Exploring the central and peripheral systems controlling appetite, food intake and digestion in Atlantic halibut (Hippoglossus hippoglossus) larvae- Tissue distribution, ontogeny and effects of food attractants

Atlantic halibut (Hippoglossus hippoglossus) is a cold-water belonging to the Pleuronectidae family and is native to Norwegian waters. Halibut is a high valued food species that have been overfished and wild-stock collapses, creating, therefore, a large potential species for the aquaculture industry. However, problems in the production cycle still exists as farmers have to rely on the use of live feed for ~40 days, which is expensive, labour intensive, and poses a risk of introducing bacterial contamination to the halibut larvae. In this context, he main aim of this thesis was to test if feed intake of inert particles (mimicking dry feed) can be stimulated in halibut larvae by adding attractant extracts in the rearing tanks. We used three different sources of attractants: water soluble extracts of Otohime, Gemma Micro and Northern prawn. Attractant extracts were given to halibut larvae together with food particles (Artemia nauplii cysts). Feed intake results, determined based on gut fullness visualization, showed no significant differences between the different attractant extracts and control groups of Atlantic halibut larvae at 10 and 20 days past first feeding (dpff). Subsequently, we decided to analyse the mRNA expression of genes known to be involved in control of feed intake, appetite and digestion in teleost species. We investigated the spatial (brain and gut) and temporal (10 and 20 dpff) expression profile of cholecystokinin paralogues (cck1 and cck2) and its receptors (cck1r, cck2r1 and cck2r2), neuropeptide y and peptide yy paralogues (pyya and pyyb). In addition, we also analysed how their expression profile was affect by feeding. Our results suggest that cck1 is involved in the regulation of digestion and acts as peripheral anorexigenic and central orexigenic factor, while cck2 seems to behave as a feed-forward signal, regulating digestive processes before feed ingestion. Npy mRNA expression indicated a central orexigenic role. The expression profile of pyyb indicate that this hormone is involved in the regulation of digestion and act as a peripheral anorexigenic factor. We hope that the results of this master’s thesis will contribute to a greater understanding of appetite, feed intake and digestion control of fish larva. When the key attractants for Atlantic halibut larvae have been identified, this will form a basis to formulate diets that optimize early weaning or enable formulated feeds from onset of exogenous feedin