THE EFFECT OF PROLONGED SWIMMING ACTIVITY ON THE GROWTH, PROXIMATE BODY-COMPOSITION AND CALORIFIC CONTENT OF O-AGE GROUP WHITING (MERLANGIUS-MERLANGUS L, GADIDAE)

Juvenile whiting were kept in four groups of 60 specimens each in a fl ow-through system for two weeks. Two of the four experimental groups w ere fed with chopped smelt at a ration of 7.5% bw . d(-1), and the oth er two groups were not fed at all. From each fraction (the fed and the non-fed fish) one group was kept swimming at 1 body length per second (bl . s(-1)) against a constant current in a raceway; the others rema ined in group compartments where they swam only marginally. (In total there were four groups: two swimming constantly, one of them fed, the other not; and two non-swimming groups, one fed and the other not). As expected, the condition factor of the groups which were fed increased , whereas that of the non-feeding groups decreased. After having swum constantly for two weeks, the feeding fish were slightly heavier than the reference group which was also fed but: not swimming. The better g rowth of the swimming fish was due to increased muscle growth with a h igher dry weight than that of the non-swimming fish. The livers were f ound to have a lower dry weight, and a lower energy and fat content. I t is concluded that the growth-enhancing effect of prolonged exercise on whiting is not the result of increased water content in the muscles , but reflects true growth of the biomass. The gross- and net-cost of transport (G-COT, N-COT) for an 15g whiting swimming at 1 bl . s(-1) ( forced swimming) was found to be 8895 J . kg(-1) km(-1) (= 2430 J . kg (-1). h(-1)) and 5602 J kg(-1) km(-1) (= 2625 J . kg(-1). h(-1)) respe ctively. However, swimming at 0.2 to 0.3 bl . s(-1) (undirected, spont aneous swimming) was relatively more costly than forced swimming and d emanded 2846 J . kg(-1). h(-1) (Gross Cost per Swimming Hour) and 1307 J . kg(-1). h(-1) (Net-Cost per Swimming Hour). By means of compariso n, no difference between the calculated G-COTs derived from regression on data from respirometry and the results from direct body calorimetr y was found, even though the data derived from body calorimetry were s cattered much less than those from respirometry.