T.F. Galloway, H. Kryvi, K. Hoehne, E. Kjorsvik
Larvae of cod (Gadus morhua L.) were fed rotifers (day 5-20) and Artemia nauplii (day 17-31) that were short term enriched, so that the DHA:EPA ratio was high (~2.0) or low (~0.2). The temperature was 8 C at hatching and was gradually increased to 12 C during the yolk sac stage. Treatment 1 (T1) received a high DHA:EPA ratio in both the rotifer and the Artemia phase, treatment 2 (T2) received a high ratio in the rotifer phase and a low ratio in the Artemia phase and treatment 3 (T3) got a low ratio in the rotifer phase and a high ratio in the Artemia phase. In this communication we report on the effect of these treatments on survival, increase in dry weight and muscle growth.
At the end of the experiment T1 had a 5% higher survival than T2 and T3. During the rotifer phase the relative growth rate was 8% per day in T1 and T2 but only 5% in T3. During the Artemia phase T1 and T2 grew by approximately 14% per day and T3 grew by 10% per day. T3 therefore had a significantly lower dry weight than T1 and T2 at day 31. A reduced DHA:EPA ratio in the rotifer phase therefore seemed to have a negative effect on growth in later stages, whereas this effect was not seen when the low DHA:EPA ratio was given in the Artemia phase.
Morphometric analyses of histological sections from the swimming muscle indicated that the reduced larval growth in T3 was related to a lower proportion of white muscle-fibres of a certain size. Also, at day 31 the white muscle fibres of larvae from T1 had a significantly higher volume fraction of myofibrils than T3. This could explain why the latter group had a lower weight at this stage.
(Dept. of Zool., Norwegian University of Science and Technology, Trondheim, Norway)