Water temperature manipulation can induce oocyte maturation and ovulation in the common Japanese conger, Conger myriaster

To develop seed production techniques for aquaculture of the common Japanese conger, Conger myriaster, we attempted to induce ovarian maturation and ovulation without exogenous hormone treatment and examined the changes in steroid hormones to understand the endocrine condition of the fish during their maturation process. Juveniles caught in November 2001 (20 to 30 cm in total length) were reared for 1.5 years in seawater at temperatures ranging from 10 to 20 °C before the experiments. Then, 141 randomly selected female eels (58 to 82 cm in total length) were maintained at a water temperature of 6 °C from April 2003 to March 2004. The initial GSI values (5.0 to 12.0) and oocyte diameters (300 to 400 μm) recorded in April 2003 had increased to maximum values of 30.0 and 650 μm, respectively, at the end of experiment, in March 2004. In late December 2003, 12 fish with oocytes measuring over 600 μm in diameter were selected and divided into an elevated temperature group from 6 to 10 °C and a control group of 6 °C for 43 days. The mean oocyte diameter in the elevated temperature group increased significantly, and one fish (estimated GSI, 47.0) ovulated eggs with an average diameter of 995 ± 46 μm on the 41st day at a water temperature of 10 °C, in contrast, the control group showed no significant changes in oocyte diameter, and no fish ovulated. Plasma T levels, examined via ELISA, increased with oocyte maturation, with the highest level detected at the migratory nucleus stage, and declined after ovulation. In contrast, plasma E2 was maintained at high levels during the vitellogenic stage, exhibiting the highest value after ovulation. The present study demonstrates that ovarian maturation and ovulation in the common Japanese conger could be achieved through water temperature manipulation.

► Ovarian maturation and ovulation can be achieved through water temperature manipulation.
► A low water temperature is an important condition for advancing ovarian development.
► An elevated water temperature stimulates ovulation.
► The mean diameter of ovulated eggs was 995 ± 46 μm (mean ± SE).