Kemp’s ridley turtles might not understand why scientists are so keen to test their hearing, but they might benefit from the experience over time.
The sea turtle species is among the most endangered on Earth, living mainly on the eastern and Gulf coasts of North America alongside some of the world’s busiest shipping lanes.
The effect on them of human-generated noise is the subject of a new study by researchers from Duke University Marine Laboratory, the National Oceanic & Atmospheric Administration (NOAA) and North Carolina State University.
According to study author Charles Muirhead the turtles face a variety of threats, including accidental capture in fishing-gear, vessel strikes, ingestion of plastic debris and degradation of nesting beaches and coastal habitat.
“Because they occupy nearshore and shelf waters of the Gulf Coast and western North Atlantic – areas with intense human activity – they are frequently exposed to overlapping stressors throughout much of their life-cycle,” he says.
Sound waves, especially low-frequency waves produced by ships and other machinery, travel long distances under water, and tend to overlap with the sounds on which turtles and other marine animals rely for navigation.
Low-frequency band
Non-invasive sensors were placed on the turtles’ heads to measure electrical signals transmitted along their auditory nerves. The research team then played them sounds ranging from 50Hz – close to the low end of human hearing – up to 1,600Hz. (Humans hear best in the 2,000-5,000Hz frequency range.)
The turtles were found to hear best at around 300Hz, and their hearing started to decline at higher frequencies,
“Our findings indicate that the turtles are most sensitive in the same low-frequency band where much industrial and vessel noise occurs,” says Muirhead. “This does not automatically mean that harmful effects are occurring, but it highlights where further monitoring and targeted impact studies should be focused.”
Physical effects
The researchers now plan to study how turtles respond to sounds in real environmental conditions, as well as understanding the relationship between measured sound signals and the physical effects of noise on turtles.
“These efforts will help refine our understanding of how human-caused noise interacts with sea turtle sensory systems,” says Muirhead. “They will also support evidence-based management approaches aimed at minimising unintended impacts while balancing human activities in coastal and offshore waters.”
The study was published on 3 February in the Journal Of The Acoustical Society Of America (JASA).
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