No apparent genetic bottleneck in the demographically declining European eel using molecular genetics and forward-time simulations

Abstract

The stock of the European eel is considered to be outside safe biological limits, following a dramatic demographic decline in recent decades (90-99% drop) that involves a large number of factors including overfishing, contaminants and environmental fluctuations. The aim of the present study is to estimate the effective population size of the European eel and the possible existence of a genetic bottleneck, which is expected during or after a severe demographic crash. Using a panel of 22 EST-derived microsatellite loci, we found no evidence for a genetic bottleneck in the European eel as our data showed moderate to high levels of genetic diversity, no loss of allele size range or rare alleles, and a stationary population with growth values not statistically different from zero, which is confirmed by finding comparable value of short-term and long-term effective population size. Our results suggest that the observed demographic decline in the European eel did not entail a genetic decline of the same magnitude. Forward-time simulations confirmed that large exploited marine fish populations can undergo genetic bottleneck episodes and experience a loss of genetic variability. Simulations indicated that the failure to pick up the signal of a genetic bottleneck in the European eel is not due to lack of power. Although anthropogenic factors lowered the continental stock biomass, the observation of a stable genetic effective population size suggests that the eel crash was not due to a reduction in spawning stock abundance. Alternatively, we propose that overfishing, pollution and/or parasites might have affected individual fitness and fecundity, leading to an impoverished spawning stock that may fail to produce enough good quality eggs. A reduced reproduction success due to poor quality of the spawners may be exacerbated by oceanic processes inducing changes in primary production in the Sargasso Sea and/or pathway of transport across the Atlantic Ocean leading to a higher larval mortality.

Detecting genome-wide gene transcription profiles associated with high pollution burden in the critically endangered European eel

“…A high contaminant burden could also impair normal reproduction or affect larval development, since lipids and lipophilic contaminants are mostly mobilized toward the gonads during the spawning migration (Palstra et al, 2006). This is in agreement with the recent genetic study of Pujolar et al (2011b), in which the observation of a stable genetically effective population size suggests that the eel crash was not due to low numbers but poor quality of spawners. Due to the detrimental effect of pollutants on fitness and fecundity, the spawning stock may fail to produce a sufficient number of good quality eggs, leading to a decline in the recruitment of the stock while not yet impacting the effective population size of the European eel.…”

Population Genomics of Marine Fishes: Next-Generation Prospects and Challenges

“…In European eel, for example, SNP markers developed from sequenced RAD tags were recently used to estimate longterm effective population size to range between 132,000 and 1,320,000 individuals (depending on the mutation rate used; Pujolar et al, 2013a). These estimates are markedly higher than those obtained in previous studies that applied a limited number of microsatellite markers (Wirth and Bernatchez, 2003;Pujolar et al, 2011). Differences between studies are potentially attributable to microsatellite mutational properties and the lack of adequate mutation models to describe microsatellite evolution, which might have biased previous estimates (Pujolar et al, 2013a).…”

Assessing patterns of hybridization between North Atlantic eels using diagnostic single-nucleotide polymorphisms

“…At equilibrium, F ST should be approximately equal to 1/(1 þ 4N e m) (Wright, 1931), where N e denotes effective population size and m is migration rate per generation. Estimates of long-term N e in Atlantic eels derived from microsatellite markers are on the order of 4000-10 000 (Wirth and Bernatchez, 2003;Pujolar et al, 2011), whereas a recent study employing RAD sequencing suggests that historical N e in European eel may be as high as ca. 100 000 to 1 000 000 (Pujolar et al, 2013).…”