The effect of acute hypoxia on swimming stamina at optimal swimming speed in flathead grey mullet Mugil cephalus

Flathead grey mullets Mugil cephalus are commonly found in Mediterranean lagoons, which are regularly subject to high environmental variations. Oxygen is one of the factors that shows extremely high variation. The objective of this study was to test the effects of acute hypoxia exposure at two experimental temperatures (i.e. 20 and 30°C) on the stamina (time to fatigue) in M. cephalus swimming at the minimal cost of transport (i.e. optimal swimming speed; Uopt). At each temperature, a relationship was established between swimming speed and oxygen consumption (MO2). This allowed estimation of Uopt at 45 cm s−1 (~1.12 Body Length s−1). Independent of temperature, stamina at Uopt was significantly reduced in severe hypoxia, i.e. at 15% of air saturation (AS). In these conditions, oxygen supply appears therefore to be insufficient to maintain swimming, even at the low speed tested here. After the stamina test, MO2 measured in fish tested at 15% AS was significantly higher than that measured after the test in normoxia. Therefore, we suggest that in hypoxia, fish used anaerobic metabolism to supplement swimming at Uopt, leading to an oxygen debt. Since flathead grey mullet is a hypoxia-tolerant species, it is possible that hypoxic conditions less severe than those tested here may reduce stamina at low speed in less tolerant species. In addition, we suggest that testing stamina at these speeds may be relevant in order to understand the effect of hypoxia on behavioural activities carried out at low speed, such as food searching.