Promiscuity vs. Fecundity: How Diversity Can Compensate for Sexual Selection

The Nivelle River in Southwest France (43°N) holds a small Atlantic salmon population located near the southern limit of the species distribution in Europe. Returning adults have been monitored continuously since 1983; they are mostly grilse (1 winter at sea, 80% of the run), and multi sea winter salmon (MSW) which spend essentially 2 winters at sea. A large proportion of the male parr matures at one year (30%) or two years (100%) of age, before seaward migration.  Anadromous individuals from three cohorts (born in 1993, 1996 and 2000) and their putative anadromous parents (male and female adults present in the river during the breeding season in 1992, 1995 and 1999) were sampled for scales which were genetically analyzed at 6 non-coding hypervariable microsatellite loci to determine parentage. Age of maturation was heritable for females but not for males. In spite of large differences in fecundity (4200 eggs for grilse and 8500 for MSW) the fitness of the two types of females was about the same overall. However, taking into account the fact that a large proportion of grilse females (37.9% vs. 13.4% for MSW) had no identifiable offspring, the corrected fitness of grilse was larger than that of MSW females. Grilse mothers were less selective for mates than MSW females. Assuming that most of the unassigned fathers were mature male parr, grilse females had more progeny with unknown mates (48.9%) than MSW females (16.5%). Offspring of grilse mothers had a higher allelic richness than those of MSW mothers. Thus, with a higher individual fitness, grilse would be able to maintain an early maturation strategy in the Nivelle thanks to a higher diversity due to promiscuous behavior.