Effects of metamorphosis timing and the larval growth rate on the latitudinal distribution of sympatric freshwater eels, Anguilla japonica and A. marmorata, in the western North Pacific

Background: Early life history traits of the temperate eel Anguilla japonica and tropical eel Anguilla marmorata were examined to determine the possible reason why these two species have similar spawning areas and oceanic larval transport in the North Equatorial Current and yet are recruited to different but partly overlapping continental growth habitats in northern East Asia. To understand the segregative migration of these two sympatric eel species, their glass eels were collected from nine estuaries in the Philippines, Taiwan, Japan, and China. The age at metamorphosis from leptocephalus to glass eel (T-m), the age at estuarine arrival (T-t), the time between metamorphosis and estuarine arrival (Tt-m), and the growth rate (G(t)) of glass eels were calculated from daily growth increments in their otoliths. Results: Results indicated that the G(t) was faster and the T-m was younger in A. marmorata than in A. japonica. On the other hand, fish length and the T-t at estuarine arrival were larger in A. japonica than in A. marmorata, indicating that elvers of A. japonica experience a longer oceanic drift than those of A. marmorata. In addition, the Tt-m also indicated that A. japonica experienced a longer coastal migration than A. marmorata. Conclusion: This study validated that the T-m and G(t) seem to play important roles in the segregative migration and latitudinal distribution of these two sympatric freshwater eel species in the northwestern Pacific.