Fish populations in the Delaware River in Pennsylvania are making a striking comeback following a major river cleanup effort. In the last 20 years, levels of harmful ammonia and bacteria in the Delaware have been cut by about half, and the level of dissolved oxygen in the river, necessary for fish to breathe, has quadrupled. Still, the level of pollution draining into the river from sewage treatment plants and urban and suburban storm runoff is on the rise, and the river bottom remains contaminated with toxic metals and chemicals from years of industrial dumping. In other fish news, the largest fish kill in a decade has hit the Chesapeake Bay area, where an estimated 1 million dead fish were found yesterday in the lower Pocomoke River in Maryland. Investigators have yet to pin down the cause.