Reduced oxygen consumption after critical oxygen level does not immediately impair optimal swimming performance in the Pile Perch, Rhacochilus vacca

Abstract

Hypoxia is a stressor that occurs naturally in the environment. Hypoxic episodes are increasing globally due to anthropogenic nutrient loading and further exacerbated by global warming. This study examines the effect of progressive hypoxia, i.e. the continuous depletion of oxygen levels, on swimming performance at optimal speed of the pile perch, Rhacochilus vacca. R. vacca is a median-paired fin swimmer that exhibits obvious gate transition from pectoral fin swimming to anaerobic burst swimming using the caudal fin.. Oxygen consumption was measured during recovery from hypoxic conditions to determine whether an oxygen debt was associated with exposure to reduced O2 levels. The optimal swimming speed was estimated to be 1.7 bls-1, using strictly pectoral fin swimming, and gate transition occurred at 2.5 ± 0.13bls-1. When exposed to progressive hypoxia at optimal speed, critical oxygen tension (Pcrit) was determined to be 10.29 ± 4.07 kPa. When approaching Pcrit, burst swimming began at 7.07 ± 0.24 kPa O2. Pectoral fin beat frequency, significantly increased at 6.3-8.4 kPa oxygen saturation. Surprisingly, swimming speed was maintained beyond Pcrit, and the average oxygen levels at which fish quit swimming was 6.24 ± 0.30 kPa. There was a significant oxygen debt measured during recovery from hypoxia, which lead to a 200% increase in metabolic rate and took approximately two hours) to return to original levels. These unexpected results suggest that R. vacca are able to maintain optimal swimming speed in hypoxic conditions by resorting to anaerobic pathways, which leads to a significant oxygen debt.