Anglers are advised not to eat fish from part of the Souhegan River in Milford.
The Environmental Protection Agency wants people to throw back any fish they catch on a roughly one-mile stretch of river near downtown.
That area is next to an EPA Superfund site called Fletcher's Paint Works. The EPA has limited fishing near the site in the past.
Removal and covering of contaminated soil and sediment wrapped up at the Superfund site last year.
But officials say fish near the site could still be contaminated with high levels of chemicals called PCBs. They tend to accumulate through the food chain, and have been linked to cancer and other human health issues.
The state Department of Fish & Game says it has stopped stocking that section of the river with trout for now.
The EPA plans to do more testing to see how long the catch-and-release order should last.
This story has been updated.
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