Effects of putative stressors in public aquaria on locomotor activity, metabolic rate and cortisol levels in the Mozambique tilapia Oreochromis mossambicus

Mozambique tilapia Oreochromis mossambicus were housed individually during 7 days in a continuous flow-trough respirometry system and daily exposed to one of three treatments: (1) a series of knocks on the side of the aquarium, (2) a series of photo-flashes and (3) control group. Exposure to photo-flashes did not change locomotor activity but decreased both nighttime and daytime oxygen consumption throughout the experiment. Knocking induced a shortlived increase in locomotor activity and tended to increase oxygen consumption, but this latter effect was not significant. Night-time oxygen consumption was not affected by knocking exposure. Cortisol levels assayed from fish-holding water collected at the end of the experiment were significantly lower in subjects exposed to photo-flashes than in subjects exposed to knocks or controls. Males did not respond differently than females to the treatments in any of the measurements taken. In summary, the data reported here suggest that exposure to repetitive photo-flashes, but not knocking, suppressed normal energy metabolism and cortisol levels. These effects were present hours to a half day after exposure to the flashes