False killer whale's foraging behaviour captured on video
Using video and audio recordings, a team of researchers and fishermen has directly observed for the first time how Hawaiian false killer whales remove fish from longline fishing gear. It is hoped that the study would offer insight into how to reduce impacts to animals and fishing operations.
False killer whales consume popular game fish like yellowfin tuna and mahi-mahi. They forage in the same open-ocean regions where commercial fishermen set fishing lines (as long as 30 to 60 kilometres) to catch the same fish. This competition has caused them to occasionally end up as by-catch. Such entanglements between fishing gear and offshore false killer whales take place so often in Hawaii that the population has been designated a “strategic stock” under the Marine Mammal Protection Act.
A team, coordinated by Scripps Institution of Oceanography at the University of California San Diego scientist Aaron Thode, managed to observe false killer whales removing fish from a longline fishing hook, a behaviour known as depredation.
Using an underwater camera, sound recorder and vibration detector on long-line fishing gear deployed by fishing vessels off Hawaii, they wanted to learn more about the animals’ behaviour, such as what attracts them to the gear, whether they make sounds as they approach, and if they removed bait or the targeted species from the hook. They also wanted to measure how far away the animals could be heard to provide new information into future passive acoustic surveys of the population.
“This study addresses some important questions about the nature of this depredation, and whether underwater sound can be used to study or possibly alleviate the issue,” said Thode, a researcher with the Scripps Marine Physical Laboratory. He is the lead author of a paper on this issue, recently published in the Journal of the Acoustical Society of America.
In the video footage, the false killer whale made distinct clicks and whistles as it approached the longlines and took baitfish off the hook. Vibrations on the fishing line when the animal removed three fish also enabled the researchers to estimate how far away the animal was from the recorder to calculate how loud the animal’s sounds were.
“Further acoustics studies could help us understand more about these animals’ behaviour, which could be used by fishermen to hear the animals make these whistles or echolocation noises before they deploy their gear,” said Thode. “The study can also be helpful in designing future experiments to estimate the actual number of animals in the region.”
He also suggested that the tugs and jerks that a given species makes on the fishing line could differ from other species, and might lead to new ways to recognise and reduce by-catch, protecting both animals and fishermen from unintentional encounters.
“This was a true collaborative effort with fishermen, fisheries managers and scientists from across the North Pacific working together that made this project a success,” said co-author Janice Straley, professor of biology at the University of Alaska Southeast. “Fishermen modified techniques used for depredation in Alaska for use in Hawaii and everyone worked on design and implementation.”
The research team hopes that this study may help test the concept of a so-called “smart hook” to reduce the impact to the animals and fishing operations.