The Physiology of Smoltification and Seawater Adaptation in Rainbow Trout

Abstract

Seawater-transferred rainbow trout (Oncorhynchus mykiss) has become an attractive aquaculture product in recent years. Industrial interest is mainly due to its resistance to infectious pancreatic necrosis and its adaptability to brackish water, which allows farming in otherwise unexploited locations. However, most practices for the aquaculture of this species have been imported from Atlantic salmon (Salmo salar) without evidence data supporting their suitability for the species. These include protocols to induce their preadaptation to seawater (smoltification). As a result, following seawater transfer, large numbers of fish die (around 10%) or become growth-stunted (GS; 10-60%). Therefore, species-specific rearing protocols and seawater-readiness biomarkers are needed. In the present PhD thesis, the effects of different photoperiod and temperature protocols for rearing in freshwater were assessed on the development of smoltification traits and subsequent seawater performance. This was achieved by using an array of molecular tools to measure osmoregulation-, growth- and haematopoiesis-related genes, proteins and hormones. Moreover, the discovery of potential seawater-readiness biomarkers and the study of smoltification, seawater adaptation and GS fish development were performed using several mass spectrometry proteomic and lipidomic approaches. Results suggest that winter light signals are inadvisable for the species, while all tested summer signals produced similar good results. Moreover, increased temperature protocols not only failed to improve smoltification and growth but potentially compromised the immune system of the fish. Overall, continuous light seems an advisable light regime, irrespective of temperature. Moreover, new promising potential biomarkers for seawater-readiness were identified using proteomics, while also suggesting a previously unknown role of these proteins in smoltification and seawater adaptation. Finally, GS development was shown to be related to low insulin-like growth factor 1 levels following seawater transfer. Moreover, other related factors to the phenotype were higher stress levels, possibly caused by bullying by bigger fish, and hepatic anomalies related to oxidative stress.