BY TOMOYUKI YAMAMOTO STAFF WRITER
An eel egg just before hatching, found off the Mariana Islands, is 1.6 millimeters in diameter. (Provided by the University of Tokyo's Atmosphere and Ocean Research Institute and the Fisheries Research Agency)
A team of researchers in Japan has found naturally spawned eel eggs in the ocean for the first time, a discovery they say will contribute to the aqua-farming of eels from eggs.
The researchers, led by Katsumi Tsukamoto, a professor of marine life science at the University of Tokyo's Atmosphere and Ocean Research Institute, collected the eggs in May 2009 off the Mariana Islands in the western Pacific.
The team included researchers from the Fisheries Research Agency, an independent administrative institution. The finding was reported in the Feb. 1 online edition of the British journal Nature Communications.
The researchers collected the eggs in a trawling survey using plankton nets.
DNA analyses found that 31 eggs, all fertilized, were those of the freshwater eel Anguilla japonica, or Japanese eel. They measured an average 1.6 millimeters in diameter.
The team said their success was the world's first in collecting naturally spawned eggs of any of the 19 eel species and subspecies.
The team focused their survey on dark new-moon nights, when they expected eels to spawn in the ocean area based on their past research.
Fertilized eggs float in seawater for only a day and a half before hatching.
The sea area, near the West Mariana Ridge, is about 3,000 to 4,000 meters deep and dotted with seamounts. The eel eggs were collected in relatively shallow waters of about 200 meters.
"We used to think eels lay eggs on the deep-sea bottom, but in fact spawning takes place in rather shallow waters," Tsukamoto said.
Japanese researchers have been probing the mystery of eel spawning for about four decades. In 2005, they succeeded in locating spawning areas by capturing just-hatched eel larvae off the Mariana Islands. But eggs were never found.
Because it's common for Japanese to eat eel to stave off fatigue from the summer heat, the fish are raised in aqua farms using nearly 100 million glass eels caught in the wild annually.
But the annual catch of young Japanese eels has dropped in recent years to only 10 to 20 percent of levels from around 1970, increasing the need to develop ways to mass-cultivate them.
The Fisheries Research Agency in spring 2010 successfully raised eels from eggs, but the rate of survival for young eels was very low, making commercial farming impractical.
The researchers say the data on water depth, temperature and salt density in the area where the eggs were found will prove valuable in attempts to efficiently raise eels from eggs.