Physical and behavioural controls on the oceanic distribution and migration of leptocephali

A pelagic, long-lived leptocephalus is common to families of eels, whether the juveniles inhabit the open ocean, shelf waters, or estuarine and fresh waters. Successful leptocephali of continental species must leave the open ocean, while those of oceanic species must avoid advection from the open ocean. The movements of leptocephali of migratory species should be considered conceptually as a dispersal-mortality problem. On the basis of the distribution of small (recently hatched) leptocephali, the Sargasso Sea in the western North Atlantic Ocean is identified as a spawning area for the catadromous American and European (angui llid) eels, western Atlantic shelf-dwelling Conger eels, and numerous oceanic species, e.g. Nemichthys scolopaceus, Serri voiner beanii, Derichthys serpentinus and Nessorhamplms ingolfianus. Within the Sargasso Sea, the Subtropical Convergence Zone is a narrow latitudinal zone where northern and southern bands of thermal density fronts occur, especially in winter. Spawning of the two catadromous eels and the two western conger eels seems to be limited to areas in or south of the southern band of fronts, where southern Sargasso Sea water occurs. Spawning of most of the oceanic species occurs on both sides of these fronts. Some feature of the frontal zone or the southern waters, such as odour, may serve as a signal to trigger cessation of migration and initiation of spawning in catadromous eels. Thus the source of leptocephali of the two species of Anguilla may be zotially restricted. Because of confluence of water into fronts and resulting eastward jet-like currents, fronts may be sinks for a portion of the leptocephali of numerous shelf-dwelling species, which may be advected away from suitable juvenile habitat. The size distributions of leptocephali suggest that gyres in the southwestern Sargasso Sea, an Antilles Current, and the Florida Current north of the Bahamas are routes of exit for anguillid and congrid eels. Shorter larval duration and more northerly spawning facilitates retention of the oceanic species D. serpentinus and N, ingolfianus in the western Sargasso Sea. Shorter duration of the leptocephalus stage of the American eel than the European eel, and plasticity in length of time prior to metamorphosis in both American and European eel leptocephali cause most leptocephali drifting on the Gulf Stream-North Atlantic Current system to arrive at the appropriate continent. This current system does transport both species, but most American eel leptocephali are found only in the west, while European eel leptocephali are found all across the North Atlantic current and its branches. A dine of increasing lengths of European eel leptocephali from south to north off the continent of Europe has been used as evidence that drift on the North Atlantic Current cannot be the transport mechanism and that active swimming must occur. Recent indications are that there is northward transport of water in the Canary Basin. Eastward jets in the Sargasso Sea and evidence for a basin-wide eastward countercurrent in the Pacific Ocean lead to the hypothesis of a possible eastward countercurrent in the Atlantic that would be a second more southerly route of drift toward Europe. Strong genetic evidence shows the presence of hybrids of the two Anguilla species among the mostly European eels in Iceland, presenting a paradox because the existence of unique currents connecting the Sargasso Sea and Iceland is highly problematic.