Aquacultural Engineering 43 (2010) 108–113 Contents lists available at ScienceDirect Aquacultural Engineering journal homepage: www.elsevier.com/locate/aqua-online Stocking homogeneous size groups does not improve growth performance of Senegalese sole (Solea senegalensis, Kaup 1858) juveniles: Individual growth related to fish size E. Salas-Leiton ∗ , V. Anguís, A. Rodríguez-Rúa, J.P. Ca ˜ navate IFAPA Centro “El Toru˜ no”, CAP, Junta de Andalucía, Apartado 16, 11500 El Puerto de Santa María, Cádiz, Spain article info Article history: Received 17 May 2010 Accepted 10 August 2010 Keywords: Solea senegalensis (Kaup) Growth Size Grading Social structure abstract Growth of homogenous sized groups was studied in captive populations of Senegalese sole (Solea sene- galensis, Kaup) juveniles. An initial sole population was graded and three different sole stocks were obtained according to their respective size to form the homogenous sized experimental groups: small, medium and large, with mean weights (±SD) of 125.5 ± 22.3, 187.8 ± 34.0 and 285.9 ± 39.3 g, respec- tively. A fourth sole population was established from randomly selected juveniles from every stock to constitute the ungraded control subpopulation (mean weight of 155.1 ± 57.8 g). Fish were individually tagged and their specific growth rates (SGR) registered throughout two months. Growth of the three subpopulations originally graded in small, medium and large sized soles, was always similar (P > 0.05) at any time (approximately 0.50% day -1 ). After 60 days of culture, the SGR of the ungraded group was higher (P > 0.05) than that for the sole groups with more uniform size. Grading soles also affected to SGR distributions, revealing significantly narrower profiles (P < 0.05) from day 20 in graded groups with respect to the one that remained ungraded. A correlation between individual SGR and sole weight revealed the independency (P > 0.05) between fish size and respective growth. This lack of correlation between individual growth and fish size is in accordance with the observed tendency of every subpopulations to achieve similar size dispersal (evalu- ated in terms of delta coefficient of variation, CV) after 60 days of culture, regardless whether or not a previous grading was performed. Results from this work support that group heterogeneity improves the overall growth, probably associated to an efficient social arrangement. Furthermore, a hierarchy estab- lishment independent of fish size is indicated. It is suggested that fish size distribution is the consequence of the hierarchical structure, instead of the cause of it. Results obtained in present work might be valuable in the design of improved growing strategies for S. senegalensis. © 2010 Elsevier B.V. All rights reserved. 1. Introduction Size grading is a routine practice in intensive fish farming which purpose is to achieve homogenous size groups, optimizing tank management and growth performance. Size grading may have rel- evant consequences for the overall production of a farm. More especially it may improve the growth of the stock thereby sup- pressing the putative negative effect of established hierarchies. Studies on the effects of size grading on growth of juvenile fish have been performed in several commercial pelagic fish species such as Atlantic salmon (Salmo salar)(Gunnes, 1976), eel (Anguilla anguilla)(Kamstra, 1993), Arctic charr (Salvelinus alpinus)(Jobling and Reinsnes, 1987; Baardvik and Jobling, 1990), channel cat- ∗ Corresponding author. Tel.: +34 956 01 13 37, fax: +34 956 01 13 24. E-mail address: emilio.salas.ext@juntadeandalucia.es (E. Salas-Leiton). fish (Ictalurus punctatus)(Carmichael, 1994) and African catfish (Clarias gariepinus)(Martins et al., 2005). In flatfish, the influ- ence of size grading on growth has been studied both in turbot (Scophthalmus maximus)(Strand and Øiestad, 1997; Sunde et al., 1998) and Atlantic halibut (Hippoglossus hippoglossus)(Stefánsson et al., 2000) juveniles. Success in terms of growth in such stud- ies is however reduced. Growth was found even to decline in Arctic charr (Baardvik and Jobling, 1990) and turbot (Stefánsson et al., 2000) when size homogeneity of the reared population increased. Several authors have pointed out to a higher level of conflicts in social interactions as the main cause to explain lower overall growth rates when similar sized specimens are cultivated (Knights, 1987; Baardvik and Jobling, 1990; JØrgensen and Jobling, 1993). At this regard, since the effect of size grading on growth has been revealed as highly species-dependent, increasing knowl- edge referred to cultivated populations formed by homogenous sized fish would allow to evaluate if size sorting is an ade- 0144-8609/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.aquaeng.2010.08.001