During 2006, plasma hormone analyses from large-scale feeding trials on rainbow trout and Atlantic salmon have generated new data showing that plasma GH levels are affected by dietary inclusion of plant proteins, which is reflected by the growth performance and condition factor of the fish. Further, the results show that circulating plasma levels of the novel “adiposity hormone” ghrelin increase in fish that are in a compensatory growth phase. Plasma ghrelin levels are related to the condition and growth of the fish, and seem to respond to dietary protein levels, depending on energy status. Plasma IGF-I levels respond to changes in dietary protein levels and oil source. There also seems to be a strong seasonal effect on plasma GH, IGF-I and ghrelin levels. These experiments provide new information on how growth and metabolic regulatory hormones respond to dietary composition at different seasons, and the effect of the energy status on lipid deposition and growth performance of salmon fish. Further, the demonstration of the presence of the rainbow trout GHS-R (the ghrelin receptor) in the hypothalamus in the brain, stomach, intestine and vagal nerve in rainbow trout, supports that ghrelin may function in appetite regulation and food processing in rainbow trout.
Research on the importance of collagen crosslinking for flesh firmness has been extended to Atlantic halibut. Crosslinks are even more important for flesh firmness in halibut than salmon with the concentration of the mature trivalent crosslink, hydroxylysyl pyridinoline, explaining 67% of the variation in fillet firmness. Research on the genes controlling fibre number has resulted in the characterisation of a novel gene in Atlantic salmon and several other fish species which we have named and registered as Cee (conserved edge expressed gene). This highly conserved gene, from worms to man, has a complex expression pattern in muscle and nervous tissue during development. Its expression is strongly up-regulated following the inhibition of myotube formation in the fast muscle of several fish species, making it a candidate for the control of fibre number. Fibre number affects the density of muscle fibres which is positively correlated with fillet firmness in salmon and halibut.
The activity regarding water soluble components in feed ingredients, has focused on nitrogen compounds that may have beneficial effects for fish and for fish as a healthy food for humans. The activity has documented a large difference in the content in various feed ingredients as taurine and anserine. Feed ingredients based on marine origin have in general a much higher level of these potential bioactive components. This is interesting when expensive fish meal with limited availability is exchanged with vegetable protein sources. The performance of feed for fish and also the impact of farmed fish as healthy food for humans may be improved by optimizing the level of small nitrogen-compounds in the feed. These small molecular weight compounds are further shown to have significant positive affect on growth and feed efficiency in a feeding experiment with trout. The eating quality of fish filet from trout fed low levels of small water-soluble compounds is not affected, however, as evaluated by the filet content of taurine and anserine. These compounds seem to be homeostatically regulated. Tailor-made trout with high filet level of taurine and anserine can therefore not be produced by changing the dietary level of taurine and anserine. On the other hand, fish may be fed high levels of vegetable protein sources without affecting the eating quality as evaluated by the filet level of taurine and anserine.