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.