DO LEPTOCEPHALI OF THE EUROPEAN EEL SWIM TO REACH CONTINENTAL WATERS - STATUS OF THE QUESTION

We examined recent arguments that leptocephali of the European eel, An guilla anguilla, swim in an oriented manner, rather than drift, to rea ch the continental shelf of Europe and the Mediterranean Sea. There is a dine of increasing body length of leptocephali from south to north and from west to east from the Mid-Atlantic Ridge (30 degrees W) to th e continental shelf, which could represent migration from south-west t o north-east, transport eastward at all latitudes, or increased growth rate with latitude. Evidence that this dine is a trend in age of arri ving glass eels along the European coast, and that the duration of mig ration is less than one year, is weak. Ages reported in the literature for specimens from Morocco to The Netherlands were based on the unval idated assumption that rings in otoliths were deposited daily. The ass umption is unwarranted because of low metabolic rate and uncertainty o f nutritional mode of leptocephali. If the assumption were accepted, c alculated hatching dates of eels arriving at the European coast imply year-round spawning. Lengths of leptocephali in the Sargasso Sea at va rious times imply that eels spawn only in late winter and spring. Lept ocephali contain tiny amounts of muscle, especially aerobic muscle for sustained swimming. They probably have insufficient capability to swi m across the Atlantic in the less than 1-2 y reported by others. Recen t studies show net east or north-eastward flow in the north-east Atlan tic at all latitudes from south of the Azores to north of the British Isles, which could transport leptocephali directly into the eastern No rth Atlantic without transiting more southerly waters first. Informati on is scanty on circulation and distribution of leptocephali between t he Sargasso and the Azores. Asymmetrical tides in the Strait of Gibral tar, including a rapid, eastward internal tidal bore, could carry vert ically migrating leptocephali into the Mediterranean. Available eviden ce should not be used to deny passive drift of leptocephali, modulated by diurnal vertical migration, across the Atlantic to the European co ast and the Mediterranean.