Stocking may increase mitochondrial DNA diversity but fails to halt the decline of endangered Atlantic salmon populations

Abstract

Over the last 50 years, Spanish Atlantic salmon (Salmo salar) populations have been in decline. In order to bolster these populations, rivers were stocked with fish of northern European origin during the period 1974-1996, probably also introducing the furunculosis-inducing pathogen, Aeromonas salmonicida. Here we assess the relative importance of processes influencing mitochondrial (mt)DNA variability in these populations from 1948 to 2002. Genetic material collected over this period from four rivers in northern Spain (Cantabria) was used to detect variability at the mtDNA ND1 gene. Before stocking, a single haplotype was found at high frequency (0.980). Following stocking, haplotype diversity (h) increased in all rivers (mean h before stocking was 0.041, and 0.245 afterwards). These increases were due principally to the dramatic increase in frequency of a previously very low frequency haplotype, reported at higher frequencies in northern European populations proximate to those used to stock Cantabrian rivers. Genetic structuring increased after stocking: among-river differentiation was low before stocking (1950s/1960s F ST = -0.00296-0.00284), increasing considerably at the height of stocking (1980s F ST = 0.18932) and decreasing post-stocking (1990s/2002 F ST = 0.04934-0.03852). Gene flow from stocked fish therefore seems to have had a substantial role in increasing mtDNA variability. Additionally, we found significant differentiation between individuals that had probably died from infectious disease and apparently healthy, angled fish, suggesting a possible role for pathogen-driven selection of mtDNA variation. Our results suggest that stocking with non-native fish may increase genetic diversity in the short term, but may not reverse population declines.

Back to the future: museum specimens in population genetics

“…Initially collected for use in age determination of individuals, several extraction protocols have been developed for scales [35,40]. Because they are plucked from fresh material, dermic and epidermic cells remain attached to the scale and dry up sufficiently fast to prevent DNA degradation [74].…”

“…For example, the cryptic invasion of a non-native genotype of the common reed (Phragmites australis) in North America during the past century was detected by sequencing samples collected worldwide at two noncoding regions of the chloroplast [34]. Similarly, archival fish scales have been used to quantify the genetic consequences for wild fish populations of stocking or of aquaculture escapees [35,36]. Stocking of Spanish Atlantic salmon (Salmo salar) since the 1970s appears to have increased the mitochondrial DNA diversity of four endangered populations.…”

“…Stocking of Spanish Atlantic salmon (Salmo salar) since the 1970s appears to have increased the mitochondrial DNA diversity of four endangered populations. However, it failed to halt the ongoing population decline [35].…”

Use of multiple markers demonstrates a cryptic western refugium and postglacial colonisation routes of Atlantic salmon (Salmo salar L.) in northwest Europe

“…The dominance of the 'Atlantic' (BBBB and BBBA) mtDNA haplotypes in Spain and high frequency of these haplotypes in Britain and Ireland has been noted previously (Verspoor et al, 1999;Consuegra et al, 2002;Ciborowski et al, 2007;Campos et al, 2008). However, the dominance of the 'Baltic' (AAAA and AABA) haplotypes in northwest France has not been observed before, although none of the previous studies had sampled from this region.…”

Contrasting responses to selection in class I and class IIα major histocompatibility-linked markers in salmon

“…Fifth, the Atlantic salmon is one of the few species where MH polymorphism has been experimentally shown to be associated with resistance/susceptibility to specific pathogens (Grimholt et al, 2003). Furthermore, historical samples are widely available from Atlantic salmon populations, often as dried scales that are suitable for extraction of DNA, allowing analysis of temporal variation within populations (Nielsen et al, 1999;Consuegra et al, 2002;Ciborowski et al, 2007).…”