The Japanese eel, Anguilla japonica, is one of 19 species in the freshwater eel family Anguillidae (all in genus Anguilla). Despite their common name anguilid eels don’t spend their whole lives in fresh water, they have a unique catadromous lifecycle in which they travel thousands of miles from inland water bodies, sometimes crawling over land at night, then swimming to far-out, restricted oceanic spawning spots (Tsukamoto 2006). The larvae hatch at sea and return to shallow waters where they metamorphose into the glass eel larval stage and travel in schools to freshwater inland rivers, lakes, streams and estuaries where they complete their development. The Japanese eel is native to the area from Japan to the East China Sea, Taiwan, Korea, China and northern Philippines. It is snake-like, with small scales, uniform brown coloration and coated with mucus. A carnivore, it feeds on benthic crustaceans, insects and fish, and grows to a maximum length of 150 cm (5 ft.) although more typically found at about 40 cm (16 in.; Food and Agriculture Organization of the UN 2013a, b; Froese and Pauly 2013).
Extremely popular in Asian cuisine, especially smoked and used in sushi (unagi), the Japanese eel is said to be the most expensive fish in Japan (Froese and Pauly 2011). Japanese eels have long been farmed, however the aquaculture industry is dependent on culturing glass eel stage larvae captured in the wild; breeding in captivity has not been successful. Currently demand for Japanese eel exceeds availability, and population levels have declined dramatically, to the extent that Monterey Aquarium Seafood Watch and Greenpeace International have declared the Japanese eel unsustainable (Greenpeace 2013; Halpin 2007). There is worry that there may be too few fish left to successfully reproduce (FAO fisheries 2013). Furthermore, aquaculture practices have significant problems that make the industry questionable. Market pressure for unagi has extended to other Anguilla species, especially the European eel (A. anguilla) and the American eel (A. rostrata) which are also now critically endangered (Freyhof and Kottelat 2010).
The spawning site of Anguilla japonica adults has only recently been discovered around several seamounts just west of the Mariana Islands, and the full-moon timing of their spawning described, a culmination of 50 years research on larval hatch dates and appearances. The location of their spawning site is crucial to positioning the larvae in the Kuroshio current, the mechanism whereby the larvae can return to their appropriate Asian habitats. Knowing this information about their spawning site is important in order to help protect their migrations from disruption (Tsukamoto 2006; Froese and Pauly 2011).