Millions of recreational fishing aficionados across America dodged a bullet last week in a federal lawsuit in North Carolina that had the potential to dramatically change the rules for the popular catch-and-release style of angling.

The lawsuit, from an environmental group named the North Carolina Coastal Fisheries Reform Group, sought to prohibit shrimpers at Pamlico Sound from throwing the so-called bycatch — the fish and marine organisms, some of them dead, inadvertently caught in their nets along with the shrimp — back into the bay after the shrimp have been removed. The environmentalists alleged that this practice is polluting the sound and a violation of the Clean Water Act.

“This large-scale disposal of dead and decomposing fish and marine species results in significant increases in organic matter and nutrient pollution in the marine environment,” the group’s original lawsuit said. “Such pollution encourages eutrophication, which decreases dissolved oxygen levels in the water, among other deleterious pollution effects.”

A district court hearing the case dismissed it, saying that releasing such bycatch did not amount to pollution, but the environmentalists appealed to the Fourth Circuit, where the court heard oral arguments last fall. Last week, the appeals court unanimously upheld the district court’s earlier ruling and raised the possibility of a startling side effect had it decided to side with the plaintiffs.

In his ruling, Judge Julius Richardson suggested that had the plaintiffs prevailed, recreational fishing enthusiasts across America would potentially be required to obtain permits from the EPA in addition to local fishing permits if they intend to catch a fish and release it unharmed. “Almost every commercial or recreational fisherman in America would be subject to EPA’s new regulatory control,” the judge warned.

Under the environmentalists’ reading of the law, Judge Richardson wrote, “when my daughter fishes on a boat by casting a hooked mud minnow into the sea, she has discharged a pollutant. She has taken a biological material (the minnow) and added it to the navigable waters (the sea) from a point source (the boat). And because she has done so without a permit, she faces crushing consequences.”

“So too if she — like most any fisherman — returns a fish that she caught, whether because she was targeting certain species, practicing catch and release, or complying with local harvesting limits or bans,” the judge added. “At argument, Fisheries sought to assure me that the EPA would not exercise its discretion to lock her up or take her allowance. Small comfort.”

An industry group representing fishermen in the state, the North Carolina Fisheries Association, hailed the decision as a common-sense one. The group’s director, Glenn Skinner, said the plaintiffs are motivated by a desire to eliminate what he said was a critical part of North Carolina’s economy.

“This is a huge win for all fishermen, commercial and recreational,” Mr. Skinner said. “If the courts had decided with Mr.  Joseph Albea and the Coastal Fisheries Reform Group, the results would have been devastating for both sectors.”

The plaintiffs have 14 days to appeal the decision and have not stated publicly whether they intend to do so. The group did not respond to a request for comment.