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Growth, fatty acid composition and Na⁺/K⁺-ATPase isoform physiology of juvenile chinook salmon (Oncorhynchus tshawytscha) fed diets supplemented with anchovy or blends of anchovy and canola oil Grant, Amelia Anne May

Abstract

This research was designed to assess the effects of alternate lipid diets on chinook salmon, Oncorhynchus tshawytscha, where canola oil (CO) was used as a vegetable lipid alternative to marine lipid. Diets supplemented with anchovy oil (AO) or CO blended with AO so that CO composed to a maximum of 41% dietary lipid content were fed to juvenile salmon twice daily. The salmon grew to a maximum of 86 grams over the study with no significant effect of diet treatment on fish growth. Similar results were obtained for an array of whole body physiological parameters including feed efficiency, hematocrit, percent muscle water and plasma Na+ and CI- concentrations. Examination of whole body fatty acid composition reflected the respective dietary lipid composition. Salmon were kept in freshwater during the majority of the study and-were periodically challenged to 24 hour seawater transfers. Gill physiology was assessed in response to the transfer and in relation to dietary treatment. Gill Na+/K+-ATPase, a dominant ATP- consuming enzyme present in chloride cells of the fish gill, was used as an indicator of salinity tolerance. Na+/K+-ATPase activity was measured and found not to be significantly different across dietary groups kept in freshwater or between salinity challenged groups. In the gill, two differentially expressed Na+/K+-ATPase isoforms, ala and alb were determined to differ in response to increased environmental salinity. Na+/K+-ATPase αla mRNA expression demonstrated sensitivity to seawater exposure and was down-regulated, while alb mRNA expression increased in response to salinity transfer. At the end of the freshwater phase of the study, salmon were transferred to full strength seawater. At this time, Na+/K+-ATPase physiology was assessed for dietary effects. Na+/K+-ATPase activity did not differ between salmon fed diets based on AO or the maximum amount of CO blended with AO . In response to increasing levels of CO in the diet, Na+/K+-ATPase αla mRNA levels did not change in fish exposed to freshwater or seawater. However, as CO content increased in the diet, Na+/K+-ATPase αlb mRNA decreased in a linear manner. Fish fed a diet containing only marine lipid had higher Na+/K+-ATPase αlb mRNA levels than those of fish ingesting the diet with the higher CO content. These data suggest that cellular fatty acids may play a role in Na+/K+- ATPase αlb expression in salmon held in freshwater. To determine the effect of time and diet on the osmoregulatory capacity o f these fish, Na+/K+-ATPase activity and isoform expression were analyzed using the fish fed with the two most extreme differences in fatty acid composition over 97 days. Na+/K+-ATPase αlb mRNA was found to be more highly expressed in fish fed a marine oil-based diet relative to those fish fed the diet with higher CO content during specific times during development. However, these molecular differences were not translated into detectable physiological differences among juvenile salmon. It is concluded that CO is a viable alternative to marine oils for juvenile chinook salmon provided essential fatty acid requirements of the salmon are met through inclusion of some marine oil.

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