Abstract
The Norwegian production of Atlantic salmon (Salmo salar) has since 2012 been stable, mainly due to problems with salmon lice (Lepeophtheirus salmonis). To combat the lice, lice-grazing cleaner fish has been deployed into the net-pens. The most used cleaner fish today is lumpfish (Cyclopterus lumpus), which of the majority stems from commercial farming. Nevertheless, the lumpfish production has been characterised by variable growth and survival, especially in the larval stage. This is due to the fact that there is poor knowledge about the species’ biology and its nutritional needs. There are also only a few studies that have taken a closer look at the lumpfish’ liver, which is necessary to interpret the fish’ nutritional status, as this is an important organ in the digestive system.
The purpose of the present study was to contribute to optimizing start-feeding regimes for lumpfish larvae in commercial farming. This was achieved by comparing effects of different start-feeding regimes with respect to the growth and survival of the fish larvae. In addition, the nutritional status of the fish was examined by histological analysis of the liver. A total of five different start-feeding regimes were examined, where the larvae were introduced to the feeding regimens at 2 days post hatch (dph) and lasted until 35 dph. Larvae from one group received enriched Artemia before weaned to formulated diet, while another group were given cirripeds (Semibalanus balanoides) before the weaning. Two larval groups were fed copepods (Acartia tonsa) in the beginning, of which one group had an early weaning to formulated diet, whereas the other group were given cirripeds and then weaned to formulated diet. The last group were given formulated diet throughout the whole start-feeding experiment. Common for all the feeding regimes were weaning to the same formulated diet before the experiments end at 35 dph.
Lumpfish larvae fed enriched Artemia had the best growth and survival throughout the experiment and showed early signs that excess energy was stored in the form of vacuoles in the liver, the latter in contrast to the other larval groups. Larvae fed cirripeds grew slow and had a slight lower survival rate before the weaning to formulated diet but ended up at almost the same size as the larvae fed Artemia due to a higher growth rate at weaning. In contrast, the larvae fed cirripeds had few signs of high nutritional status in the liver before weaning, as they had significantly smaller hepatocytes and a lower vacuole fraction than the larvae fed Artemia. This suggests that the rapid growth was due to an increased ability to digest and assimilate the nutrients in the formulated diet after the period fed cirripeds. Both groups given copepods had lowered growth and survival compared to the larvae fed Artemia and cirripeds, especially the group that had early weaning to formulated diet. Despite the fact that the other group received cirripeds after the copepod-period, this group did not show the same tendency for rapid growth during weaning to formulated diet like the larvae fed only cirripeds prior to weaning, indicating that the copepod-period was the reason for this. Larvae fed formulated diet throughout the whole experiment also had lower growth and survival than the groups given Artemia and cirripeds. The results therefore indicate that a late weaning to formulated diet after a longer period of live feed could be beneficial, and that perhaps a combination of Artemia and cirripeds before formulated diet could be an optimal diet for lumpfish larvae. At the experiments end, when all the larval groups received the same formulated diet, the larvae showed no significant differences in neither hepatocyte size, hepatocyte nucleus size nor vacuole fraction. Thus, it is evident that changes in the liver are reversible up to a certain point and can give an indication of what the larvae eats. On the other hand, the hepatocyte size and vacuole fraction may be better suited than hepatocyte nucleus size in evaluating the lumpfish larvae’s nutritional status, as the findings among the former two correlated better with the fish’ growth and survival during the experimental period.