions of low–energy-density prey and experienced the highest rates of population decline. Manuscript accepted 20 September 2002. Fish. Bull. 101:147–167 (2003). Prey consumption of Steller sea lions (Eumetopias jubatus) off Alaska: How much prey do they require?