The Energy Cost Of Free Existence For Bering Sea Harbor And Spotted Seals

Abstract

Energy-flow models were developed to assess the net and gross energy requirements of natural populations of Bering Sea harbor seals (Phoca vitulina richardsi) and spotted seals (Phoca largha), and to estimate their impact on two commercially-important fishes, walleye pollock (Theragra chalcogramma) and Pacific herring (Clupea harengus pallasi). Energy requirements were estimated from long-term studies of food consumption and proximate composition, food energy content and digestibility, and metabolic effects of temperature, feeding, activity, molt, and reproduction in captive representatives of each species. Captive seals adjusted their ad libitum intake of food to maintain caloric equivalence with diets of varying fat content. The mean digestible energy of pollock and herring was 96.7 (+OR-) 0.2% and 91.2 (+OR-) 0.7% of gross ingested energy, respectively, and the estimated net energy available from both diets was 80.3% of gross energy. Basal metabolism of both seal species remained constant with season and declined with age. Maximal metabolic effort in water was achieved with harbor seals carrying an 8-kg weight load at an oxygen consumption rate of 32.8 (+OR-) 2.8 ml O(,2)/kg(.)min, or approximately four times basal rate. Metabolism during molt in harbor seals was about 10% less than pre-molt levels, accompanied by a rise in plasma cortisol and decline in plasma thyroxine. Reproductive energy costs were estimated at 2.4 x 10('5) and 2.2 x 10('5) kcal for individual harbor and spotted seals, respectively. The annual gross energy required by both populations combined was estimated at 5.6 x 10('11) kcal, corresponding to an annual consumption of 8.16 x 10('4) metric tons of pollock, 5.17 x 10('4) metric tons of capelin (Mallotus villosus), 3.73 x 10('4) metric tons of herring, and 4.61 x 10('4) metric tons of invertebrate species, four important prey of these seals. These results suggest that the annual pollock and herring intake of both populations may be about 9% and 20%, respectively, of the present commercial take of these fishes.