Factors influencing effective population size in commercial populations of gilthead seabream, Sparus aurata

Gilthead seabream (Sparus aurata) is one of the most important species among intensively reared fish in the Mediterranean region. Although there is a considerable interest in the genetic improvement of this species, many of the genetic parameters of commercial stocks have yet to be investigated. The effective population size (N-e) of a commercial gilthead seabream broodstock was determined using microsatellite analysis. Eight microsatellite loci were used to ensure a high confidence to the parental assignment of offspring from several mass-spawning events. The N-e was consistently low, ranging from 14 to 18, between photoperiod-controlled broodstock groups. This equated to an inbreeding rate of 2.7-3.5% per generation. The primary constraint on N-e was the high variance in family size and fewer males than females contributing to each spawning. The contribution of male parents to spawning was also more variable than females. Log-linear modelling of offspring counts revealed a significant quadratic relationship, with intermediate parental weights being optimum. The relationship between parental weight and contribution may be evidence of an optimum age structure to the broodstock, or stabilising selection. Low N-e in commercial stocks of gilthead seabreant could lead to an increased risk of inbreeding through the practice of broodstock replacement from within same-farm populations. (c) 2005 Elsevier B.V. All rights reserved.