Long-term stocking practices threaten the original genetic diversity of the southernmost European populations of Atlantic salmon Salmo salar

A Almodóvar,B Elvira,Gg Nicola,Jl Hórreo,S Leal,E García-Vázquez

doi:10.3354/esr01029

Abstract

Many Atlantic salmonSalmo salarpopulations in Europe are threatened by previous stocking with foreign hatchery strains. Temporal patterns of genetic characteristics of salmon from northern Spain, the southernmost European populations, were compared before and after species decline and heavy stocking with specimens from northern Europe. Eleven microsatellite loci were analysed in archival (scales from 1958-1960) and contemporary (2007-2008) samples from the River Sella. Temporal analyses revealed a similar heterozygosity between archival and contemporary samples, despite a drastic decrease in population abundance, while the contemporary sample showed a higher allelic richness due to the occurrence of foreign alleles. Considering only the alleles with at least 4% frequency in the archival sample, 2 alleles exclusive to the River Sella were absent in the contemporary sample, and 14 alleles showed a decrease of at least 4% frequency. Four alleles common in Scotland showed a high occurrence in the contemporary sample, so they are good candidates as markers of introgression of foreign genes. The heavy stocking with non-native Scottish broodstocks between 1970 and 1990 caused the introgression found in the contemporary sample when compared with the pristine population. An abrupt decrease was evident when the estimates of effective number of breeders were adjusted to take into account overlapping generations (NbAdj), effective population size (NeAdj) estimated fromNbAdj, and number of breeders estimated using the sibship assignment method (NbSIB). The very low effective size values found in the contemporary sample, together with the detrimental synergy between genetic drift and high rates of introgression, represent a severe risk for the conservation of native salmon.

Highlights

Preventing the loss of genetic diversity is essential for the conservation and long-term preservation of species, because high genetic diversity is a requisite for evolutionary adaption to ongoing rapid environmental change (Schindler et al 2010)
Eleven microsatellite loci were analysed in archival sample of Atlantic salmon Salmo salar, with an average successful amplification ranging from 88 to 99% and a variation between polymerase chain reaction (PCR) replicates of 96 to 100% depending on the locus
Our results indicate a deterioration of the River Sella population, since 3 main changes can be observed: (1) the loss of native and distinctive alleles, (2) introgression in the contemporary River Sella sample, and (3) a reduction in the effective number of breeders and effective population size

Summary

Introduction

Preventing the loss of genetic diversity is essential for the conservation and long-term preservation of species, because high genetic diversity is a requisite for evolutionary adaption to ongoing rapid environmental change (Schindler et al 2010). Human-induced changes in water temperature or quality, overfishing, and alterations in fluvial connectivity may result in steep reductions in populations as well as modifications of population structure (Hórreo et al 2014a). Another consequence of returning to their natal rivers is the minimal genetic flow among local populations, so salmon that inhabit neighbouring rivers are to a large degree reproductively isolated. This isolation facilitates the genetic differentiation among wild Atlantic salmon populations (Ayllón et al 2006) and creates the potential for local adaptation (Ciborowski et al 2007)

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