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Researchers test morphometric method to determine sex of Japanese eels

Japanese eel (Anguilla japonica) is a commercially important aquaculture species in Eastern Asia, including Japan, Taiwan, Eastern China, Korea and Vietnam. However, most of the eels supplied by the aquaculture industry are grown from wild glass eels. Therefore, to fulfill the demand for glass eels in aquaculture artificial breeding is essential to conserve wild eel populations.

At the silvering stage, female eels are usually longer than males, but in the yellow eel stage, it is difficult to predict the sex of eels because of their long slender body shape.

At the Aquaculture Europe conference held last fall in Rotterdam, Netherlands H.M.V. Udayantha and colleagues Yucheol Kim, Thanthrige Thiunuwan Priyathilaka, Hyung-Bok Jeong, Jehee Lee, Shinkwon Kim, and Bong-Soo Lim from the School of Marine Biomedical Sciences, Jeju National University, Republic of Korea, described an easy morphometric method to determine the sex of Japanese eels.

To facilitate the process 180 Japanese eels from an eel farm in Younggwang, South Korea, were separated into two groups according to their body length (301-400 mm and 401-500 mm) and the following measurements were made: total length (TL); fin to fin length (FFL); fin height (FH); and fin length (FL). From these, the total length-to-fin width ratio (TL/FW) and total length to fin height ratio (TL/FH) were calculated for each eel. Body and gonad weights were measured to calculate the gonado-somatic index (GSI), and histological investigation to confirm their sex.

Total length-to-fin width ratios were calculated for eels in each group. In both cases females showed significantly higher TL/FW ratio than males. In the 301-400.mm group the female ratio was more than 31 while the ratio for males was lower than 31. In the other length group (401-500 mm) the females ratio was above 28, while that of males was less than 28.

Histological observations confirmed the sex of the each of the sampled eels. GSI of all sampled female and male eels were less than ≥ 0.22 and ≥ 0.17 respectively, confirming that the eels were in the yellow eel stage. The authors suggest that the TL/FW ratio can be used reliably for sex determination of yellow-stage Japanese eels.

For more information contact H.M.V. Udayantha by email: virajud@gmail.com