Stocking homogeneous size groups does not improve growth performance of Senegalese sole (Solea senegalensis, Kaup 1858) juveniles: Individual growth related to fish size

Growth of homogenous sized groups was studied in captive populations of Senegalese sole (Solea senegalensis, 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, respectively. 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 (evaluated in terms of delta coefficient of variation, ΔCV) 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 establishment 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.