Decline of North Atlantic eels: a fatal synergy?

Abstract

Panmictic species pose particular problems for conservation because their welfare can be addressed effectively only on a global scale. We recently documented by means of microsatellite analysis that the European eel (Anguilla anguilla) is not panmictic but instead shows genetic isolation by distance. In this study, we extended the analysis to the American eel (A. rostrata) by applying identical analytical procedures and statistical power. Results obtained for the American eel were in sharp contrast with those obtained for the European eel: the null hypothesis of panmixia could not be rejected, and no isolation by distance was detected. This implies that the species must be managed as a single population. Using Bayesian statistics, we also found that the effective population sizes for both species were surprisingly low and that the populations had undergone severe contractions, most probably during the Wisconsinan glaciation. The apparent sensitivity of eels to climatic changes affecting the strength and position of the Gulf Stream 20 000 years ago is particularly worrying, given the effects of the ongoing global warming on the North Atlantic climate. Moreover, additional short-term stresses such as surging glass eel prizes, overfishing and lethal parasitic infections negatively affect eel population size. The fascinating transatlantic migration and life cycle of Atlantic eels is also their Achilles' heel as these negative short-and long-term effects will probably culminate in a fatal synergy if drastic conservation measures are not implemented to protect these international biological resources.

Panmixia in European eel revisited: no genetic difference between maturing adults from southern and northern Europe

“…In comparison, studies of genetic structure in other Anguilla species with a similar life history have given mixed results. Like our present results, no deviations from apparent panmixia have been found in A. rostrata (Wirth and Bernatchez, 2003), A. reinhardtii (Shen and Tzeng, 2007a) or A. japonica (Ishikawa et al, 2001; N Yoshizawa et al, in preparation; but see Tseng et al, 2006), whereas genetically distinct subpopulations have been identified in A. australis (Shen and Tzeng, 2007b), A. marmorata (Minegishi et al, 2008) and A. bicolor (Y Minegishi et al, in preparation).…”

Panmixia in the European eel: a matter of time…

“…Wirth & Bernatchez (2003) used the procedure of Beaumont (1999), which estimates several genealogical and demographic parameters from microsatellite data using Bayesian statistics, and presented values of the current effective size of the European eel stock that ranged between 1050 and 6000. These values correspond to remarkably low numbers of spawning eels that succeed in reproduction each year.…”

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

“…Molecular studies in both species have demonstrated that they are panmictic Côté et al, 2013). Remarkably, although mitochondrial DNA lineages of the two species are reciprocally monophyletic (Avise et al, 1986), low differentation is found at microsatellite loci, with F ST values of 0.055 and 0.018 reported in previous studies (Mank and Avise, 2003;Wirth and Bernatchez, 2003).…”

“…Nevertheless, even very low gene flow between the species may be biologically significant and could explain the surprisingly low genetic differentiation between North Atlantic eels observed in previous studies (Mank and Avise, 2003;Wirth and Bernatchez, 2003). 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.…”

“…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).…”