First published in Landings, July, 2013.
In a report released in May, Woods Hole Oceanographic Institute (WHOI) scientists provided a new assessment of how fishing lines change a whale’s diving and swimming behavior. They found that an entangled whale suffers from the effects of added buoyancy and drag, expending critical energy as it swims with that gear.
A two-year old female right whale, called Eg 3911, was first sighted entangled by an aerial survey team on December 25, 2010, near Jacksonville, Florida. Fishing gear was around her mouth, wrapped around both pectoral fins, and trailed about 100 feet behind her tail.
Disentanglement teams attempted to cut away the fishing gear on December 29 and 30, but were not successful because the whale was evasive. Another team tried again on January 15, 2011. This team attached a cellphone-size device called a Dtag, developed at WHOI, to the whale. The Dtag recorded the whale’s movements before, during, and after at-sea disentanglement operations.
The disentanglement team also administered a sedative with a dart gun developed for large whale drug delivery by Paxarms NZ in collaboration with Dr. Michael Moore, director of the Marine Mammal Center at WHOI and a marine mammal veterinarian. The sedated whale allowed the team to approach and remove nearly all the fishing gear.
The Dtag then measured 152 dives that Eg 3911 took over six hours. Immediately after Eg 3911 was disentangled from most of the fishing gear, she swam faster, dove twice as deep, and for longer periods. “The whale altered its behavior immediately following disentanglement,” the scientists reported in their paper. “The near-complete disentanglement of Eg 3911 resulted in significant increases in dive duration and depth.”
Unfortunately, when the whale was next observed through an aerial survey on February 1, she was dead.
After analyzing the data gathered by the monitor, the WHOI scientists concluded that entanglement in buoyant gear may overwhelm a whale’s ability to descend to depths to forage on preferred prey. Increased drag also can reduce swimming speeds, delaying whales’ timely arrival to feeding or breeding grounds. “Most significant, however, is the energy drain associated with added drag,” said Julie van der Hoop, lead author of the study and a graduate student in the MIT/WHOI Joint Program in Oceanography.
To calculate that drain, the scientists towed three types of fishing gear from a skiff, using tension meters to measure the drag forces that may have affected Eg 3911. They then calculated how much more energy a whale would require to compensate for the drag. Based on that test, van der Hoop and her colleagues estimated that entangled whales have significantly higher energy demands, requiring 70 to 102 percent more power to swim at the same speed as they would unentangled. Many whales spend months and even years entangled in fishing gear, leading to an inexorable drain on their energy stores. 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.