It’s estimated that more than 100,000 sea ducks are killed worldwide annually when caught in large fishing nets known as gillnets. A recent study identifies the frequencies at which sea ducks can hear underwater, and the findings can help inform decisions on the use of sound devices to deter ducks from nets and other threats.
New insight on sea duck hearing could help protect them from fishing nets
“Different species can hear at various frequencies and intensities underwater, and prior to this study, there was limited knowledge on underwater hearing of sea ducks and other seabirds,” said U.S. Geological Survey research wildlife biologist Alicia Berlin, who is a coauthor on this study. “Previous studies, including several by the USGS, have focused on hearing in air.”
Peak hearing between 1 to 3 kilohertz
Scientists found that peak underwater hearing sensitivity for the sea ducks in this research is between 1 to 3 kHz. The findings suggest that an auditory device, known as a pinger, emitting a 2 kHz tone has the potential to work as a deterrence for all species studied. The research included five long-tailed ducks, three surf scoters and one common eider.
“This study provides a baseline understanding on the underwater hearing range of diving ducks and is one of the first steps in determining whether an acoustic device could be effective in achieving deterrence and protecting them from underwater human activities,” continued Berlin.
Sea ducks dive for food and are vulnerable not only to entanglement in gillnets but other potential dangers, including ships, construction and seismic surveys. These types of interactions between marine life and humans continue to increase due to human population growth and industrialization.
Scientists monitor brain signals
The birds were raised as ducklings at the USGS Eastern Ecological Science Center in Maryland and the research was done at the center’s seabird colony.
As one part of the study, and after USGS scientists trained the ducks for several months, researchers recorded whether the ducks pecked a target after tones were played at certain frequencies. If they did, that indicated they could hear the sound. In addition, researchers monitored the ducks’ brain signals as sounds were emitted underwater.
“This is the most comprehensive study to date as we looked at several species of sea ducks, and the first time that their brainstem response was measured underwater,” said Berlin.
All research procedures were approved by the Animal Care and Use Committee at the USGS Eastern Ecological Science Center.
Studies by other organizations have shown that sound devices can be effective in reducing the number of marine mammals caught in gillnets. Those devices have been designed to emit tones outside of the audible range for most fish species, which hear at lower frequencies.
Limitations and acknowledgments
While researchers suggest a device emitting 2 kHz frequencies might be a good “catch all,” other frequencies might be more effective. While the species shared a common range, long-tailed ducks exhibited greatest hearing sensitivity at 2.96 kHz, surf scoters were at 1 kHz and common eiders showed comparable responses between 1 and 3 kHz.
Additionally, existing pingers such as a 3kHz one already in use for whales might be sufficient for sea ducks but the development of new devices with different frequencies could be more effective mitigation strategy. The USGS and partners are also involved in other related projects, including investigating whether visuals would be another potentially effective approach.
The research was authored by the USGS, the Virginia Maryland College of Veterinary Medicine, the University of Delaware and the Naval Undersea Warfare Center.
The study was published in the Journal of Experimental Biology and is available online at https://doi.org/10.1242/jeb.243953.
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