New study measures effects of entanglement
By Summit Voice
FRISCO — Stray fishing gear has long been a problem in the ocean, and a new collaborative study shows exactly how whales struggle when they get wrapped up in abandoned lines. By carefully tracking tangled whales, the scientists documented how the predicament hinders whales’ ability to eat and migrate, depletes their energy as they drag gear for months or years, and can result in a slow death.
The special tracking device, called a Dtag, was developed at Woods Hole Oceanographic Institution. The researchers attached the tracker to a two-year-old female North Atlantic right whale called Eg 3911 to monitor the whale’s movements before, during, and after at-sea disentanglement operations. Immediately after Eg 3911 was disentangled from most of the fishing gear, she swam faster, dove twice as deep, and for longer periods.
The study, by scientists at WHOI, the Georgia Department of Natural Resources, the Florida Fish and Wildlife Conservation Commission, and NOAA Fisheries, was published online May 21 in the journal Marine Mammal Science.
“The Dtag opened up a whole new world of Eg 3911’s life under water that otherwise we weren’t able to see,” said Julie van der Hoop, lead author of the study and a graduate student in the MIT/WHOI Joint Program in Oceanography.
North Atlantic right whales were nearly eradicated by whaling and remain endangered today, with a population of 450 to 500. About 75 percent bear scars of fishing lines that cut into their flesh.
Born in 2009, Eg 3911 was first sighted entangled and emaciated by an aerial survey team on Christmas Day 2010, near Jacksonville, Florida. Fishing gear was entangled around her mouth, wrapped around both pectoral fins, and trailed about 100 feet behind her tail.
Teams aboard boats attempted to cut away the fishing gear on Dec. 29 and 30, 2010, but the whale evaded the would-be rescuers. A multiagency team tried again on Jan. 15, 2011. First, they applied a Dtag. Then they administered a carefully calculated sedative with a dart gun The becalmed whale allowed the team to approach and remove nearly all the fishing gear.
The Dtag measured 152 dives that Eg 3911 took over six hours. There were no significant differences in depth or duration of dives after sedation, but “the whale altered its behavior immediately following disentanglement,” the scientists reported. “The near-complete disentanglement of Eg 3911 resulted in significant increases in dive duration and depth.”
“Together, the effects of added buoyancy, added drag, and reduced swimming speed due to towing accessory gear pose many threats to entangled whales,” the scientists wrote. Buoyant gear may overwhelm animals’ ability to descend to depths to forage on preferred prey. Increased drag can reduce swimming speeds, delaying whales’ timely arrival to feeding or breeding grounds. “Most significant, however, is the energy drain associated with added drag,” they said.
The researchers also took detailed measurements of the drag forces by towing gear behind a skiff, calculating that the entangled whales have significantly higher energy demands, requiring 70 to 102 percent more power to swim at the same speed unentangled. Alternatively, they need to slow down their swimming speed by 16 to 20.5 percent.
On Feb. 1, 2011, an aerial survey observed Eg 3911 dead at sea. The whale was towed ashore for a necropsy.
“She didn’t make it,” van der Hoop said. “We showed up on the beach that night. I remember walking out there and seeing this huge whale, or what I thought was huge. She was only 10 meters long. She was only two years old. And all these people who had been involved in her life at some point, were there to learn from her what entanglement had caused.”
The necropsy showed that effects of the chronic entanglement were the cause of death.
“No fisherman wants to catch a whale, and I wish no fisherman a hungry day,” said Moore. “There needs to be a targeted assessment of how the fishery can still be profitable while deploying less gear so we can reduce the risk of marine mammals encountering fishing gear in the first place. At WHOI, we have hosted workshops talking with fisheries managers and fishermen about what might change so that they can continue to catch fish and stop catching whales.”