The run of young trout up the Tama River this summer is expected to top last year's figure, the most promising in almost a decade, according to the Tokyo Metropolitan Fisheries Experiment Station.

At least 640,000 trout had already made the upstream journey by the end of May, according to the station, and the figure is expected to top 1 million before the migration winds down later this month.

In 2002, an estimated 1.13 million trout made the journey from Tokyo Bay. Before that, the number had not topped 1 million since 1993, when the station recorded 1.3 million migrating trout.

Scientists at the experiment station have been conducting annual studies on the early April through mid-June trout run since 1983 by setting up nets in a section of the Tama River in Ota Ward.

Scientists attribute the increase in trout this year to heavy rainfall during the 2002 trout spawning season, which they say cleaned the rocks on the riverbed, making it easier for the eggs to stick to the stones and hatch.

Trout spawn between October and December. The young trout swim down to Tokyo Bay and start the run up the Tama River beginning in late March. Trout fishing season on the Tama River begins in June, drawing around 360,000 anglers annually.

The type of trout bred in Tokyo Bay is known as "Edo-mae ayu," and trout caught in the Tama River are particularly prized among anglers. During the Edo Period, trout caught in the Tama River were reportedly used as an offering to the shogun.