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To test the countergradient variation (CnGV) hypothesis and determine any genetic differences among populations, we conducted an experiment with juvenile cod originating from 3 regions in the NW Atlantic, 3Ps (Placentia Bay, Newfoundland, Canada; 48°N, 54°W), 4T (southern Gulf of St. Lawrence; 46°N, 61°W), and 4X (44°N, 67°W), at 2 temperatures (7 and 11°C) for 15 wk. Results indicated that temperature influenced the growth rates of individuals in all 3 populations, and juveniles reared at 11°C were significantly bigger (length and weight) than those reared at 7°C. No significant differences in growth and food conversion efficiency were found among the 3 populations. Although 3Ps juveniles were significantly smaller in length and lighter in weight than 4T and 4X juveniles at the start of the experiment, length and weight did not differ among populations at the end of the experiment. Survival of both 3Ps and 4T juveniles was significantly higher than that of 4X cod in both high and low water temperature treatments. Although statistical analyses showed no significant differences in growth and food conversion efficiency among juveniles from the 3 populations, reaction norm analysis showed a strong genotypic influence on growth and survival (no genotype × environment interactions) but a genotypic × environment interaction in food conversion efficiency. Our results suggest that Atlantic cod juveniles from these 3 regions do not show CnGV in growth and food conversion efficiency. However, performance differences among populations may be stage specific, and this may have masked CnGV in traits among Atlantic cod populations in our study.

The effect of changing temperature on muscle fatty acid composition was examined in juvenile steelhead trout (~120 g) with complete substitution of fish oil (herring oil; HE) with either flax seed oil (FLX; also known as linseed oil) or sunflower oil (SF). The temperature was increased from~10.0°C to 18.0°C following seasonal temperature changes in~2°C incremental steps with plateaus. Dietary lipid analysis showed the HE diet was rich in marine lipids, docosahexaenoic acid (DHA, 22:6ω3), eicosapentaenoic acid (EPA, 20:5ω3) and docosapentaenoic acid (DPA, 22:5ω3). In contrast FLX was rich in linolenic acid (ALA, 18:3ω3) and SF in linoleic acid (LA, 18:2ω6). Both diet and temperature had no significant effect on growth but significant effects were observed on muscle fatty acid composition. Proportions of marine fatty acids in HE fed fish were significantly higher than in both SF and FLX fish at the end (41, 31 and 31% respectively), with significantly lower levels of terrestrial plant fatty acids compared to the beginning of the experiment. Substitution of SF and FLX had a direct influence on muscle EPA:AA and ω6:ω3 ratios which could be detrimental to the health of the fish, as well as to the quality of the end product for human consumption. Aquaculture 433 (2015) xxx-xxx DOI of original article: http://dx.Please cite this article as: Wijekoon, M.P.A., et al., Reprint of "Effect of dietary substitution of fish oil with flaxseed or sunflower oil on muscle fatty acid composition in juvenile..., Aquaculture (2015), http://dx.

Fish regulate cellular membrane fluidity in response to temperature by restructuring membrane lipid composition. This study evaluated the effect of diet and temperature on liver membranes in steelhead trout, an important aquaculture species. Oncorhynchus mykiss were fed three commercial diets with different levels of PUFA (lower (L‐n3); medium (M‐n3); higher (H‐n3) omega 3) from marine, terrestrial, and vegetable oils; the effect of temperature changes on liver membrane was measured. Fish fed the H‐n3 diet had the most linear response in Raman spectroscopy, indicating that they can adapt to changes in temperature with the least effect on liver membrane, due to the higher polyunsaturate:saturate ratio in the diet, counteracting the influence of low temperature. L‐n3‐fed fish presented increased membrane fluidity at all temperatures, highlighting the influence of terrestrial fatty acids on membrane properties. These results underscore changes in sterol:phospholipid ratios as a key response for membrane adaptability to environmental changes, and the necessity to include environmental variables when testing new diets. Substitution of fish oil with vegetable oils may compromise sterol:phospholipid ratios, affecting membrane adaptability. This study shows changes at cellular level in liver tissue for fish fed different diets and subjected to different water temperatures.