FISHERY BULLETIN: VOL. 81, NO. 3 



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FIGURE 2. — Surface current vectors used in the simulations for the month of May. Vector in a cell 

 was computed by taking the mean of the current components at the cell's edges and calculating 

 the resultant vector. Key depicts representative current speeds. AC = Antilles Current; FC = 

 Florida Current; GS = Gulf Stream. 



irregularly spaced throughout the modeled area, so 

 their components were calculated by interpolating 

 between adjoining cells and months. Current veloc- 

 ities were necessarily taken as representative of the 

 currents that occur throughout the depth range of 

 Anguilla leptocephali. The leptocephali were 

 assumed to maintain themselves continually in sur- 

 face waters, so the finite differences were derived 

 with no assumptions regarding fluid continuity. The 

 effect of this is that leptocephali are concentrated in 

 regions of net water convergence (downwelling) and 

 dispersed from regions of divergence (upwelling). 



The diffusivity coefficient {K) was calculated as a 

 function of grid spacing. Data from numerous dye dif- 

 fusion experiments reviewed by Okubo (1971) were 

 used in a least squares regression analysis to com- 

 pute the equation K= (3 X 10" 4 ) h 11 relating the dif- 

 fusivity parameter (in m 2 /s) to the length scale (grid 

 spacing) h in meters. No spatial variation in diffusivi- 

 ty, other than that due to meridional grid narrowing in 

 more northerly latitudes, was assumed. Diffusivities 

 used ranged from 68 to 1 10 m 2 /s. 



In carrying out the simulations, American eel lep- 

 tocephali were considered to be "spawned" as point 

 sources at the various locations designated by letters 

 or stars in Figure 1. These starting locations cover 

 most of the presumed geographic range of American 

 eel spawning (Kleckner and McCleave 1980; Kleck- 



ner 5 ). Locations designated by letters are in the area 

 which, on the basis of collections of very small lep- 

 tocephali, represent the principal spawning area. 

 The distributional patterns of leptocephali started at 

 these lettered locations are discussed in detail in this 

 paper, and the various simulation runs are referred to 

 by these letters. The date 1 March is representative 

 of the peak American eel spawning period (Kleckner 

 and McCleave 1980; Kleckner footnote 5). That 

 point in time was used as the starting date for the 

 American eel drift simulations, with no additional in- 

 put of leptocephali after that date. The center of the 

 estimated European eel spawning area (Schmidt 

 1925; Schoth and Tesch 1981) is marked in Figure 1 

 with an X atlat. 27°N, long. 5 7° W. The results of one 

 simulation, in which leptocephali were started on 15 

 April at this point, are presented. The length of the 

 time step in the simulations varied by month, but was 

 always between 1.5 and 2.0 d. The simulation results 

 are presented initially on a monthly basis and then 

 later on a bimonthly basis. 



There is little information regarding actual concen- 

 trations of leptocephali in the ocean. As the 

 simulations progressed, individual cell concen- 

 trations were expressed as proportions of the start- 



*R. C. Kleckner, Research Associate in Zoology, Department of 

 Zoology, Murray Hall, University of Maine at Orono, Orono, ME 

 04469, pers. commun. July 1981. 



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