Glass Eel Recovery in Japan

All over the world, eel populations are in steep decline.  These mysterious creatures have a complex life cycle that includes a stage (following the larval stage), in which they are referred to as glass eels and are only a few inches long. In the U.S., the decline of the American Eel prompted a 2007 petition to have them added to the endangered species list.  The proposal was denied; however, there will be another chance to have them added in 2015.  In Japan, the country with the highest eel consumption rate (70% of the global catch), has listed one species as endangered. This act alone, though, is not nearly enough.

Vast quantities of glass eels are captured and sent to Japanese aquaculture farms to be raised for sushi, in addition to being threatened by “changing ocean currents, disease, pollutants, fisheries, barriers to migration [such as dams] and freshwater habitat loss.”  The issue of over harvesting, which has profoundly negative effects on eel populations, is not being helped by the rising value of the eels.  In Europe, only 1% of previous number of glass eels migrating are still doing so.

In America, the Atlantic States Marine Fisheries Commission is closing fisheries on the East Coast to allow for eel recovery.  Japanese researchers are taking perhaps more direct steps to heal eel populations. At the National Research Institute of Aquaculture in Shibushi, scientists have developed an efficient captive breeding system. Although the sustainability of the program may not be perfect due to fossil fuel heating and use of fish meal as a food source for the eels, 99% of glass eels cultured survive into the next life stage. While the glass phase seems simple enough to support, it is the life stage preceding the glass stage that proves problematic.

What makes the the larval eel, or Leptocephali, so unique is the “transparent gelatinous material [making up their bodies] that functions to store energy.”  The Leptocephali are so difficult to culture because of their unique diet, which is unlike that other marine larvae that feed on zooplankton.  At this point in their life cycle, the eels feed on marine snow, which is “composed of materials released by phytoplankton that mix with other free material in the ocean and are colonized by microorganisms.”  Naturally, this food source is difficult to develop for use in captivity.  Researchers have developed a pink paste substitute made of shark eggs, soy protein, and various vitamins. This method will only last so long, however, because the preferred shark, the Spiny dogfish, is listed as vulnerable.

Despite a few set backs, it does seem that the Japan’s captive breeding programs will, and are already, make a positive impact.


Tank of Glass Eels



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