24The natural food for Atlantic salmon (Salmo salar) in freshwater has relatively lower levels 25 of omega-3 (n-3) long-chain polyunsaturated fatty acids (LC-PUFA) than found in prey for

A common-garden experiment was carried out to compare two genetically distinct strains of Atlantic salmon (Salmo salar) fed diets with either high (CHO) or low (NoCHO) digestible carbohydrate (starch). Twenty salmon from either a commercial farmed strain (F) or a land-locked population (G) were placed in two tanks (10 fish of each population in each tank) and fed either CHO or NoCHO feeds. At the end of the experiment fish were fasted for 8 h, euthanized and blood and liver collected. Both diet and population had an effect on circulating glucose levels with G showing hypoglycaemia and dietary starch increasing this parameter. In contrast, G showed increased plasma triacylglycerol levels regardless of dietary treatment suggesting faster conversion of glucose to triacylglycerol. This different ability to metabolize dietary starch among strains was also reflected at a molecular (gene) level as most of the metabolic pathways evaluated were mainly affected by the factor population rather than by diet. The data are promising and suggest different regulatory capacities toward starch utilization between land-locked salmon and the farmed stock. Further analyses are necessary in order to fully characterize the capacity of land-locked salmon to utilize dietary carbohydrate.

Atlantic salmon populations across the world have diverse ecological and evolutionary histories, from wild anadromous or landlocked, to domestication and genetic modification. The natural host behaviours confer protection from infestation by ectoparasitic salmon lice Lepeophtheirus salmonis, yet whether genetic origin results in different behaviours and thus susceptibility to infestation is unknown. In common garden experiments, we tested antiparasite behaviours, susceptibility and retention of salmon lice in wild anadromous, wild landlocked, domesticated and genetically modified domesticated strains. Within domesticated strains, we tested two infestation histories (previously infested and naïve) and a new phenotype (albino colouring). Farmed stocks initially acquired 24%-44% higher levels of parasite density than the wild and landlocked strains. Burst swimming and displacement behaviours were higher in the domesticated groups, and jumping was more prevalent in the domesticated strains. At 34 days post-infestation, domesticated strains and the wild anadromous strain did not differ significantly from each other; however, landlocked salmon had increased infestation levels considerably. Domesticated strains lost ~20% (±9.9%-16.5%; 95% CI) of their initial parasite load, while parasite load increased by 5.5% (±30.1%) for wild salmon and 20.1% (±28.5%) in landlocked salmon. This study provides early evidence for diverged host-parasite interactions associated with domestication in this system.

Summary This study examined how triploidization and species hybridization affects skeletal related meristic characteristics in salmonids with the goal of understanding the maternal and paternal contribution. The study also investigated the hybrid sea louse infection rate in hybrid salmonids. In order to do so, a number of vertebrae, scales along the lateral line, and dorsal fin rays were measured in diploid and triploid Atlantic salmon, Arctic char, and Atlantic salmon (female) X Arctic char (male) hybrids. The success of triploidization was 100%, and some spontaneous triploid individuals (~15%) were found among the hybrids. In general, hybrids displayed intermediate counts for all three characteristics with salmon displaying the highest number of dorsal fin rays, and char displaying the highest number of scales and vertebrae. The effect of triploidization was always strongest among the hybrids. However, the direction of the effect differed between structures in the hybrid; triploidization increased the number of vertebrae and reduced the number of fin rays towards the level observed in diploid char, and reduced the number of scales towards the level observed in diploid salmon. We conducted a second experiment whereby diploid Atlantic salmon and Atlantic salmon (female) X Arctic char (male) hybrids were exposed to an experimental challenge with the parasitic salmon louse (Lepeophtheirus salmonis). There was no difference in the sea‐lice infection level between Atlantic salmon and the hybrids. This study shows that the present model with diploid and triploid Atlantic salmon (female) X Arctic char (male) hybrids may be a useful tool for the study of which traits are maternally and paternally inherited in salmonids. The understanding of morphological development with regard to meristic traits could be advantageous for both hybrid and triploid fish domestication.

The vertical distribution and seasonal timing of sexual maturation were investigated in Atlantic cod in four large commercial sea cages in Northern Norway during their second year of on-growth. Replicate cages were exposed to either natural light (NL) or continuous light (LL) from May 2007 to January 2008. Hydro-acoustic monitoring revealed that the cod in the NL cages were mainly distributed from 5 to 13 m depth at day while they were more dispersed and swam deeper at night. A clear ascent was observed prior to feeding events. The cod in the LL cages displayed a similar seasonal and daily pattern, but with a more dispersed vertical distribution range at both day and night. It is hypothesized that surface feeding motivation resulted in cod preferentially occupying the upper reaches of the sea cage, generally at depths < 13 m. This would also reduce the need for excessive alterations in swim bladder volume. Both the LL and the NL groups were exposed to LL the year before the study, resulting in first time sexual maturation (puberty) being delayed until the summer in both groups. However, while cod in the NL cages matured during the autumn 2007, the LL cod did not.