Impact de la variabilité du recrutement sur le stock d'oursins commercialisables de l'archipel de Glénan (Sud-Bretagne)

In Brittany, the edible sea-urchin Sphaerechinus granularis (Lamarck) is at the northern limit of its range. Small-scale exploitation of this species occurs in the Glenan islands, in the bay of Concarneau (Finistere, France). Three extensive diving surveys undertaken in 1990, 1991 and 1992 show a significant variation of the stock which was estimated at between 700 and 1400 tonnes during this study. The constant level of the annual sea-urchin harvest suggests that these fluctuations may be attributed to variation in recruitment. This can be considered as non-existent in 1989 and 1992, substantial in 1990 and moderate in 1991. The causes of this variability are complex. It is certainly directly related to the phenology of the reproductive cycle (recruitment 1989) but also to the climatic conditions occurring during the phase of settlement. However, if unusually mild temperatures favour post-larval settlement, unfavorable thermic conditions (winters 1987 and 1991) do not inhibit this process.