WASHINGTON – Mike Terrenzi is finding it harder to catch tuna in Cape Cod’s Great South Channel.

The lucrative fish, he said, are spending less time in the channel because their food supply there is dwindling. And as tuna, cod and other fish go farther afield to find the herring they feed on, fishermen are struggling to find a solution.

So Terrenzi and other New England fishermen took their fight to Washington last week, lobbying for changes in fishery oversight they hope will restore the diminished herring stocks.

In meetings with more than three dozen lawmakers, including Rep. William Delahunt, D-Mass., Atlantic fishermen said there has been a 600 percent increase in the number of large herring trawl vessels dragging nets for the fish in the Gulf of Maine.

Tens of millions of pounds of herring are brought to Maine ports each year. In 2002, the state’s herring harvest was 88.6 million pounds, valued at $6.2 million.

As the huge nets sweep up herring, they are also capturing vast amounts of other fish, ranging from cod and flounder to an occasional whale. And often that by-catch is simply thrown away.

Fisheries are feeling the impact from Maine to North Carolina, where striped bass are also being affected, said the coalition of fishermen.

The problem is, they have little data to back up their claims. Unlike other fisheries, the herring trawlers are not required to have observers on board to monitor their catch. And without the data collected by observers, federal authorities can’t tighten regulations on the trawl vessels or the gear they use.

“We want observers, and that will show the catastrophic bycatch events,” said Peter Baker, spokesman for the Cape Cod Commercial Hook Fishermen’s Association. Right now, he said, they only have anecdotal evidence.

Delahunt said the group made a persuasive argument for observers. But on Tuesday National Marine Fisheries Service Director William Hogarth said he can’t fund as many as the fishermen would like.

Instead, he said he will begin putting observers on 15 to 20 percent of the herring trawl vessels this year.

He said the fisheries’ science center will determine the percentage necessary to get an accurate sample.