Influence of genetic drift on patterns of genetic variation: the footprint of aquaculture practices in Sparus aurata (Teleostei: Sparidae)

Aquaculture finfish production based on floating cage technology has raised increas‐
ing concerns regarding the genetic integrity of natural populations. Accidental mass
escapes can induce the loss of genetic diversity in wild populations by increasing
genetic drift and inbreeding. Farm escapes probably represent an important issue in
the gilthead sea bream (Sparus aurata), which accounted for 76.4% of total escapees
recorded in Europe during a 3‐year survey. Here, we investigated patterns of genetic
variation in farmed and wild populations of gilthead sea bream from the Western
Mediterranean, a region of long gilthead sea bream farming. We focused on the role
that genetic drift may play in shaping these patterns. Results based on microsatellite
markers matched those observed in previous studies. Farmed populations showed
lower levels of genetic diversity than wild populations and were genetically divergent
from their wild counterparts. Overall, farmed populations showed the smallest ef‐
fective population size and increased levels of relatedness compared to wild popula‐
tions. The small broodstock size coupled with breeding practices that may favour
the variance in individual reproductive success probably boosted genetic drift. This
factor appeared to be a major driver of the genetic patterns observed in the gilthead
sea bream populations analysed in the present study. These results further stress the
importance of recommendations aimed at maintaining broodstock sizes as large as
possible and equal sex‐ratios among breeders, as well as avoiding unequal contribu-
tions among parents.