Except the habitat loss, there are other three main factors affecting the eel
resource, pollution, climate change and overfishing. Water quality changes associated
with increased levels of nutrients, sediments, and contaminants. For example, nutrient
loading leads to algal blooms, which can decrease the concentration of dissolved
27
oxygen (DO) in the water. Low DO can reduce the amount of suitable habitat for fish
and can impair fish growth and reproduction; and the factor, low DO, may effect eel
too (Amanda, 2009). Otherwise, the effects of persistent pollutants combined with the
eel's unusual life cycle may cause the decline in the eel population in northern Europe
in recent decades (Larsson, 1991); the other example, when the eel expose in the
contaminated environment, the strongly polluted eels detoxify less efficiently, have a
lower condition and might be less successful spawner (Feunteun, 2002).
About the climate change, global warming has affected the stability of the hence
produced shifts in plankton communities and food web structures. Two potential
sources of nutrition have been proposed for eel larvae; dissolved organic matter
(DOM) and particulate organic matter (POM) in the form of zooplankton fecal pellets
and larvacean houses (Otake, 1993; Mochioka, 1996; Pfeiler, 1999). Marine snow has
also been proposed as a potential source of nutrition (Knights, 2003); those primary
production has been considered to be a good proxy for leptocephali food (Bardonnet,
2005). Thus, recruitment declines in Japanese eel may also have been due to
starvation–advection problems (Karl, 2001; Knights, 2003).
There is also a factor about the climate change, the ENSO, Kimura (2001)
showed a certain synchrony between Anguilla japonica recruitment and salinity fronts
driven by ENSO in the Japanese eel spawning area; Kim (2007) also demonstrated
28
that the changing oceanic conditions associated with climate change have resulted in
decreased recruitment of Japanese eel.
The factor of eel resource decline worth to be discussed is overfishing. Tzeng
(1986) indicated that Japanese eel elvers have been overfishing for aquaculture in
Asian countries, thus the eel population is obviously decreased. Furthermore, Knight,
(2003) has also inferred that Japanese eel populations (and escapement of
pre-spawning silver eels) have been affected by overfishing.
The annual catch of glass eel in the 1970s in Japan about 80.6 tons and 334096
individual in Fulung in the period 1984~1995 on average; after that time, the catch
cleared showed a decrease although it was fluctuating. Annual catches in some local
fishing areas showed nearly synchronous fluctuations. This fluctuation may be caused
in part by oceanic current conditions (Kimura, 2001). On the other hand, the average
catch in Japan in the past 5 years were 6.6 tons and in Fulung in the period
2006~2013 is 14190 eels; this value was only 8% of the average catch in Japan in
1970s and was only 4% in Fulung in the period 2006~2013. It may be very difficult to
explain this rate of decline for about 30 years by only dynamic oceanic environmental
conditions (Tatsukawa, 2003). Because of some river fishermen have expressed
concern that the decline in catch might be caused by overfishing of glass eels year by
year. Furthermore, previous literature assumed that water pollution might have
29
affected the survival of glass eels too.
Although many factors affect eel stocks, unfortunately, it is difficult to separate
these potential factors. Thus we could not identify the main factor or the percentage of
each factors contributing to eel resource decline (Tzeng, 2004).
30
Conclusion
This study has demonstrated that the HQI of East Asia declines 25%. However,
the eel resource changes in Japan and Fulung of Taiwan are -92% and -96%,
respectively. Thus, although the habitat destruction should contribute to eel resource
decline to some extent, other factors such as water pollution, overfishing, and climate
change may also be important factors for the decreasing of the eel resource.
31
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