It has long been thought that sea turtles were silent and not very sensitive to noise. This is false, and their sensitivity to certain sounds can even be used to avoid the accidental capture by fishing nets of these species crucial to marine biodiversity.
Sea turtles play a crucial role in marine ecosystems by regulating the populations of other species and influencing the physical structure of habitats. They contribute to the health of the oceans by feeding on seagrass, sponges, jellyfish or crustaceans, and thus preserving seagrass beds, coral reefs, and ecological balances. They also promote nutrient recycling and provide mobile habitat for other organisms. As prey, they support the food chain and participate in the dispersal of marine species by migrating over long distances. The preservation of sea turtles is therefore crucial for the health of marine ecosystems.
Nearly 800 green turtles in fishing nets each year in Martinique
Accidental captures of marine turtles by fishing nets therefore constitute a major threat both to the survival of this species and to the economy of fisheries, particularly in the French West Indies. Fishing areas, both coastal and offshore, often overlap with turtle breeding and feeding habitats. In Martinique, around 800 green turtles are captured each year in gillnets.
This unintended interaction causes economic losses for fishermen, not only by reducing the catch of target species, but also by damaging their fishing gear. Accidental captures persist despite the ban on turtle fishing in the French West Indies since the 1990s. It is therefore urgent to develop methods to limit these interactions while ensuring the sustainability of artisanal fishing activities, which play a crucial role. in the local economy. For this, the study of the vocalizations of sea turtles can be of great help.
Turtle vocalization, a recent object of study
This field of research, like the behavioral responses of turtles to acoustic signals, remains a neglected aspect of their ecology. Although cetaceans have been widely studied for their use of sounds in communication and navigation, sea turtles, on the other hand, have long been considered silent species and not very sensitive to sound stimuli. However, recent findings on green turtle vocalizations (Chelonia mydas) in the Caribbean Sea, more precisely in Martinique, reveal that these animals produce sounds in certain situations, particularly in response to social stimuli or dangerous situations. We therefore sought to see how we could harness these sounds to reduce harmful interactions between sea turtles and fishing gear, an approach that could offer innovative solutions for the conservation of these endangered species.
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Previously ineffective techniques for preventing accidental captures
Until now, many methods have been tried to reduce accidental captures, with mixed success. Among them, visual deterrent devices (VDD), such as green and UV LEDs, have shown some effectiveness in reducing turtle captures by making nets more visible to turtles, thus encouraging them to use them. avoid.
However, these devices have several limitations: they are expensive, require constant maintenance, and the use of lithium batteries raises environmental concerns. Furthermore, their effectiveness has not yet been proven on a large scale in commercial fisheries.
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Acoustics, a tool already used to avoid cetacean captures
The approach proposed in our study is based on the idea that sounds naturally produced by turtles could serve as warning signals, encouraging turtles to avoid fishing nets. Acoustic devices (Acoustic Deterrent Devices, ADD) have already proven themselves in reducing accidental captures of cetaceans, which rely heavily on sound for their navigation and communication. However, until now it has been widely assumed that acoustic communication in sea turtles is negligible. We therefore set out to prove the opposite by demonstrating that sounds produced by turtles themselves can trigger behavioral responses that could be used to reduce incidental captures.
Ten types of sounds recorded in sea turtles
Our study took place in five bays of Martinique between 2018 and 2023. We used on-board cameras and hydrophones (a type of microphone that can be used underwater) attached to the shell of juvenile green turtles. to record the sounds produced by these animals. We have thus identified ten types of sounds, classified into four main categories: “impulses” (pulse), “low amplitude calls” (LAC), “frequency modulated sounds” (FMS) and “squeaks” (squeak). These sounds were then played to green turtles in their natural environment to observe their reactions. At the same time, synthetic sounds and natural sounds (such as earthquakes) were also played to compare the turtles’ reactions.
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An immediate leak in 95% of cases
The results of the study show that turtles react significantly to sounds produced by their conspecifics, with an estimated reaction probability of 95%. In particular, “rumble” type sounds (rumble) caused increased alertness or immediate flight in 94.4% of the turtles tested, while “squeaks” (squeak) triggered vigilance or flight responses in 60.7% of turtles. Conversely, turtles showed no response to synthetic sounds or natural sounds such as earthquakes. This lack of reaction to synthetic sounds can be explained by the fact that sea turtles are continually exposed to anthropogenic noise in their feeding areas and therefore no longer pay attention to it. The turtles’ reactions to sounds also varied depending on the distance between the sound source and the turtle. Tests showed that the turtles were more likely to respond when the sound was played within 200 meters of their position. Additionally, the turtles showed signs of habituation, meaning that after several consecutive exposures to the same sound, they were less likely to respond.
A promising technique for sea turtle conversation
These results offer new perspectives for reducing accidental catches. By broadcasting the sounds produced by turtles, it might be possible to alert turtles to the presence of fishing nets, thus encouraging them to move away from them. This approach has several advantages: it relies on natural sounds that turtles are already sensitive to, and it could be applied to other marine species or other potentially dangerous environments for turtles, such as boat propellers or equipment. dredging.
The study also highlights the importance of continuing research into the diversity of sounds produced by different species of sea turtles and the variations in behavioral responses depending on the size and age of individuals. This research should be prioritized in order to improve the effectiveness of acoustic devices and expand their use to other fisheries around the world.
In Gascony, a similar device for dolphins
Although this acoustic approach seems promising, it is still in an experimental stage. To maximize its effectiveness, it is recommended to further research the hearing abilities of sea turtles and conduct similar studies in other regions of the world, to better understand turtle responses to different types of sounds. In addition, the development of suitable acoustic devices will require funding, but the study highlights that European and French funds are available to support these initiatives. For example, a similar project recently received €6 million in funding to equip fisheries in the Bay of Biscay with acoustic devices to reduce accidental capture of dolphins.
This article is published as part of the Science Festival (which takes place from October 4 to 14, 2024), and of which The Conversation France is a partner. This new edition focuses on the theme “ocean of knowledge”. Find all the events in your region on the Fetedelascience.fr website.
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