ANCHORAGE — When Southeast Alaska commercial fisherman Jay Skordahl prepares to lay down a 2-mile line of baited fish hooks, he keeps an eye out for 40-ton pests.
Sperm whales, the world’s largest toothed cetaceans, for more than a decade have bedeviled fishermen catching sablefish, also known as black cod, a deep-water fish that tastes like butter and sells for more dollars per pound than any other Alaska finned fish.
“Some whales are happy enough kind of jogging around behind you as you’re moving up your string and slurping up the discards. Those are the nice whales,” Skordahl says.
Then there are the “bad boys” with more sophisticated palates. They suspend themselves hundreds of feet below a boat and pick off sablefish as a fisherman retrieves his longline.
“Some of them are very well educated,” Skordahl says. “They’ve eaten black cod before and really like it. They really know what they’re doing.”
Researchers this year are using a $353,000 federal grant to continue assessing how much thievery sperm whales, an endangered species, conduct in the black cod fishery. They plan to continue using acoustic recorders to refine their estimates of how many fish are taken. They also will experiment with a cheap deterrent: acrylic beads attached to lines that are designed to confuse a sperm whale’s built-in sonar, echoing back a signal that resembles a tasty sablefish.
“It’s going to essentially light up the whole longline string,” said Jan Straley, a whale biologist at the University of Alaska Southeast.
Americans so far have largely not discovered the allure of sablefish, also known as butterfish. Their high oil content make them exceptionally flavorful and excellent for smoking. Most of the market is in Japan.
In federally managed waters, fishermen last year reported a total Alaska catch of 21.9 million pounds. Southeast Alaska fishermen sold dressed sablefish for an average of $5.96 per pound. Skordahl says large specimens this year are fetching more than $8 per pound.
They’re harvested with “longlines” — 3/8-inch nylon rope thousands of feet long. Attached every three feet or so is a ganion, a short line with a circular hook to which bait is attached. The longline is let out behind a boat and anchored to the bottom.
Estimates that whales can pluck 5 to 10 percent of the fish off a line may be low, Straley said. Before a camera captured a sperm whale gently taking a black cod from a line in shallow water, the only visual evidence of plundering was bent hooks or fish lips left on lines. Fishermen sometimes even caught more fish when whales were present, Straley said, which may prove only that both whales and fishermen know where the fishing hot spots are.
Skordahl fishes out of Sitka on the edge of the outer continental shelf, laying his line on steep canyon walls 600 to 900 feet deep.
The fishing used to be derby style, where boats rushed out to catch a set quota as quickly as they could. These days, fishermen are assigned individual quotas and can head out anytime from spring to fall. It also means that opportunistic whales have a longer season to find fishermen.
Researchers have determined that sperm whales apparently can distinguish longliners from salmon trollers or other boats by how they shift gears. Laying down a string, Skordahl said, requires pacing: moving forward, shifting to neutral or reverse, finding the best speed to pay out a line. It’s been a dinner bell for sperm whales.
They’re among the deepest diving whales and when they forage where little light penetrates, they find prey with echolocation. The behemoths send out sounds and read what comes back. Their skill is so sophisticated, they can distinguish between a sablefish and an unappetizing rockfish in the dark water.
“It’s like an individual label,” Straley said.
Acoustic work by research oceanographer Aaron Thode of Scripps Institution of Oceanography recorded sperm whale “clicks” that accelerate into “creaks,” almost like the sound of a rusty hinge, as they approach prey, including fish caught on a longline. The video shot in 2006 recorded those sounds, underscoring the importance of echolocation even when whales could use their eyes, Thode said.
Straley’s role in the latest research will be to determine whether 28mm beads attached to each ganion will throw the whales off their game. The beads, slightly smaller than a golf ball, were picked because they bounce back a signal similar to a sablefish. Researchers hope the whales will not be able to distinguish the signal returned by a fish from the signal from the beads.
“It’s inexpensive and it doesn’t damage anybody’s gear coming over the roller,” Straley said. “If it works, it will be great. It’s passive deterrent.”
It likely will take until mid-June to get the beads and make the gear, she said.
“It’s kind of like a big macrame project,” Straley said.
Underwater sound recorders will try to detect how effective the beads are.
“We’re going to try to come up with ways to distinguish how successful the whales are in depredating, under both normal haul conditions and under these various deterrent strategies,” Thode said.
Of special interest to fishermen such as Skordahl will be an assessment of how sperm whales affect the federal survey used to set fishermen’s quota. The primary tool for assessing sablefish populations is a longline fishery that federal biologists do themselves.
The take by sperm whales on the survey has not been factored. Chris Lunsford of the National Marine Fisheries Service said researchers hope to measure depredation acts through “creaks” to quantify whales’ effect on the survey.