Impact of precocious male parr on the effective size of a wild population of Atlantic salmon

Abstract

Summary 1. There is growing evidence that sexually mature but morphologically juvenile males of Atlantic salmon (precocious or mature male parr) actively participate in reproduction and, therefore, in the genetic composition of the populations of this species. The impact of mature male parr on the effective population size (Ne) of such populations has been previously studied under experimental settings, but no studies have been performed directly on natural populations. 2. Continuous monitoring and sampling of all sea returns is possible in the Lérez River (northwest of Spain). From demographic data on variances of reproductive success and genetic data from six microsatellite marker loci we carried out parentage assignment and assessed the impact of male parr on demographic and genetic estimates of Ne in two consecutive years. 3. Our results reveal that: (i) approximately 60% of the total sire paternity is attributable to mature parr; (ii) mature parr decrease the variance of reproductive success of males by a threefold factor and increase the effective population size of males by a 10‐fold factor; (iii) however, they do not substantially affect the variance of reproductive success and the effective size of females; (iv) mature parr increase two‐to threefold the overall effective size of the population but the ratio Ne/N, where N is the population size including or not mature parr in each case, is not affected.

Breeding success of four male life history types of spring Chinook Salmon spawning in an artificial stream

“…The infusion of genes from these males would increase the effective size of a recipient population and therefore help retain its genetic diversity and variation (Saura et al 2008). They could also be used as hatchery broodstock.…”

Age Structure, Changing Demography and Effective Population Size in Atlantic Salmon (Salmo salar)

“…Mature male parr reproduction has been speculated to be beneficial for N e (Saunders and Schom 1985;L'Abée-Lund 1989;Martinez et al 2000;Valiente et al 2005;Juanes et al 2007), although mature parr may ''increase'' effective size only in the sense thatN e is increased compared to when mature parr are ignored in analyses (e.g., Jones and Hutchings 2002;Saura et al 2008). Nevertheless, the reasoning implies that parr reproductive contribution increases N e by reducing the variance in (male) individual reproductive success.…”

Estimation of census and effective population sizes: the increasing usefulness of DNA-based approaches

“…Kinds of N e estimators and concepts Effective population size is whatever must be substituted in the formula (1/2N) to describe the actual loss in heterozygosity Sewall Wright (1969) Contemporary N e can be estimated using demographic (direct) methods (Caballero 1994) or genetic (indirect) methods (Table 2). Demographic estimators often overestimate the true N e because demographic estimators seldom include all the factors, such as variance in reproductive success, which can reduce N e compared to the N C (but see Saura et al 2008 for an exception)…”