Effects of density, PVC-tubes and feeding time on growth, stress and aggression in African catfish (Clarias gariepinus)

Successive exposure to aquaculture-related stressors may compromise the allostatic capacity of African catfish and lead to allostatic overload and poor welfare. Therefore, we tested the effect of (i) feeding during the light or dark phase, (ii) density (51 fish per 140 L versus 51 fish per 43 L) and (iii) altered available resting space/shelter on stress handling were studied in African catfish (Clarias gariepinus). Available resting space/shelter was manipulated by providing PVC-tubes in the tanks. Growth, baseline stress load (cortisol and osmolality), metabolism (glucose, lactate and non-esterified fatty acids) and aggression (measured by skin scar incidence) were assessed. Upon completion of the experimental period, we determined the adaptive capacity of the fish by the response to air exposure. We show that night-feeding enhances growth and lowers feed conversion ratio compared to day-feeding. No effects of density were found for baseline stress hormones, basal metabolism or aggression. Low density seems to stimulate aggressive behaviour after air exposure. Unexpectedly, providing PVC-tubes under high-density conditions increased aggression, raised baseline cortisol levels and decreased basal metabolism as well as growth; also the endocrine response to air exposure was stronger. We argue that these effects relate to a reduction in available space, i.e. a further increase in density, or to an increased competition for shelter. Overall, our data suggest that night-feeding optimizes growth and that care should be taken in using objects as shelter/enrichment for African catfish