Graphic video released this week of California commercial fishing crews slicing the fins off live sharks, beating a shark with a baseball bat, hauling in dolphins and sea lions and leaving giant sunfish to suffocate on deck has bolstered an effort by environmentalists to ban drift gill nets, a controversial type of fishing gear.

The mile-long underwater walls of nylon mesh that catch swordfish are under growing scrutiny, because they sometimes entangle sea lions, dolphins, whales and other species of marine life.

The undercover footage was shot on two drift net boats in Southern California between December 2016 and December 2017 by an animal welfare activist posing as a freelance filmmaker. It was posted online this week by the Los Angeles-based group Mercy for Animals, and comes as California’s U.S. senators, Dianne Feinstein and Kamala Harris, have introduced a bill in Congress to ban the nets nationwide. A similar bill is pending in Sacramento to phase them out statewide.

“Much of the cruelty that happens is done without any witnesses,” said Kenny Torrella, a spokesman for Mercy for Animals. “Our investigators are acting as eyes and ears for the public. Imagine what’s happening on other boats. Most people have no idea.”

The undercover activist who filmed the incidents said in an interview that he has spent 17 years making dozens of secret videos at factory farms, puppy mills, slaughter houses and other locations, usually after applying for a job and getting hired there, then smuggling in hidden video equipment. His work has led to convictions on animal cruelty charges, he said, and some facilities being ordered closed. He agreed to an interview this week with the Bay Area News Group on the condition he not be identified.

In this case, he said, he worked on two commercial fishing boats, the Albatro and the Margaret O, with a camera in plain view after the captains allowed him on board because he said he was making a documentary film.

“I convinced them to let me on and openly film a lot of the routine fishing work that they did, and whenever there was a violation they would tell me to put the camera away and I would switch to covert gear,” he said.

“On the Albatro, all three times dolphins came up in the drift nets, they would tell me to not film,” he added. “One of the crew would say ‘you’re not a cop, right? If you are a cop you’re done, we’re going to tie sandbags to your boots.’ And he pointed to the bottom of the ocean.”

Each boat had a captain and two crew members. The Albatro left from the Long Beach-San Pedro Harbor area and the Margaret O from Ventura. The footage was shot on three trips, he said, two on the Albatro between Santa Barbara Island and San Nicholas Island in December 2016 and September 2017, and the other on the Margaret O in December 2017 while it fished between Santa Rosa Island and the Channel Islands.

Gary Burke, a Santa Barbara commercial drift net fisherman who speaks for the other drift net captains, said that the state’s fishing industry has been steadily reducing the amount of unintended animals it catches, known as bycatch, by using nets with larger mesh openings 14 inches wide, placing electronic devices that send out pinging sounds to scare away dolphins and other measures.

Burke said he is familiar with the video and knows the captains of the boats, identified by Mercy for Animals and commercial harbor records as Carlos Rivas Sr., on the Albatro and Jack Bateman on the Margaret O.

“All fisheries have some bycatch if you catch anything in numbers,” Burke said.

He said that the 20 commercial drift net boats left in California — down from 129 boats in 1994, according to federal records — mostly fish between San Diego and Big Sur. If drift net fishing is banned, he said, the U.S. will import more swordfish, thresher shark, opah and other fish from other countries with fewer rules.

As for the practices in the video, Burke said he disagrees with several of them, and blamed them on a crew member who had recently come to the U.S. from South America.

“In some foreign countries there are no rules. They do whatever they want,” he said. “That guy, you’ve got to remember he’s from Argentina or Chile. The guy doesn’t know anything about anything. But in his defense, all those sharks are edible sharks. I’m not in favor of beating them with a bat. I would have used a knife. But we are allowed to catch them, and people eat them.”

The drift nets are up to 1 mile long and 100 feet deep.

“As this latest video footage shows, drift gill nets are damaging, deadly, and inhumane,” said State Sen. Ben Allen, D-Redondo Beach, whose bill, SB 1017 would phase out the nets. “We need to find a way to move the fishing industry to better equipment, and that is what my legislation aims to do.”

Last month, the group released another video that showed underwater footage of dolphins dead in the nets.

Burke, the commercial fisherman, opposes the bill. He noted that “it’s illegal” to cut the fins off a live shark, and the crews in the video should have thrown the live sunfish back overboard. But he said that despite the disturbing images, consumers eat swordfish at home and in restaurants, and his industry is heavily regulated and struggling to stay afloat.

“No matter how you catch any fish, you have to kill it some time,” Burke said, adding: “The environmentalists call it ‘Deadly catch.’ It should be ‘swordfish fishery in jeopardy.'”

Drift net opponents say that the fishermen should use baited hooks to catch swordfish instead. Last June, the Trump administration killed an Obama administration rule that would shut down the drift gill net fishery for two years if two sea turtles or large whales were killed or injured by the nets.

The undercover activist said he turned the footage over to investigators for NOAA, the National Oceanic and Atmospheric Administration.

Michael Milstein, a NOAA spokesman in Oregon, said the agency won’t comment on specifics.

‘We are aware of the video and take any potential violations seriously,” he said.

California is the last West Coast state to allow drift gill nets. Voters banned their use in state waters out to three miles offshore in 1990, but they remain legal beyond that in federal waters. Many other states already have banned them, including Washington, Oregon, Alaska and Hawaii. According to federal records, efforts to reduce the numbers of marine mammals, birds and other animals killed in the nets are showing progress.

According to NOAA records, the drift gill net fishery killed or seriously injured about 20 endangered leatherback turtles per year in the early 1990s off the West Coast. Now only one has been killed since 2009, records show. And while 50 beaked whales were injured or killed in drift gill nets in the early 1990s, only one has been injured or killed since 2002.

Opponents say there aren’t enough federal observers on the boats, so the actual number of animals indiscriminately ensnared and killed is much higher than reported. In the 2016-17 fishing season, NOAA records show, 11 dolphins, two elephant seals and a gray whale were killed by California drift net fishermen, although observers only recorded information from 114 of 618 uses that season of drift nets.

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“The use of driftnets to target swordfish harms too many endangered or protected marine animals and should be phased out,” Feinstein said last month when she introduced the bill to ban them by 2020. “It’s unacceptable that a single California fishery that uses this type of drift net is killing more dolphins and porpoises than the rest of the West Coast combined.”