| Abstrakt: |
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-1in summer and ? 0.1?mm?day-1in 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-1for 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. Les baies du golfe de Gascogne (France) h?bergent des habitats o? la sole, Solea solea, est le plus abondant des poissons plats. Cette ?tude vise ? ?valuer le fonctionnement des habitats pour leur r?le de nourriceries, en analysant la croissance et la condition des juv?niles de sole pendant leur premi?re ann?e. Les donn?es de deux s?ries, 1999 et 2000, de chalutages mensuels sont compar?es ? celles acquises lors d''une exp?rience de 6 mois en m?socosme impliquant l''?levage de poissons originaires de la m?me nourricerie. La croissance observ?e est compar?e ? la croissance potentielle, en utilisant un mod?le exp?rimental d?crivant la croissance en fonction de la temp?rature. Les soles du groupe d''?ge 0 captur?es dans la nourricerie atteignent une taille moyenne de 130?mm en septembre. Le taux de croissance est de 0,7?0,9?mm?jour-1en ?t? et ?0,1?mm?jour-1en automne, alors que le mod?le pr?dit respectivement 1,4 et 0,9?mm?jour-1. En m?socosme, la croissance aux m?mes saisons est de 0,9 et 0,4?mm?jour-1pour des densit?s (300?soles 1000?m-2) environ 10 fois sup?rieures ? celles observ?es dans la nourricerie. Les soles sauvages maintiennent une condition moyenne (K env. 1 g cm-3) pendant toute la saison de croissance alors que ces m?mes poissons atteignent un facteur K plus ?lev? pendant l''?levage en m?socosme (env. 1,2?1,3?g?cm-3). La croissance en m?socosme est d?pendante principalement de la temp?rature : les poissons atteignent 150?160 mm (TL) en novembre. Nous montrons ainsi que la croissance et la condition des soles sur la nourricerie sont alt?r?es pendant toute la saison de croissance. Cette situation est amplifi?e ? l''automne lorsque les contraintes abiotiques et la comp?tition sont accrues. Ceci montre une d?gradation du fonctionnement de la nourricerie et indique la vuln?rabilit? de ces baies face ? toute contrainte suppl?mentaire. [ABSTRACT FROM AUTHOR] |
