Reproductive cycle in reared male common Japanese conger, Conger myriaster

Annual changes in gonadal histology, gonadosomatic index (GSI), and plasma 11-ketotestosterone (11KT) levels were investigated in reared male common Japanese congers, Conger myriaster. Young fish, 20–30 cm in total length and around 20 g in body weight, caught in November 1996 (group 1) and in September 1999 (group 2) were reared for 3 years in seawater at temperatures ranging from 10 to 20 °C.

In most fish, only spermatogonia occupied the testes for 1 year and a few months after capture. Spermatocytes appeared in February, and both spermatids and spermatozoa appeared in March, in 1998 for group 1 and in 2001 for group 2. Spermiation was observed from May to September and reduction in testis was observed after October in both groups. GSI and plasma 11KT levels changed with progression of spermatogenesis. Although GSI was less than 1.0 and the plasma 11KT level less than 1.0 ng/ml, in the first year in most fish, both increased in the second year of rearing. GSI peaked in June 1998 (5.3±3.0; mean±standard deviation) in group 1 and May 2001 (2.3±1.3) in group 2, and bottomed in October (0.3±0.1) in both groups. Plasma 11KT levels peaked in March 1998 (5.8±1.9 ng/ml) in group 1 and May 2001 (4.4±2.4 ng/ml) in group 2, and bottomed in August in group 1 and September in group 2 (around 0.1 ng/ml in both groups). Spermatogenesis and changes in GSI and plasma 11KT levels were repeated the following year in both groups.

These observations indicate that males have an annual reproductive cycle under rearing conditions. It is possible that wild male common Japanese congers also have multiple spawning seasons in their lives.