An energy budget for juvenile cod, Gadus morhua L., in a Scottish loch

• Main Author: Soofiani, N. M.
• Published: University of Aberdeen 1983
• Subjects: 591.7
• Online Access: http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.593484

An energy budget derived from combined laboratory and field studies was prepared for juvenile cod in Loch Torridon fjordic sea loch on the West Coast of Scotland. Laboratory experiments were conducted on the growth of fish in relation to ration size at several temperatures. A positive linear relationship was established between specific growth rate and ration level for all size classes at each experimental temperature. Though, the size of fish had little effect upon the growth/ration relationship, temperature had a very significant effect. Highest growth for a given ration occurred at the lowest temperature. The logarithm of the maintenance ration increased linearly with temperature. Fish fed on higher rations showed higher condition factors, higher calorific content and lower water content. The growth of wild cod was measured by two independent methods; from the mean length at age and from tagging data. There were strong seasonal variations in growth rate and condition factor, both being higher in the summer months. Estimates of the daily food intake of wild cod were derived by two independent methods; from the growth rate and from stomach contents analysis. The former method generally yielded lower values, probably reflecting an underestimation of the growth in the wild. Both methods showed a pronounced seasonal variation in food intake, generally reaching a maximum in summer and autumn and declining during winter and spring. The mean food assimilation efficiency was 89.78%, 90.41% and 91.77% at 7°, 10° and 15°C. The energy lost through ammonia excretion accounted for 4.71%, 7.11% and 8.23% of the ingested energy at 7°, 10° and 15°C. A positive linear relationship was calculated between ration level and the costs of food processing at each experimental temperature. The metabolic rate was greatly increased for cod fed to satiation, resulting in a pronounced reduction in scope for activity, by between 83-97% depending on the temperature. Standard and active metabolism were investigated by measuring the oxygen consumption of fish under different conditions in the laboratory. The energy expended in activity in the wild was estimated by measuring a mean swimming speed for fish by acoustic tracking (0.09 B1 s-1) and determining the costs of swimming at this speed by comparison with oxygen consumption measurements on swimming fish in a tunnel respirometer. Discrepancies were observed in the energy budget prepared for the wild fish. The energy intake generally exceeded the energy expenditure. It is probable that the energy expended by the fish in locomotion was underestimated.