PORTLAND – The recent discovery of some 48,000 pounds of juvenile haddock mixed in with the catch from two herring trawlers has some fishermen worried about the impact on efforts to rebuild groundfish stocks.

The Maine Marine Patrol boarded the boats Aug. 10 and found that 4.5 percent of the 1.1 million-pound catch was undersize haddock, according to the New England Fishery Management Council. Federal law prohibits herring boats from landing haddock, and its presence with a catch can result in stiff penalties.

“The groundfish industry has sacrificed and some have been put out of business. To see four-inch haddock caught and thrown away with 6-cents-a-pound herring, it’s a slap in the face,” Peter Baker, spokesman for a Cape Cod fishermen’s organization, said. Mature haddock typically sells for about $1.50 a pound, he said.

Critics say herring trawlers pull up groundfish because they fish too deep. But a herring industry association says the recent incidents were unusual and came about because the small haddock were schooling at higher water levels than normal.

“Haddock, generally considered bottom dwellers, at certain life stages can school and move up in the water column,” said a statement from the herring industry group East Coast Pelagic Association. “However, intermixing with. … herring stocks is an experience that fishermen have not had in the past.” Historically, herring fishermen have not landed significant amounts of groundfish, the association said.

Demand for herring has grown and the techniques for harvesting the fish have become more sophisticated.

Modern herring boats can run in pairs, pulling huge nets stretched between them. The net is pulled through the mid-water column, that is, not along the ocean bottom. The practice should lead to relatively little bycatch of groundfish, which typically stay near the bottom.

The recent enforcement action against two herring boats is under investigation by federal authorities, in part to determine the severity of the problem.