The effect of progressive hypoxia on spontaneous activity in single and shoaling golden grey mullet Liza aurata

The hypothesis of a differential effect of hypoxia on activity in shoaling v. solitary fish was tested in golden grey mullet Liza aurata. In both solitary and shoaling fish, (1) swimming activity increased significantly at < 60% air saturation compared with normoxia, (2) aquatic surface respiration increased significantly below 15% air saturation and (3) a significant increase in ventilation frequency was observed between 40 and 10% air saturation. Swimming activity was higher in shoaling than in solitary fish, possibly due to social interactions in shoaling fish. Despite showing a higher swimming activity, shoaling individuals showed similar ventilation frequency to the less active solitary fish. This suggests that shoaling exerts a calming effect on L. aurata, probably related to increased safety while in numbers. In addition, shoaling fish spent a higher proportion of time performing aquatic surface respiration at 15% air saturation (i.e. the highest oxygen level at which aquatic surface respiration was observed) than solitary fish, possibly because of (1) synchronization of aquatic surface respiration in shoaling fish and (2) lower risk of predation perceived by shoaling fish.