COMPARISON BETWEEN THE LIFE-CYCLES OF 2 SOLEIDAE, THE COMMON SOLE, SOLEA-SOLEA, AND THE THICKBACK SOLE, MICROCHIRUS-VARIEGATUS, IN THE BAY OF BISCAY (FRANCE)

The life history of two Soleidae, the common sole, Solea solen, and th e thickback sole, Microchirus variegatus, were compared in the Bay of Biscay in an attempt to set out factors which could explain settlement styles known to be different between juveniles of each species. Commo n sole juveniles had been shown to depend on coastal and estuarine nur series, and the thickback sole to develop in open-sea nurseries, altho ugh the spawning grounds and spawning season of both species overlappe d (offshore and at springtime, respectively). For this study, data on adult, juvenile and larva distributions were obtained from cruises car ried out in the Bay of Biscay during the last decade. In addition, gro wth rate of larvae and planktonic interval duration, estimated by mean s of otolith increment analysis, were compared, as well as literature- derived information on behaviour of larvae. By comparing larval featur es, it appeared that thickback sole did not obtain the advantage of a slightly longer pelagic life span for an increased dispersal, due to a n early shift to benthic behaviour. Other evidence was given by more s pecifically stated distributions that the further and deeper offshore spawning of M. variegatus, compared to S. solen, was a likely key-fact or of the juvenile settlement process. This could explain why the thic kback sole, contrary to the common sole, never reach coastal areas and thus settle off shore, in waters deeper than 30 m.