Demographic and genetic structures of white sea bream populations (Diplodus sargus, Linnaeus, 1758) inside and outside a Mediterranean marine reserve.

We studied the white sea bream (Diplodus sargus), a protandrous hermaphroditic fish, in two protected and unprotected areas in southwestern France. We observed a significant difference in the demographic structure between the two areas. Females were present in two different age distributions inside and outside the marine reserve with younger females outside. This suggests plasticity in the age of sexual inversion in the case of an exploited population. Genetic differentiation was weak and apparent at only one locus of 26 surveyed (FST = 0.007, p = 0.04). Our data suggest that gene flow between the two areas is important, or the separation between the two sites is recent. Our data on the white sea bream show that fishes inside and outside the marine reserve are very similar genetically, which means that the 'reserve effect' is truly a demographic one, not the result of genetic differences.