First Microchemical Examination of Abnormal Otoliths of Amphidromous Ayu Plecoglossus altivelis
Tao Ma, Mari Kuroki, Rikizo Ishida, and Katsumi Tsukamoto
The ayu Plecoglossus altivelis is an annual fish that spawns in freshwater in autumn. The larvae drift downstream to overwinter in coastal waters and then migrate upstream as juveniles in spring (Tsukamoto et al. 1987). Landlocked populations are known from Biwa Lake (Azuma 1973) and several other lakes in Japan, in addition to the typical amphidromous populations. Juvenile ayu have been stocked extensively in many rivers as they are an important freshwater fishery species in Japan. However, some ayu are found to have abnormal otoliths, and little is known about their detailed morphology and microchemistry. We examined the morphology and microchemical concentrations of some common elements (Sr, Ca, Na, K, and S) of abnormal otoliths in ayu and consider the possible factors influencing otolith abnormality.
A total of 31 reared juvenile ayu (101.7–137.0 mm standard length) were sampled randomly from three local culture ponds in Wakayama Prefecture (WK), Kumamoto Prefecture (KM), and Biwa Lake (BL) during June 4–10, 2004. These ayu had been collected as larvae from the wild and then reared to a larger size in culture ponds for stocking into rivers. Sagittal otoliths were extracted from all specimens, observed using a dissecting microscope, and subsampled for life history transect and x-ray intensity map analyses using a wave-length dispersive x-ray electron microprobe (JEOL JXA- 8900R).