Sweetfish explosion sees 12 million in Tamagawa river

Some 25 years ago, the number of beloved "ayu," or sweetfish, in the Tamagawa river in the Kanto region was estimated at a lowly 20,000.

This year, that number has exploded to a record 12 million, a survey by a Tokyo metropolitan government’s research center has found, which could attest to the increasing water quality of the river and Tokyo Bay.

The figure is the largest since the survey was started in 1983 by the Tokyo Metropolitan Islands Area Research and Development Center of Agriculture, Forestry and Fisheries. It is also the first time that the estimate exceeded 10 million, center officials said.

The officials added that the increase could be due to the improved water quality and spawning environment in the river and the waters of Tokyo Bay.

Every spring, the center has conducted the survey in the upper reaches of the Gasu-bashi bridge in Tokyo’s Ota Ward, about 11 kilometers from the mouth of the river, using fixed fishing nets.

This year, members of the center caught 645,000 sweetfish from March through May. Based on the data collected through last year, the center estimated that the total number of sweetfish that swam up the river this year totaled 11.94 million.

That estimate was only 20,000 in 1985, but the number has increased over the years, topping 1 million in 2006. It has since swelled in subsequent years, reaching 7.83 million in 2011, or four times as many as that of 2010.

Sweetfish, which are prized for their delicate, sweet flavor, lay their eggs in shallow spots in the middle of rivers in autumn. Upon hatching, sweetfish fingerlings swim down the rivers into Tokyo Bay, where they winter and grow. After that, they return to the fresh water of rivers in the spring to spawn.

Unlike salmon, which return to the rivers where they were born, sweetfish are believed to swim to the rivers where conditions are most suitable for spawning. As a result, many are apparently choosing the Tamagawa river, where the water quality has improved in recent years and the quantity of water is abundant.

However, as for the sharp increase in the number of sweetfish in the Tamagawa river this year over the last, an official of the center says, “We don’t know the true reasons.”

The large jump in numbers may be attributable to two possible reasons. One is that many of their parent sweetfish swam up the river last year to spawn. Another is that the environment of egg-laying places was favorable, as the bottom of the river was washed by heavy rains over the past year.