The Effects of Metabolic Depression Induced by Food Deprivation on Hypoxia Tolerance of Juvenile Rainbow Trout (Oncorhynchus mykiss)
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Hypoxic condition is a naturally occurring environmental stressor in aquatic ecosystems. However, due to modern anthropocentric activities, hypoxia has been increasing in prevalence and severity. Rainbow trout, a keystone species in many North American lakes, is hypoxia intolerant. As a result, this species is of particular concern when studying the effects of hypoxia on an organism’s physiological functioning. Chronic starvation was used as a tool to induce metabolic depression to determine the effect that depressed metabolic rate had on hypoxia tolerance. Juvenile rainbow trout were deprived of food for five weeks at 15oC. Each week, routine metabolic rate (RMR) and critical oxygen tension (Pcrit) were measured. Concomitantly, resting and post-hypoxia fish (8 h at ~50% air saturation) were sampled to measure metabolites in blood, liver and muscle, as well as enzyme activities in select tissues. Food deprivation resulted in a decrease in routine metabolic rate (RMR) and shift towards an increased reliance on aerobic metabolism. Pcrit decreased significantly following four weeks of food deprivation respectively, indicating that metabolic depression induced by food deprivation may confer an increased tolerance to low environmental oxygen concentration ([O2]). However, marginal metabolic scope (MMS), another indicator of hypoxia tolerance, did not change in response to metabolic depression. Furthermore, subjecting trout to O2 limitation resulted in mobilization of carbohydrates from the liver subsequently leading to hyperglycemia. This was likely a survival technique ensuring that if severe hypoxia ensues, anaerobic substrates are ready for transport to the necessary tissues.