A century of research on the larval distributions of the Atlantic eels: a re‐examination of the data

Abstract

The spawning areas of the Atlantic freshwater eels were discovered about a century ago by the Danish scientist Johannes Schmidt who after years of searching found newly hatched larvae of the European eel, Anguilla anguilla, and the American eel, Anguilla rostrata, in the southern Sargasso Sea. The discovery showed that anguillid eels migrate thousands of kilometers to offshore spawning areas for reproduction, and that their larvae, called leptocephali, are transported equally long distances by ocean currents to their continental recruitment areas. The spawning sites were found to be related to oceanographic conditions several decades later by German and American surveys from 1979 to 1989 and by a Danish survey in 2007 and a German survey in 2011. All these later surveys showed that spawning occurred within a restricted latitudinal range, between temperature fronts within the Subtropical Convergence Zone of the Sargasso Sea. New data and re-examinations of Schmidt's data confirmed his original conclusions about the two species having some overlap in spawning areas. Although there have been additional collections of leptocephali in various parts of the North Atlantic, and both otolith research and transport modelling studies have subsequently been carried out, there is still a range of unresolved questions about the routes of larval transport and durations of migration. This paper reviews the history and basic findings of surveys for anguillid leptocephali in the North Atlantic and analyses a new comprehensive database that includes 22612 A. anguilla and 9634 A. rostrata leptocephali, which provides a detailed view of the spatial and temporal distributions and size of the larvae across the Atlantic basin and in the Mediterranean Sea. The differences in distributions, maximum sizes, and growth rates of the two species of larvae are likely linked to the contrasting migration distances to their recruitment areas on each side of the basin. Anguilla rostrata leptocephali originate from a more western spawning area, grow faster, and metamorphose at smaller sizes of <70 mm than the larvae of A. anguilla, which mostly are spawned further east and can reach sizes of almost 90 mm. The larvae of A. rostrata spread west and northwest from the spawning area as they grow larger, with some being present in the western Caribbean and eastern Gulf of Mexico. Larvae of A. anguilla appear to be able to reach Europe by entering the Gulf Stream system or by being entrained into frontal countercurrents that transport them directly northeastward. The larval duration of A. anguilla is suggested to be quite variable, but gaps in sampling effort prevent firm conclusions. Although knowledge about larval behaviour is lacking, some influences of directional swimming are implicated by the temporal distributions of the largest larvae. Ocean-atmosphere changes have been hypothesized to affect the survival of the larvae and cause reduced recruitment, so even after about a century following the discovery of their spawning areas, mysteries still rema...

The ecology of oceanic dispersal and survival of anguillid leptocephali

“…Thin Layer of Muscle (myomeres) Esophagous Gall Bladder/Stomach Shinoda et al 2011;Kuroki et al 2014;Feunteun et al 2015;Miller et al 2009Miller et al , 2015aMiller et al , 2015bMiller et al , 2016Pepin 2016), so only selected or recent literature is emphasized here. With increased worldwide concern over the welfare of anguillid eels (Jacoby et al 2015), our aim is to facilitate a better understanding of their larval dispersal and ecology.…”

“…Anguillid leptocephali grow to sizes of about 55 mm in tropical species and about 60-90 mm in temperate species, with those of A. anguilla being by far the largest, at about 70-90 mm (Kuroki et al 2014;Miller et al 2015a). In comparison, some marine eel leptocephali grow to sizes >250 mm (Miller et al 2013b).…”

“…Their transparent leptocephali must in many cases move across vast distances of the oligotrophic ocean to reach their recruitment areas, and low survival of their poorly understood larval stage has been hypothesized to influence recruitment levels (Knights 2003;Bonhommeau et al 2008aBonhommeau et al , 2008bMiller et al 2016). The leptocephali of the American eel (Anguilla rostrata) and the European eel (Anguilla anguilla) become widely distributed after the adults spawn in the southern Sargasso Sea of the western North Atlantic (Schmidt 1922;McCleave 2003;Miller et al 2015a). Their larvae then move to each side of the North Atlantic subtropical gyre where they either recruit to North America (A. rostrata) or Europe and North Africa (A. anguilla) (Tesch 2003).…”

“…For example, the spawning areas of the Atlantic eels discovered by Schmidt (1922) were further studied by surveys starting in 1979 as overviewed recently (Miller et al 2015a). In the western North Pacific, after the spawning area was discovered in 1991 (Tsukamoto 1992), sampling surveys for eggs, prefeeding stage preleptocephali, and adults have confirmed that A. japonica spawns in a narrow area along the southern tip of the West Mariana Ridge in the few days before new moon periods Aoyama et al 2014).…”

Animal Navigation: The Eel’s Magnetic Guide to the Gulf Stream

“…Under these conditions, low gene flow increases genetic differentiation, and especially so if population size remains low for several generations. European eels experienced a major collapse in recruitment that occurred during the early 1980s and the population failed to recover its population size to the pre-decline levels [20]. Together with their newly discovered geomagnetic-sensing ability, scenarios of population structure at the Sargasso sea driven by habitat fragmentation become plausible.…”