SIMULATION OF THE NORTH ATLANTIC OCEAN DRIFT OF 



ANGUILLA LEPTOCEPHALI 



James H. Power 1 and James D. McCleave 2 

 ABSTRACT 



A numerical simulation model of surface current drift was developed in order to simulate the poorly un- 

 derstood drift migration of Anguilla leptocephali in the North Atlantic Ocean. The model was based upon the 

 advection- diffusion equation, which was approximated by finite differences. Currents for the model were 

 calculated from ships' drift data. Leptocephali were "started" at various points in the presumed American 

 and European eel spawning areas, and the model produced spatiotemporal patterns of leptocephalus con- 

 centrations resulting from surface current drift and turbulent diffusion. In the American eel drift simulations 

 the patterns followed a sequence of four phases: 1) Initial northwest drift on the presumed Antilles Current; 

 2) the formation of a "patch" of leptocephali offshore of Florida and the Gulf Stream; 3) dispersal along the 

 North American coast resulting from the continued input of larvae intothe Gulf Stream from the patch; and 

 4) transport eastward into the Atlantic on the Gulf Stream. In the European eel drift simulation, the lep- 

 tocephali slowly spread throughout the Sargasso Sea region of the North Atlantic, and there was little Gulf 

 Stream transport by the eighth month of drift. The patterns of distribution produced by the model correspond 

 well with the limited collection data for both species, though it remains for future sampling efforts to verify 

 whether the features present in the simulations actually occur. 



Schmidt (1925) summarized over two decades of 

 work to provide what has since become known as the 

 "classical solution" to the Atlantic eel problem. 

 Schmidt proposed that adult European eels, An- 

 guilla anguilla, and American eels, A. rostrata, 

 migrate in the fall from their freshwater habitats and 

 travel to spawning areas in the Sargasso Sea. The 

 adults spawn in the early months of the year and then 

 die. The resulting larvae (termed leptocephali) are 

 presumed to drift passively on surface currents 

 toward their respective coasts. Schmidt stated that 

 American eels, having a shorter distance to traverse, 

 drift about a year as leptocephali before meta- 

 morphosing to the glass eel phase and commencing 

 their migration toward freshwater. European eel lep- 

 tocephali are presumed to take 3 yr to complete their 

 journey. This scenario was challenged by Tucker 

 (1959), who hypothesized that the two Atlantic 

 anguillid eel species are in fact only one. Tucker pro- 

 posed that all adult European eels die during their 

 migration, that all anguillid leptocephali are the pro- 

 geny of eels originating in North America, and that 

 European eel stocks are replenished by leptocephali 

 that simply drifted across the Atlantic after failing to 



'Migratory Fish Research Institute and Department of Zoology, 

 University of Maine at Orono, Orono, Maine; present address: 

 Southwest Fisheries Center La Jolla Laboratory, National Marine 

 Fisheries Service, NOAA, P.O. Box 271, La Jolla, CA 92038. 



; Migratory Fish Research Institute and Department of Zoology, 

 University of Maine at Orono, Orono, ME 04469. 



Manuscript accepted December 1982. 

 FISHERY BULLETIN: VOL. 81, NO. 3, 1983. 



land on the North American coast. Tucker felt that 

 differences in the vertebral counts used to dis- 

 criminate between the two species could be ex- 

 plained by a thermal shock suffered by developing 

 embryos in part of the spawning area. Tucker's 

 hypothesis has been largely discounted, and recent 

 electrophoretic (Jamieson and Turner 1980; Com- 

 parini and Rodino 1980) and karyotypic (Passakas 

 1981) evidences indicate the existence of two 

 anguillid eel species in the North Atlantic and 

 associated freshwaters. 



Nonetheless, there are persistent unanswered ques- 

 tions concerning the migrations of larval, juvenile, 

 and adult eels (Vladykov 1964; McCleave and Har- 

 den- Jones 1979). One of these questions concerns 

 the location and timing of American and European 

 eel spawning. Schmidt (1925) identified the Euro- 

 pean eel spawning area as lying between lat. 22° to 

 30°N and long. 48° to 65° W, and stated that spawning 

 "commences in late winter or early spring and lasts 

 well on in summer." He based these limits on the dis- 

 tribution of the smallest leptocephali (<10 mm) he 

 collected. To the present time no adult eel has been 

 captured away from the continental shelves, and no 

 identified anguillid eel eggs have been collected. 

 Schmidt did not collect many small American eel lep- 

 tocephali, and consequently his delineation of the 

 American eel spawning area and time is much less 

 precise. Recently there have been several systematic 

 sampling efforts for small leptocephali with the ob- 



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