The critical oxygen threshold of Yellowtail Kingfish (Seriola lalandi)

Low concentrations of dissolved oxygen are one of the most limiting abiotic factors in land-based and marine aquaculture, impacting the welfare of target-species. Yellowtail Kingfish (Seriola lalandi) (YTK) is a high energy demanding species and its commercial aquaculture is rapidly expanding globally yet no information on its hypoxia tolerance is available. YTK is commonly cultured in sea pens, in which abiotic factors such as temperature and ambient oxygen can fluctuate substantially. The move away from marine fish oils to more sustainable terrestrial oil sources in aquafeeds implies a change in fatty acid intake. This shift in fatty acid concentrations and temperature fluctuations can impart physiological effects, impacting the animals stress tolerance. The critical oxygen threshold is a common method to quantify the lower, tolerated threshold of oxygen concentrations for an organism. This study assessed the critical oxygen threshold in fasted, juvenile YTK with respect to acclimation temperature (15 degrees C Sr 20 degrees C) and dietary lipid source (fish oil Sr poultry oil). Additionally, observations on the visual and behavioral hypoxia responses in YTK were made. This study demonstrated that YTK could regulate their oxygen consumption down to 2.92-1.84 mg dissolved oxygen L-1, but this strongly depends on the acclimation temperature, and to a lesser extent dietary oil source. At dissolved oxygen concentrations below this level, YTK became oxyconformers, unable to maintain an optimum rate of oxygen uptake. Warmer acclimation temperatures led to significantly less hypoxia tolerance compared to YTK held in cooler temperatures. Dietary oil source had no significant effect on the critical oxygen threshold; however, YTK fed a poultry-oil based diet displayed less hypoxia tolerance and greater deviation around the mean, attributing the non-significant difference to YTK fed a fish oil-based diet. Additionally, hypoxia triggered behavioral responses were initiated earlier in YTK fed the poultry oil diet. First behavioral responses, after passing the critical oxygen threshold, were attempted aquatic surface respiration, increased opercular frequency, and gulping, followed by darkening of skin coloration. We recommend rapid oxygenation of the rearing system if dissolved oxygen levels approach 2.92 mg L-1 at 20 degrees C or at first sign of these changes. Further onset, such as rush or rest behavior, may rapidly lead to the final stages of hypoxia. These results expand knowledge on YTK physiology and behavioral responses to low dissolved oxygen environments and provide information for farm managers to ensure adequate levels of dissolved oxygen throughout rearing, handling, bathing or transportation procedures.