The northern limit of spawning by Atlantic eels (Anguilla spp.) in the Sargasso Sea in relation to thermal fronts and surface water masses

American and European eels (Anguilla rostrata and A. anguilla) spawn in a large poorly defined area east of the Bahamas between about longitude 50W and 75W in the Sargasso Sea. We use the distribution of tiny Anguilla larvae taken in ichthyoplankton collections and associated characterizations of hydrography to test two hypotheses concerning distribution of water masses and the northern limit of spawning by both species. Data are presented from four transects of closely spaced stations conducted during February and April 1983 which refute our hypothesis that a positive correlation exists between the distribution of the Subtropical Underwater and spawning by Anguilla. Larvae ≤5.5 mm were taken on both sides of fronts at the northern edge of the Subtropical Underwater. This result was supported by data from two longer transects conducted during March 1985. Though Subtropical Underwater was not encountered along these transects tiny Anguilla larvae were present. Our second hypothesis, that fronts along the northern edge of the warm, saline surface water mass of the southern Sargasso Sea form the northern limit of spawning by Anguilla, was strongly supported by the March 1985 collections. Tiny Anguilla larvae were taken in all collections south of fronts separating southern Sargasso Sea surface water from mixed Subtropical Convergence Zone water to the north. Anguilla larvae ≤5.5 mm TL were not taken in collections at stations where mixed Subtropical Convergence zone water was present.