Growth, energy consumption and prey density requirements in first feeding larvae of cod (Gadus morhua L.). In: The propagation of cod Gadus morhua L.: an international symposium, Arendal, 14 - 17 June 1983

Growth, oxygen uptake, swimming activity, feeding ability and energy content were investigated in cod larvae from several groups under different experimental conditions. At yolk absorption the larvae consumed 0.090-0.120 ul O2/larva/hour depending on the larval group examined. The larval energy consumption either measured as oxygen uptake or calculated from larval weight decrement, was of the same magnitude. The oxygen uptake was 20-30% higher under 100 lux illumination than in darkness, a difference which was of the same magnitude (20%) as the difference in weight decrement between larvae held at a constant 500 lux and larvae held in constant darkness. Light-adapted larvae showed a 6-10 times higher swimming frequency (bursts of swmming per min) than dark-adapted ones. The overall utilization of yolk for somatic growth varied between 65 and 70 % depending on the larval group examined. The utilization seemed independant of temperature and light intensity when these parameters were kept within the natural range observed during hatching in the Lofoten area. The specific energy content of eggs and larvae showed a steady decrease through the experimental period from 5.8 cal/ mq ash free dry weight in newly fertilized eggs to 4.8 cal/ mg ash free dry weight in larvae 12 days after hatchlng. The larvae showed maximum swimming speeds at days 5, 6 and 7 post-hatching (10-20 cm/min). There was a significant correlation between mean swimming speed (cm/min) and mean swimming frequency (number of swimming bursts/min) throughout the experimental period. The larvae showed maximum feeding success (22%) at day 7 post-hatching. The results are discussed in relation to larval prey density requirements at the onset of feeding