Study finds deep-set Loggerhead turtles generally spend most of their time at or near the ocean's surface and seldom dive deeper than 75 feet, National Marine Fisheries Service scientists have discovered.
lines could spare
most turtles
Since loggerhead turtles stay near
the surface, they would avoid
fishers' lines, scientists sayDeeper lines could still snag turtles, critics say
By Helen Altonn
haltonn@starbulletin.comAs a result, they concluded, turtles are not likely to be caught by longline fishing gear if it is set deep in northern waters.
This was among findings in a study of diving behavior of loggerhead and olive ridley turtles by ocean ecologist Jeffrey Polovina and turtle specialist George Balazs.
Their research confirms that the present management plan for longline fishermen will substantially reduce loggerhead turtle catches, said Polovina, who directs ecosystem environmental investigations at the Fisheries Service's Honolulu Laboratory.
He said the plan, which prohibits shallow longline gear in northern waters, will protect endangered loggerheads while allowing longliners to set deeper gear to go after tunas or perhaps swordfish.
Polovina teamed with Balazs to study the turtles' behavior -- how deep they dive and how much time they spend at different depths. Evan Howell and Denise Parker, with the University of Hawaii Joint Institute for Marine and Atmospheric Research, worked with them. Parker, now in La Jolla, Calif., still does computer work for the project, Balazs said.
The researchers discovered some "curious phenomena," Polovina said, noting the turtles often change diving habits.
He said loggerheads did not dive generally for more than 45 minutes or go below 75 feet. Yet one animal, caught in a big storm north of Hawaii last winter, changed its diving time at night to spend two to three hours underwater.
"We saw one loggerhead swimming against the wind and currents into a strong storm and it dives deeper to avoid faster-moving surface water," he said.
"It goes deeper but encounters colder water and ends up spending a longer time. Cold water slows down oxygen utilization. It can spend two to three hours under water. Other dives can be less than an hour."
Balazs has been tracking turtles for years with satellite tags to see how long they would survive if hooked by a fisherman's longline."We thought, if there was a hook anywhere on an animal, it probably would go off the air in a couple weeks because the animal would die. But things didn't turn out that way," Balazs said. "Many of them, we learned, do survive, and apparently survive very well for long periods of time."
That discovery resulted in longer tracks, over weeks and months, he said. "I tossed some of the things Denise Parker and I gathered at Jeff, and he got excited about it."
Observers on longline fishing boats had outfitted 50 turtles with electronic transmitters for Balazs' program, Polovina said.
More sophisticated recorders were used to transmit data to National Atmospheric and Oceanographic Administration satellites with Argos receivers, allowing Polovina to study oceanographic features and the animals' dive profiles.
Satellite-linked time-depth recorders were placed on two loggerheads and two olive ridleys to see if loggerheads are diving deep enough in the central North Pacific to be caught in deep longline gear.
Polovina said the deepest loggerhead dive recorded was 584 feet, "which is pretty deep nevertheless for turtles we're used to seeing around Oahu. Green turtles are always in a shallow area."
Olive ridley turtles, which aren't endangered, often dive below 830 feet, he said. However, that's the deepest the recording device goes, so they may dive even deeper, he said.
"They really do operate differently," he said, explaining loggerheads are found in colder, shallower northern waters with swordfish and olive ridleys are in warmer, southern water where the tuna are.
"Loggerheads generally are on the edge of oceanic fronts, generally feeding near the surface. Olive ridleys tend to go deeper and take advantage of the vertical structure in the ocean while loggerheads take advantage of the horizontal structure."
Loggerheads make shorter dives in the day and longer dives at night, he said. They drop down and sleep underwater, naturally buoyant, but spend most of their time at a shallower depth.
"We don't know if they're just exploring the water column to look, maybe, for the best foraging area, or maybe they're looking for the bottom because they come from a coastal habitat," Polovina said.
When the present longline fishing management plan was drawn up, Polovina said there were no data to indicate what would happen if deep gear was used in the northern waters because shallow sets were always used to catch swordfish.
Now, it looks as though turtles wouldn't be affected if the industry moves further north and sets gear below 330 feet, which is permissible.
But there are exceptions to the diving pattern, Polovina said. "One loggerhead spent a month and a half riding around the edge of an eddy and every day it dove to 200 to 300 feet.
"This is a depth where it might begin to encounter some deep-set longlines, particularly when the currents of the eddy may keep longlines from sinking deeper."
The researchers also discovered that the turtles aren't just randomly going across the ocean, Polovina said. Olive ridleys go back and forth, but loggerheads always move westward toward Japan.
There is a loggerhead population off Mexico, and the scientists think the animals are starting to leave the area and move across. Loggerheads have nesting beaches in Japan, "but these animals out there generally are too young to lay eggs in Japan," he said. "They're just spending several years out there in an oceanic habitat."
They travel along a wall of chlorophyll, a transition zone discovered by the Honolulu fishery biologists between low-chlorophyll subtropical waters and high-chlorophyll subarctic waters. "So they're using this as foraging pasture, but the olive ridleys are going in both directions ... We don't understand the total migration pattern," Polovina said.
But thousands of remote sensing observations help them learn where the turtles go and what they're doing.
"It's a phenomenal thing," Polovina said. "Now we're able to look at the ocean in ways we never could see it before, by putting instruments on animals to see where they're going, and by looking at remotely sensed data, seeing what the habitat looks like."
This has "really revolutionized" fisheries research, he added.
"Turtles are not supposed to be killed and hurt," Balazs said. "We want to come up with science-based ways management can use to have fisheries prosper with minimal or virtually no impact to species that are not target for food fish."
Setting longlines deeper for swordfish may not protect loggerhead turtles, according to a fisherman and an environmental lawyer. Deeper lines could still
snag turtles, critics sayBy Helen Altonn
haltonn@starbulletin.comPaul Achitoff, the Earthjustice Legal Defense Fund attorney who sued the National Marine Fisheries for failing to protect turtles, said he doesn't question results of recent research by two NMFS scientists.
"I only question extrapolating too far with it to the point where you're suggesting you can't catch loggerhead turtles with deep-set gear," he said.
"You may not catch as many of them, relative to shallow gear, but you still would catch some."
Isaac Harp, Lahaina fisherman and member of the Hawaiian Environmental Alliance, said he's not sure deeper lines, which are used for tuna, would be effective for swordfish. And it's questionable whether they would protect turtles, he said.
"Turtles probably dive on the bait while they're setting the lines. They're setting out thousands of hooks. Turtles see the bait going down and they'll dive on it. I think that's how they get hooked."
Achitoff said turtles may rarely dive as deep as swordfish gear, as NMFS scientists Jeff Polovina and George Balazs discovered. "But as Isaac points out, it doesn't mean turtles will never find their way to the gear."
When boats are setting and retrieving gear, it won't be shallow any more, Achitoff said. "Who knows? We will find out. There will be observers (on the boats). If they come up with any loggerheads, we will know.'
Harp, who served on six advisory panels to the Western Pacific Regional Fishery Management Council, said he offered the council a solution to incidental turtle catches by longline boats. He recommended running the line down from the boat in a long metal tube with a weight so it will sink faster and stay down.
"The combination of those two things would help a lot in eliminating the impact on seabirds and probably turtles, although I'm not really sure on the turtles."
So far, he said, the only thing he knows that the longline fishermen tried was dying the bait blue.
"They used that to lessen the impact on seabirds. They thought if the bait was blue birds couldn't see it as well."
It didn't work, he said, because the birds knew what the boats were for. "I think the birds know what is going on."
He said the council "should start thinking of what's best for the resources ... or eventually we're not going to have anything left out there.
"The fish are smaller and smaller because we're depleting the stock. The breeding size is getting smaller. The full-grown size is shrinking. Eventually we're gonna be catching little fish that are called 'adults.'"