Influence of environmental temperature and water oxygen concentration on gas diffusion distance in sea bass (Dicentrarchus labrax, L.)

Gas diffusion distance (GDD) of sea bass was measured in fish bred under farm conditions, at different dissolved oxygen concentrations (DO): normoxia condition (80–100% of the saturation value) and `mild' hyperoxia condition (120–130% of the saturation value). Measures were carried out two times in a year (beginning of summer and autumn) in order to evaluate the effect of water temperature on GDD at the two different dissolved oxygen concentrations. There was a significant influence of both dissolved oxygen concentration (p 0.001) and environmental temperature (p 0.001) on GDD. In summertime it was 1.75 m and 2.31 m for fish reared under normoxia and hyperoxia, respectively, and in autumn 2.51 m and 2.96 m for fish reared under normoxia and hyperoxia, respectively. When DO was reduced at the higher temperatures, GDD decreased as well. Results lead to the conclusion that GDD increased with the increasing of DO, both due to reduced water temperature and to the mild oxygen hypersaturation following application of pure oxygen. The advantage for fish may be found in the compromise between maximising O<sub>2</sub> diffusion at the gills and ions/water intake/loss, known as `osmoregulatory compromise'.