抄録
The total length frequency, age and gonadal maturity in wild loach Misgurnus anguillicaudatus were monitored over a 12-month period in a small stream in Gifu Prefecture, central Japan, in order to clarify the annual reproductive cycle and life history. Age assessment from otolith analysis indicated that fish in the research area reached 3+ years. Annual changes in total length frequency indicated that newly born fish (less than 40 mm) joined the population from late June through late September, suggesting a spawning period from May to August. Nonyolky oocytes occurred in the ovaries through the year. Oocyte development was asynchronous, the onset of vitellogenesis indicating the beginning of the next reproductive cycle. Vitellogenesis occurred from October until March. From April to July, females had ovaries containing full-grown oocytes, suggesting that active spawning occurred during those months. Spermatozoa were seen in the testis seminal lobules throughout the year, the proportion being high in winter, low in summer. Spermatogenesis is apparently active from May to September, based on the presence of spermatid cysts in the testis. It was thought that most of the spermatozoa occupying the testis during the non-reproductive period were residual, being absorbed in May at the onset of active spermatogenesis.
