Stocking density affects Senegalese sole (Solea senegalensis, Kaup) growth independently of size dispersion, evaluated by an individual photo-identification technique. * Pablo S´ anchez, Pedro Pablo Ambrosio and Rosa Flos Departament d’Enginyeria Agroaliment` aria i Biotecnologia Universitat Polit` ecnica de Catalunya Xarxa de Refer` encia en Recerca i Desenvolupament en Aq¨ uicultura de la Generalitat de Catalunya September 2011 Abstract To assess how initial size dispersion affects Senegalese sole (Solea senegalensis, Kaup) growth, 128 fish were stocked either under homogeneous (HOM) or heterogeneous (HET) initial size dispersion (standard length, CV 11.8% and 29.7% respectively), and also under low (LD) and high (HD) stocking density (60% and 180% of bottom occupation). After 105 days, growth parameters, evaluated at an individual level by linear mixed-effects models, were not significantly affected by initial size dispersion, LD fish growing significantly faster than HD fish. Variance of individual growth rate distributions of weight and standard length were lowest for LD-HOM fish, indicating the most regular growth, and highest for LD-HET. The diminution of the coefficient of variation along time in HET groups (between 13% and 15.4% for weight, and between 4.3% and 5.8% for standard length), and its slight decrease in the HOM groups (between 1% and 3% for both parameters), is consistent with the absence of strong competition between individuals. —Keywords: Senegalese sole, Stocking density, Individual growth, Social hierarchy. 1 Introduction Stock size composition can be behind aggressive behavior and competition over resources as food or territory, promoting the onset of hierarchies and affecting fish growth in several ways (Gunnes, 1976; Irwin et al., 1999, 2002; Dou et al., 2004; Hatlen et al., 2006). Large size variation is still an issue in the aquaculture of Senegalese sole (Solea senegalensis, Kaup; Dinis et al., 1999; Flos et al., 1995, 2001; Rueda-Jasso et al., 2004; S´ anchez et al., 2010), and the influence of initial size distribution on growth has been little studied (Salas-Leiton et al., 2010). Besides genetic differences between individuals, environ- mental parameters such as feed availability (Hatlen et al., 2006) or stocking density (Irwin et al., 1999) among others could be behind differences in growth through the development of complex social interac- tions, from which strong hierarchization can emerge. Mechanisms linking stocking density and growth are not fully understood, but it is generally accepted that, when water quality is not affected by density, and food is provided in sufficient amounts, differences in growth performance could be attributed to the * This work has been accepted for publication in Aquaculture Research (2011); doi:10.1111/j.1365-2109.2011.03026.x 1