Daily age and hatching time of Japanese eel elvers from southeast coastal estuaries of China

The Japanese eel, Anguilla japonica, is one of the most important cultured species,widely distributed in rivers of the northeastern Asian countries’ Taiwan,China,Korea and Japan.The spawning grounds were recently discovered in the North Equatorial Current west of the Mariana Islands,15°N,140°E.From January to March 2008,there were 538 Japanese eel elvers collected from nine estuaries along Guangdong Province to Jiangsu Province which were measured on total length and long radius of sagitta otolith.Furthermore,observations were based on growth increments in sagittal otoliths of elvers collected at 9 sites.These results indicated that total length of elvers were 47.40 mm to 62.43 mm,the average value was(54.77±2.21) mm and the mean total length of each site was significantly different(ANOVA, P<0.001).As for long radius of sagitta otolith,it was from 133.22 μm to 175.95 μm,the average value was(154.55±9.09) μm and there was no significant difference among each site(ANOVA,P>0.05).Daily age of elvers ranged from 124 d to 158 d and the average value was(135.78±6.84) d,indicating that migration of eel larvae from their oceanic spawning ground to the estuary requires 4 to 5 months.An obvious trend of daily age was increasing from southern to northern areas and the maximum difference was 15.9 d.Elver hatching dates,back-calculated from estimated daily ages,indicated that the spawning season lasted 2 months(from Sep.6 to Oct.20),mainly concentrated in middle of October and approximately 85.98% samples accorded with New Moon Hypothesis.Finally,the inverse correlation between daily age and mean daily growth rates of fish length and otolith radius indicated that the time taken on migration from oceanic spawning ground to the estuary was shorter for fast-growing larvae than fast-growing elvers which arrived at the estuary earlier than slow-growing ones.