The work with leptin started by developing proteomic strategies for quantifying leptin in teleost species. A radioimmunoassay (RIA) procedure was established for Atlantic salmon on rainbow trout, and Arctic charr, but also for non-salmonid species such as Atlantic cod and turbot. This suggest that the RIA will be a powerful tool for quantifying circulating leptin in fish, and may have contribute substantially to our understanding of how fish regulate energy balance. Preliminary experimental data indicate similar, but unexpected results; that there is a negative correlation between plasma leptin levels and condition factor in Atlantic salmon, indicating that leaner fish have higher circulating levels of leptin. This is opposite to the situation in mammalian species, where there is a positive correlation between leptin levels and fat stores/body weight. Also, individually held rainbow trout that were treated with ghrelin decreased their food intake and subsequently growth rates, compared with control fish. Again, this is opposite to most other species where ghrelin stimulates appetite, but confirms our previous data that also centrally administered ghrelin suppresses food intake. Thus, the future clarifying of the endocrine regulation of food intake and energy balance in salmonid species represents an exciting challenge.
Muscle fibre density is a heritable trait that is positively correlated with flesh firmness. Research to identify the genes controlling fibre number has continued. Candidate genes have been identified using subtracted cDNA libraries and whole genome microarray experiments with RNA from myotube producing and post-fibre recruitment growth stages. In order to exclude candidates associated with changes in body-size that not related to the inhibition of myotube formation we are studying expression in two species, the rainbow trout and Atlantic salmon, which stop fibre recruitment at markedly different body sizes. Factors influencing the post-harvest quality of Atlantic halibut in collaboration with Aga Marin AS have been extended to seasonal investigations of proteolytic enzymes. Winter depletion and sexual maturation in males were shown to result in an increase in cathepsin H, cathepsin D and cathepsin B & L activities resulting in a decreased water holding capacity (WHC) of the flesh. The results indicate that halibut should be harvested in the autumn or early winter when WHC, texture and nutritional state are optimal.
During 2007 variation of the aromatase gene in Atlantic cod has been investigated. Aromatase has been shown to play important regulatory roles fish, in relation to sex determination, sex behaviour, gametogenesis and development of the central nervous system (CNS). Accordingly, aromatase can play a key role for quality, which is often dependent on the sexual stage of the individual. We identified a DNA microsatellite which affects the number of leucine residues in the protein. We screened 250 individual cod originating from five wild cod populations (Baltic Sea, North Sea, Northeast Arctic, Faeroe Island and Iceland) for variation at this microsatellite. Cod aromatase microsatellite variation was contrasted with 10 other random microsatellites not known to be associated with any genes. A much higher level of genetic differentiation among populations was found for the aromatase microsatellite compared to the random microsatellites. These results strongly suggest that there is differential local selection on cod aromatase, or alternatively but less likely, to other genes linked to aromatase. In particular between the North Sea and the Baltic Sea large genetic differentiation was found, most likely reflecting local selection driven by the highly different environments (e.g. salinity and temperature) found in the two areas. This case study strongly suggests that cod populations are adapted to local environmental conditions and, consequently, that genetic variation associated with quality is distributed among potential broodstocks for aquaculture.
A growth experiment with Atlantic cod was conducted using feeds containing different inclusion levels and fractions of salmon by-product hydrolysate. The experiment has been completed and the resulting growth and feed efficiency data have been collected and are currently under evaluation. Also, fish and feed samples are currently being analysed for the evaluation of the composition of fish fillets in specific potential growth and health promoting nitrogenous compounds such as free amino acids, with special attention to taurine and anserine.