Modeling the drift of European (Anguilla anguilla) and American (Anguilla rostrata) eel larvae during the year of spawning

Abstract

The distribution of the leptocephalus larvae of European (Anguilla anguilla) and American (Anguilla rostrata) eels collected during recent Sargasso Sea surveys was used to model larval drift. The drift trajectories of individual larva were back-calculated to the estimated time of spawning, using current data from two global oceanographic assimilation models. The results of both models give the same overall result; widespread spawning extended in time from December to March. The drift was also calculated forwards for approximately 1 year. The forward drift modelling showed that most leptocephali remained in the area south of the Subtropical Frontal Zone. One conclusion is that the majority of leptocephali remain trapped and possibly die in the retention area. A small proportion of leptocephali are entrained into the Gulf Stream system. An implication is that the spawning success may be highly sensitive to oceanographic and climatic factors that alter the dispersion of leptocephali out from the retention area. An alternative interpretation is that the surveys were made too late after the peak spawning period and that the core spawning area was missed.

The ecology of oceanic dispersal and survival of anguillid leptocephali

“…This type of strategy seems to make sense, because otherwise large anguillid leptocephali could frequently be retained offshore by eddies and may never re-enter the appropriate current systems to take them to recruitment habitats. Their possible need for active swimming to get out of the Sargasso Sea gyre was illustrated by a modelling study that found only a small proportion of A. anguilla leptocephali entered the Gulf Stream after 1 year (Westerberg et al 2017). It makes less sense, though, for premetamorphosis size leptocephali to use directional swimming because this would probably slow down their growth due to the extra energy expenditure and less time spent feeding.…”

Oceanic Environmental Effects on American Eel Recruitment to the East River, Chester, Nova Scotia

“…Although American Eels are panmictic (Côté et al 2013), genetically distinct ecotypes related to rearing habitat (brackish or saltwater and freshwater) occur (Pavey et al 2015). American Eels may begin spawning as early as mid‐January but do so mostly between February and April (Miller et al 2015), although Westerberg et al (2018) concluded that spawning occurs between December and March, peaking in early February. Spawning occurs at the western side of and between temperature fronts (northern temperature front at the 22.5°C isotherm) within the Subtropical Convergence Zone, the location of which may position larvae for access to west‐flowing currents (Kleckner and McCleave 1988; McCleave 1993; Friedland et al 2007; Munk et al 2010; Westerberg et al 2018).…”

“…American Eels may begin spawning as early as mid‐January but do so mostly between February and April (Miller et al 2015), although Westerberg et al (2018) concluded that spawning occurs between December and March, peaking in early February. Spawning occurs at the western side of and between temperature fronts (northern temperature front at the 22.5°C isotherm) within the Subtropical Convergence Zone, the location of which may position larvae for access to west‐flowing currents (Kleckner and McCleave 1988; McCleave 1993; Friedland et al 2007; Munk et al 2010; Westerberg et al 2018). Larval eels (leptocephali) migrate from the Sargasso Sea to the Florida Current, perhaps via the poorly defined, winter‐seasonal Antilles Current (Kleckner and McCleave 1985; McCleave 1993) but more generally at latitudes between northern Florida and South Carolina (about 30°N to 33°N).…”

“…Next was examined larval and glass eel migration times between spawning and stream arrival and during various migration segments. The total migration period was calculated as the time between the mean spawning period (estimated as February 6 from Westerberg et al [2018]) and the mean time of continental arrival based on the start of the run to the East River. Migration times to continental streams from the Gulf Stream were derived from the distances from the Gulf Stream to shore and glass eel swimming speeds from Wuenschel and Able (2008).…”

Paths to the unknown: dispersal during the early life of fishes

“…Another approach by Westerberg et al (2018) to locate the spawning area and time of European (Anguilla anguilla) and American (Anguilla rostrata) eels used backtracked trajectories and spawning times back-calculated from individual length and growth rates to show that a large portion of the larvae are trapped in a retention area south of the Subtropical Frontal Zone. There is large interannual variability in the leptocephali that escape and become entrained in the Gulf Stream system, due to prevailing oceanographic and climatic factors that alter the dispersion from the retention area.…”