Growth performances of juvenile sole Solea solea under environmental constraints of embayed nursery areas

Tidal embayments in the Bay of Biscay (France) host nursery grounds where common sole, Solea solea, is the most abundant flatfish species. This study aimed to appraise the way those habitats function as nurseries through juvenile sole's responses in somatic growth and condition (Fulton's K) during their first year of occupancy. Field data, two yearly trawling series, taken monthly, were compared with a 6-month-long mesocosm experiment involving reared fish of the same wild origin. Growth rates were compared with predicted maximum growth according to an experimentally established model in relation to temperature. In the field, 0-group sole total length (TL) averaged 130 mm from September onwards in 1999 and 2000. Mean growth rates were 0.7-0.9 mm day-1 in summer and 0.1 mm day-1 in autumn, when the model predicted 1.4 and 0.9 mm day-1, respectively. In the mesocosm, the growth rates were 0.9 and 0.4 mm day-1 for the same seasons and for stocking densities (300 fish 1000 m-2) about 10 times higher than usually recorded in the field. Wild sole remained in medium condition (K ca. 1 g cm-3) during the entire growing season, whereas they attained a higher K after being released into the mesocosm (1.2-1.3 g cm-3). During the mesocosm experiment, growth was shown to be primarily temperature-controlled: fish reached 150-160 mm (TL) in November. From this investigation, it can be concluded that sole's responses in condition and growth are altered over the whole growing season on nursery grounds. The situation is exacerbated in autumn when the sum of abiotic constraints increases as the competitive biomass does. This indicates impairment of the nursery functioning which, in the context of fish habitat conservation, emphasises the particular vulnerability of embayments to the addition of any further constraint