Why Has the Japanese Eel Not Declined to the Levels of the European and American Eels? A Possible Explanation from the Movement and Age Structure of Growth-Phase Eels
Kazuki Yokouchi, Jun Aoyama, and Katsumi Tsukamoto
Catadromous temperate eel recruitment in the northern hemisphere has been declining drastically in some areas, but the degree of the decline appears to differ. Glass eel recruitment of the European eel Anguilla anguilla and American eel A. rostrata has dropped as much as 99% in some areas since the early 1980s, while the Japanese eel A. japonica has remained at levels as great as 20% of the levels of the early 1970s (Dekker 2003). The characteristics of the life cycle have an effect on the traits of the stock, and the demographic attributes during the growth phase of eels (e.g., sex, growth, and age at maturation) are important determinants for their life cycles that may differ among species. The objective of this study was to explore why the Japanese eel stock has not declined to the level of the European and American eel stocks by comparing the demography of the growth phase between Japanese eels and the Atlantic eels.
A total of 4,925 Japanese eels were collected by electrofishing, eel pots, set nets, longline, and traditional traps from nine prefectures of Japan (Ibaraki, Chiba, Shizuoka, Aichi, Tokushima, Kochi, Ohita, Miyazaki, and Kagoshima) from 1999 to 2004. Mean total length (±SD, mm) of females and males was 510 ± 118.2 and 420 ± 81.9, respectively (Figure 1). Mean age (±SD, years) of females and males was 5.0 ± 2.0 and 3.6 ± 1.7 (Figure 1; Table 1). Mean size-at-age growth rate (±SD, mm/ year) of females (n = 4,077) and males (n = 1,038) was 96.3 ± 37.8 and 118.0 ± 63.5 (Table 1). The age of Japanese eels was less than that of European eels (mean age range: 8.5–20 years; Vøllestad 1992) and of American eels (range: 1–39 years; Morrison and Secor 2003) after eel stocks began to decline (European eel after 1985, American eel after 1990). The size-at-age growth rate of the Japanese eel was also faster than the Atlantic eels (28–60 mm/year; Vøllestad 1992; Moriarty 2003; Morrison and Secor 2003). These data on the age and growth of northern temperate anguillid eels suggest that the life cycle of the Japanese eel is shorter than that of European and American eels.
The movement patterns of eels are also important factors in fish stock assessment, and Japanese eels had restricted movement (81% of recaptured eels had moved less than 150 m from the release points) in the Nishijinden River in Shizuoka, Japan, inferred from a mark–recapture experiment from 2003 to 2004. This result was similar to previous studies of European and American eels (Oliveira 1997; Feunteun et al. 2003; Morrison and Secor 2003). Therefore, it was suggested that these three species have limited movements.
Although eels have been studied in freshwater and estuarine environments, it is not known if the causes for the declines of the temperate eels are changes in oceanic conditions, overfishing, or the loss or deterioration of growth habitats. However, since all temperate eels may experience similar effects of changing oceanic conditions during their larval dispersal by oceanic currents, some of the differences in the levels of declines of temperate eels may be due to factors during their yellow eel growth phase. European and American eels are characterized by a sedentary growth phase and a long life span, compared with Japanese eels. These characteristics would increase the time in which Atlantic eels would be more vulnerable to fishing pressure, and anthropogenic impacts on their growth habitats, before reaching maturation. It is hypothesized that this may be one of the reasons that the European and American eel stocks have declined more than the Japanese eel stock.