Total body length (cm, A), body mass (g, B), coefficient of variance (CV) of body length (C) and CV of body mass (D) of male and female zebrafish kept at different stocking densities, 1 fish/L (n = 148), 3 fish/L (n = 442) or 6 fish/L (n = 852). Values are shown as individual fish (A,B) and individual tanks (C,D) with mean marked as line and SD in whiskers. ***p < 0.001 compared to fish kept at 3 or 6 fish/L.

The number of aggressive acts per week performed by zebrafish during 9 weeks (W1–W9, A–I) of rearing at different stocking densities with or without environmental enrichment. **p < 0.01 compared to fish kept at 3 or 6 fish/L. Values are shown as individual tanks with mean marked as line and SD in whiskers.

Cortisol secretion to surrounding water by zebrafish kept at different stocking densities with or without environmental enrichment. Groups of fish were subjected to confinement by lowering of the water level to 1 cm for 30 min. Values are shown as individual tanks with mean marked as line and SD in whiskers. ***p < 0.001 compared to fish kept at 3 or 6 fish/L.

Cortisol secretion after confinement stress in 50 mL Falcon tube for 30 min by individual zebrafish kept at different stocking densities with or without environmental enrichment. Values are shown as individual fish with mean marked as line and SD in whiskers.