Demographic history of two endangered Atlantic eel species, Anguilla anguilla and Anguilla rostrata

European (Anguilla anguilla) and American eel (A. rostrata) are panmictic catadromous fish species, which have experienced recent drastic population declines amounting to just a few percent recruitment relative to levels prior to 1980. However, little is known about the extent of recent population declines relative to historical fluctuations. We analyzed demographic histories of the species using a method for reconstructing skyline plots based on site frequency spectra, in this case derived from restriction site-associated DNA (RAD) markers. The results showed very high effective population sizes ranging in the millions for most of the time span covered. Both species experienced ancient declines coinciding with the time of speciation (ca. 170,000 generations ago) and at a later stage where secondary contact occurred (ca. 90,000 generations ago). Whereas the demographic histories of the species were similar most of the time, they followed widely different trajectories from ca. 70,000 to 40,000 generations ago. However, for the past ca. 30,000 generations both species have shown demographic stability, even across glacial and interglacial periods. We discussed the possible environmental factors, including ocean current changes and geomagnetism reversal that could have affected demographic history and further suggest that southward displacement of spawning regions and continental distribution could explain the apparent stability even during glaciations. The recent declines appear unprecedented against a backdrop of long-term demographic stability, underpinning concerns that low density of spawners in the huge spawning region could lead to detrimental Allee Effects.

Automated summary

European and American eel are catadromous fish species that have experienced recent population declines. The demographic histories of both species show ancient declines and stable periods, with possible environmental factors including ocean current changes and geomagnetism reversal.