A treatment plant-fish hatchery in one.

Chinook salmon are spawning in the East Chicago Sanitary District Wastewater Treatment Plant near Lake Michigan, scientists announced in October.

This is the first reported case of salmon spawning in a treatment plant, Peter S. Baranyai of the sanitary district and his colleagues assert in an unpublished report. They first noticed the fish in the plant in 1993. Previous attempts by fishery managers to encourage salmon reproduction in Lake Michigan's southern basin had failed.

A tertiary treatment plant such as this one makes a sensible spawning area, says ecologist William K. Hershberger of the University of Washington in Seattle. Only very clean water flows through the areas the salmon use. As in the wild, the salmon must swim upstream and jump up a wall of water.

To get to the plant, the salmon travel from Lake Michigan up the Grand Calumet River to the plant's 40-foot-wide, 700-foot-long effluent channel. Most swim up that channel; through a 12-foot-wide, 150-foot-long pipe; over a 4.5-foot-high wall; and into a 260-foot-long chamber, where they lay their eggs. Some salmon spawn, instead, in the channel leading to chamber.

The salmon's offspring leave by the reverse route. Preliminary genetic tests suggest that salmon born in the plant in previous years have returned there to spawn.

In June, Baranyai and his colleagues saw as many as 50 salmon in the channel at one time and 15 in the chamber.