Abstract
Understanding how aquatic species respond to extremes of DO and temperature is crucial for determining how they will be affected by climate change, which is predicted to increasingly expose them to levels beyond their optima. In this study we used novel animal-borne DO, temperature and depth sensors to determine the effect of extremes of DO and temperature on the vertical habitat use of Atlantic salmon Salmo salar in aquaculture cages. Salmon showed a preference for temperatures around 16.5 to 17.5 °C, however, selection of preferred temperatures was trumped by active avoidance of low DO (<35% saturation) at the bottom of the cage. In addition to low DO, salmon also avoided warm surface waters (>20.1 °C), which led to a considerable contraction in the available vertical habitat. Despite their avoidance behavior, fish spent a large amount of time in waters with suboptimal DO (<60% saturation). These results show that vertical habitat contraction could likely be a significant consequence of climate change if the reduction in DO outpaces the increase in hypoxia tolerance through local adaptation. They furthermore highlight that site-specific environmental conditions and stock-specific tolerance thresholds may need to be considered when determining stocking densities.
Highlights
Oxygen is the final receptor in the electron transport chain underlying the aerobic respiration of most organisms[1]
The aim of this study was to use newly developed acoustic telemetry tags which record swimming depth and environmental dissolved oxygen (DO) and temperature experienced by Atlantic salmon Salmo salar in aquaculture cages in Macquarie Harbour, Tasmania, Australia to determine the behavioral response of salmon to seasonal extremes of DO and temperature during the hottest summer on record in Tasmania[23] and identify the trade-offs between the Number of fish
Fish in cage 1 swam on average approximately 1 m deeper than fish in cage 2 and experienced cooler mean temperatures and lower DO values than fish in cage 2. In both cages fish spent a considerable amount of time in waters with DO values between 35 and 60% (Fig. 1) and in cage 2 the distribution of DO values experienced by the fish resembled a uniform distribution with a lower bound of 30 to 40% and a higher bound of 95 to 100% whereas the temperatures experienced by the fish were approximately normally distributed, centered around a mean of 16.5 to 17.5 °C (Fig. 1)
Summary
Oxygen is the final receptor in the electron transport chain underlying the aerobic respiration of most organisms[1]. Temperature is the most important environmental factor governing the metabolic rate in ectotherms[4] and the effects of DO and temperature on marine fish are intrinsically linked as higher temperatures increase the metabolic rate and oxygen demand and lower the solubility of oxygen, reducing supply[5] Together, these two environmental variables have a profound effect on the ecology of marine species, from the growth and health of individuals[6] to the spatial distribution of populations and species[5]. Studies of the response of salmonids to extreme DO and temperature values in the field have been carried out using echosounders to determine swimming depth of salmon in aquaculture cages in concurrence with high resolution environmental sensors[17,18,19]. While more intrusive than echosounders, biotelemetry tags have been used successfully to measure DO21 and temperature values[22] experienced by free-swimming salmonids
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