Challenges for Diadromous Fishes in a Dynamic Global Environment

The Early Life History and Migration of Japanese Eel Larvae

Akira Shinoda and Katsumi Tsukamoto

doi: https://doi.org/10.47886/9781934874080.ch67

The Japanese eel Anguilla japonica is a catadromous species that is widely distributed in the rivers and estuaries of East Asia. The spawning area of the Japanese eel has been determined to be in the North Equatorial Current near seamounts to the west of the Mariana Islands, 2,000–3,000 km away from their growth habitats (Tsukamoto 2006). The leptocephali of the Japanese eel are transported by surface flows in the North Equatorial and Kuroshio currents from the spawning area toward the coastal waters of East Asia. Therefore, long-term or short-term changes in hydrographic structure of the ocean have the potential to cause population fluctuations in the Japanese eel by influencing the timing or location of recruitment and the yearly recruitment success. The early life history parameters such as hatching date, duration of the marine migration, and timing and duration of metamorphosis may also have an effect on the location and timing of recruitment of each individual. We analyzed the otolith daily increments of glass eels collected throughout its range in East Asia during three fishing seasons to understand the key factors of the migration success of Japanese eel larvae.

Otoliths were analyzed from 307 glass eels collected from November 1997 to April 2000 in nine estuaries in China, Taiwan, and Japan. The average total length of the glass eels was 57.9 mm (range: 51.2–64.6 mm), and their pigmentation stages ranged from 5A–6A3 (mode: 5B). Sectioned sagittal otoliths were viewed and photographed with scanning electron microscopy, and thereafter, otolith increments were counted and their widths measured. We classified six stages (embryo, preleptocephalus, leptocephalus, metamorphosing larva, oceanic glass eel, and coastal glass eel) of the early life history of the Japanese eel, using morphological otolith characteristics of checks and patterns of change in increment width.

The average age at recruitment was 156 d (range: 98–227 d), and individual differences in age at recruitment within the samples were large (maximum: 84 d). These data suggest that larvae hatched in different months frequently recruit simultaneously to the same location. The duration of the leptocephalus stage varied widely from 59 to 170 d (mean = 106 d) and accounted for 68% of the average age at recruitment. The average duration of the other early life history stages were relatively shorter than the leptocephalus stage: 9 d for preleptocephali, 21 d for metamorphosing larvae, 21 d for oceanic glass eels, and 6 d for coastal glass eels. The large variation in ages at recruitment may originate from the long and variable duration of the leptocephalus stage. Drifter buoys released near the spawning area of the Japanese eel in 1998, and tracked by the ARGOS satellite, showed complicated trajectories (Tsukamoto et al. 2003), suggesting that the leptocephali were frequently mixed by the fine- or mesoscale eddies in the North Equatorial Current. Indeed, the ages of leptocephali had a 27-d age range in the North Equatorial Current, although they were collected from only one net tow (Tsukamoto et al. 1989).