FEEDING ENERGETICS AND METABOLISM IN DEMERSAL FISH SPECIES FROM ANTARCTIC, TEMPERATE AND TROPICAL ENVIRONMENTS

The energetics of feeding has been investigated in demersal fish with similar sedentary lifestyles from the Antarctic (Notothenia neglecta Nybelin), North Sea (Myoxocephalus scorpius L.) and Indian Ocean (Cirrhitichys bleekeri Bleeker). In general, the metabolic rates of fasting individuals were positively correlated with adaptation temperature: values for a standard 100 g fish (mg 02/h) were 3.3 for N. neglecta at around 0-degrees-C, 2.7 for winter-acclimatized M. scorpius at 5-degrees-C, 4.3 for summer-acclimatized M. scorpius at 15-degrees-C, and 7.0 for C. bleekeri at 25-degrees-C. In all species, following a single satiating meal, oxygen consumption increased to a peak of 2 to 3.5 times the fasting values. Maximum rates of oxygen consumption after feeding were several-fold higher in the warm-than in the cold-water species. After controlling for the effects of body mass and energy intake by analysis of covariance, the duration of the increase in metabolic rate, referred to as ''specific dynamic action'' (SDA), was found to be 3 to 4 times shorter in the warm- than in the cold-water fish, ranging from 57 h in C. bleekeri to 208 h in N. neglecta. In contrast, the SDA was not significantly different in the various species, corresponding to 15 to 23 % of the energy ingested. Seasonal influences on metabolism and feeding were also studied in N. neglecta acclimated to simulated winter (-1.0 to -0.5-degrees-C; 3 h light:21 h dark) or summer (0 to 0.9-degrees-C; 21 h light:3 h dark) conditions. The metabolic rates of fasting and fed individuals, and the characteristics of the SDA were found to be independent of acclimation conditions. This suggests that N. neglecta is capable of processing food at similar rates throughout the year. Energy stores and enzyme activities were measured in the swimming muscles and liver of fish fed ad libitum. Summer-acclimated fish had higher concentrations of liver triglyceride stores and elevated activities of some enzymes of intermediary metabolism relative to winter-acclimated fish. The observed changes in intermediary metabolism are probably related to annual cycles of growth and reproduction. It is suggested that the low aerobic scope for physiological performance in Antarctic fish may necessitate the seasonal switching of energy allocation between growth and reproduction.