The response of striped surfperch Embiotoca lateralis to progressive hypoxia: Swimming activity, shoal structure, and estimated metabolic expenditure

When exposed to moderately low O-2 conditions (75-30% O-2 saturation), shoaling striped surfperch Embiotoca lateralis show no changes in swimming activity, uniform levels of metabolic expenditure and an unchanging shoal structure. As O-2 levels become more hypoxic surfperch reduce their locomotor activity, initially with a decrease in turning rate at 20% O-2 saturation, followed by a reduction in swimming speed at 15% O-2 saturation. Reductions in activity occur in combination with individuals adopting more uniform turning behaviors, which translate into an increase in the distance traveled per unit time (i.e. net displacement). This increase in net displacement may enhance the ability of surfperch to distance themselves from discrete zones of hypoxia. Measures of shoal structure, including nearest neighbor distances and shoal polarity, were constant throughout low O-2 exposure despite changes in swimming activity. Estimation of the energetic costs of these behaviors reveals that metabolic expenditure (routine metabolic rate) remained steady at similar to 120 mg O-2 kg(-1) h(-1) between 100 and 30% O-2 saturation, dropping to similar to 108 mg O-2 kg(-1) h(-1) at 20 and 15% O-2 saturation. Given that surfperch possess a critical oxygen saturation (S-crit) of 15.2%, reductions in swimming activity and metabolic energy expenditure clearly occur as individuals reach their aerobic metabolic limits. These results identify that surfperch demonstrate a multi-faceted coping strategy when exposed to low oxygen conditions, which may prove advantageous when the species experience hypoxic episodes in their natural habitat. (C) 2014 Elsevier B.V. All rights reserved.