Effects of density, PVC-tubes and feeding time on growth, stress and aggression in African catfish (Clarias gariepinus) Jeroen G J Boerrigter 1 , Ruud van den Bos 1 , Hans van de Vis 2 , Tom Spanings 1 & Gert Flik 1 1 Organismal Animal Physiology, IWWR, Radboud University Nijmegen, Nijmegen, The Netherlands 2 IMARES, Wageningen UR, Yerseke, The Netherlands Correspondence: J G J Boerrigter, Organismal Animal Physiology, IWWR, Radboud University Nijmegen, Heyendaalseweg 135, 6525 AJ Nijmegen, The Netherlands. E-mail: J.Boerrigter@science.ru.nl Abstract Successive exposure to aquaculture-related stres- sors 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 osmolal- ity), metabolism (glucose, lactate and non-esteri- fied fatty acids) and aggression (measured by skin scar incidence) were assessed. Upon completion of the experimental period, we determined the adap- tive 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 aggres- sive 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 shel- ter. Overall, our data suggest that night-feeding optimizes growth and that care should be taken in using objects as shelter/enrichment for African catfish. Keywords: African catfish (Clarias gariepinus), stress, aggression, density, enrichment, growth Introduction The growing human population increasingly demands food, e.g. protein. One of the growing sources for protein is fish. The quantity of fish caught in the wild has stabilized over the past years (FAO 2012). In contrast, the aquaculture sector is fast growing (FAO 2010, 2012) and expected to do so till 2025 (Diana 2009). In 2012, the sector provided 47% of the world’s fish consumption (FAO 2012). The impact of increas- ing numbers and volume of fish farms on the envi- ronment and welfare of fish in aquaculture causes public concern (Grigorakis 2010; Bovenkerk & Meijboom 2012; EATIP 2012; Bergqvist & Gun- narsson 2013). Sustainable aquaculture is a focus point of the European Union (COM 299 2013); noteworthy, animal welfare is an element of sus- tainability (Gamborg & Sandøe 2005). Animal welfare is defined in different ways (Spruijt, van den Bos & Pijlman 2001; Korte, Oli- vier & Koolhaas 2007; Broom 2011; Hagen, van den Bos & de Cock Buning 2011; Ohl & van der Staay 2012). Key to all definitions is that poor welfare is associated with overtaxing the coping capacity of animals (allostatic overload; McEwen & Wingfield 2003). Allostatic overload results from chronic stress and leads to pathologies and mortal- ity (Koolhaas, Bartolomucci, Buwalda, de Boer, Flugge, Korte, Meerlo, Murison, Olivier, Palanza, Richter-Levin, Sgoifo, Steimer, Stiedl, van Dijk, Wohr & Fuchs 2011). Successive or cumulative © 2015 John Wiley & Sons Ltd 1 Aquaculture Research, 2015, 1–16 doi: 10.1111/are.12703