Growth differences of Japanese eels Anguilla japonica between fresh and brackish water habitats in relation to annual food consumption in the Kojima Bay-Asahi River system, Japan

To learn about the relationships between feeding and growth of temperate eels in freshwater and brackish water habitats, we analysed 533 yellow-phase Japanese eels Anguilla japonica collected in both types of habitats in southeastern Japan. Because male eels were very rare in each habitat (FW, N = 1; BW, N = 20), characteristics of female eels were compared between the different habitats. Annual food consumption was evaluated with the consideration of instantaneous food consumption and annual activity period. Stomach fullness index (stomach content weight/body weight) was used as an indicator of instantaneous food consumption. The ratios of number of months with eel catch to those when eel sampling was conducted were used as an indicator of activity period. Female yellow eels tended to be older and slower growing in fresh water (N = 78; age, mean +/- SD = 7.9 +/- 2.4 years; growth rate, 59.8 +/- 14.0 mm year-1) than in brackish water (N = 229; age, 5.5 +/- 1.8 years; growth rate, 90.1 +/- 24.4 mm year-1). Irrespective of sex, yellow eels in brackish water had a higher stomach fullness index and a greater ratio of months with eel catches, indicating greater annual food consumption by brackish water eels. These results indicate that greater annual food consumption contributes to the greater growth rates of Japanese eels in brackish water habitats.