The discovery emerged from a passive acoustic monitoring system deployed on a static gill net off the coast of Cornwall. Passive acoustic monitoring, commonly known as PAM, uses underwater instruments to listen to sounds produced by marine animals without actively transmitting signals into the water. Harbour porpoises rely heavily on sound because their underwater environment is often dark, murky and visually limited. They produce extremely high-frequency echolocation clicks, typically far above the range of human hearing, and interpret returning echoes to detect prey, obstacles and other features of their surroundings. The monitoring equipment was designed to study porpoise activity near the fishing gear, but it unintentionally recorded a bycatch event in exceptional detail. The resulting data allowed researchers to compare the animals’ acoustic behaviour with their movements around the net and during the entanglement.

At first, the recordings showed two harbour porpoises foraging close to the fishing gear. Their repeated movements and echolocation activity suggested that the animals were searching for prey in the area while navigating around the net. The porpoises appeared capable of detecting and avoiding the structure for several minutes, a finding that complicates the simple assumption that animals fail to sense fishing nets. Gill nets are made from thin, nearly transparent monofilament or multifilament lines that can be difficult for marine mammals to perceive, particularly when water conditions, light levels or background noise reduce the clarity of the returning echoes. The animals may have recognized the net as a potential hazard, but the researchers believe that a brief loss of attention—possibly during a rapid pursuit of prey—may have brought one porpoise into contact with it. In a moment, an animal that had been successfully navigating around the gear became caught.

Once entangled, the porpoise began a sustained effort to free itself. Movement data indicated that it was able to generate enough force to lift the net, but not enough to tear through or break the material. This distinction is important because the physical properties of fishing gear strongly influence whether an animal survives an encounter. A net that is flexible but exceptionally strong may hold an animal in place while allowing it to drag or raise a section of the gear, increasing energetic demands and restricting access to the surface. Harbour porpoises must breathe air, so every dive is constrained by the need to return to the surface. An entangled animal can become trapped below the water, lose efficient swimming ability, exhaust its oxygen reserves and drown. Even when it can reach the surface, the additional drag and stress may prevent normal breathing, escape or feeding.

The acoustic recordings provided evidence that the second porpoise remained nearby during the crisis. Both animals continued to produce echolocation clicks, which would have helped them sense the net, the surrounding seabed and one another. However, the researchers also detected rapid sequences of clicks that sounded like a buzzing signal after the recordings had been shifted into a frequency range audible to humans. Porpoise clicks are normally ultrasonic and cannot be heard directly by people. The rapid click trains identified in the recording are thought to represent communication signals, potentially associated with distress, agitation or attempts to maintain contact. Although scientists cannot determine the precise meaning of the signals from a single event, the contrast between ordinary echolocation and unusually rapid acoustic activity offers a rare glimpse into how porpoises may respond socially to danger.

The event lasted several minutes, long enough for the instruments to capture changes in movement, sound production and interaction between the two animals. Such data are extraordinarily difficult to obtain because researchers cannot safely or ethically recreate an entanglement, and direct observation in the open ocean is nearly impossible. Monitoring only a fraction of the thousands of kilometres of fishing nets deployed in UK waters, researchers say the probability of recording a bycatch event at the exact moment it occurs is extremely small. The observation therefore acts like an underwater black box, preserving a sequence that would normally vanish without a trace. It shows the animal approaching and working around the net, becoming trapped, attempting to escape and remaining acoustically connected with its companion. For conservation scientists, that sequence is more informative than a simple record that an animal was found dead or missing.

The findings could influence the design of fishing gear and acoustic deterrence technologies. One possibility is to make nets more acoustically reflective, increasing the strength or clarity of the echoes returned to a porpoise and making the net easier to detect. Researchers are also investigating modified net materials and construction methods that could allow larger or stronger animals to break free rather than remain trapped. These approaches must be tested carefully because a change intended to protect porpoises could alter the net’s performance, reduce target catches or create new hazards for other species. Another potential strategy involves biomimicry: developing warning signals inspired by naturally meaningful sounds or by the acoustic cues marine mammals already use to assess danger. The challenge is to discourage porpoises from entering a hazardous area without causing harmful stress, disrupting essential behaviour or driving animals away from important feeding grounds.

The study also highlights why practical solutions must be developed with fishing communities rather than imposed without considering the realities of working at sea. Static net fishing supports coastal livelihoods and is an important part of the fishing industry, meaning that eliminating all such gear is neither a simple nor necessarily realistic conservation strategy. The more immediate goal is to understand precisely how and why entanglements happen, then reduce the risk through evidence-based changes that fishers can use. Researchers at St Andrews work with fishing communities through the UK Bycatch Monitoring Programme, funded by the Department for Environment, Food and Rural Affairs, and through CIBBRiNA, a European Union-funded initiative focused on reducing bycatch of endangered, threatened and protected species. Collaboration can help determine which modifications are affordable, effective and compatible with real fishing conditions.

The scale of the problem makes those improvements urgent. In the United Kingdom alone, approximately 1,000 harbour porpoises are estimated to die each year after becoming trapped in static fishing nets, while global losses affect numerous species of dolphins, porpoises and whales. Harbour porpoises are small, fast-moving cetaceans with high metabolic demands, so an entanglement can become fatal quickly. Their dependence on high-frequency echolocation also means that the acoustic properties of fishing gear may be central to both the problem and the solution. The new recording does not provide a complete explanation for every bycatch event, and the meaning of the porpoises’ communication signals remains uncertain. It does, however, transform an invisible conservation threat into a documented behavioural sequence. By listening to the final minutes of an entangled animal’s struggle and the possible distress calls of its companion, scientists may be closer to designing fishing nets that marine mammals can detect, avoid or escape—turning a tragic accidental recording into a potential blueprint for saving thousands of animals.

Subject of Research: Animals, specifically harbour porpoises

Article Title: First detailed insights into harbour porpoise behaviour during a bycatch event

News Publication Date: 26 August 2026

Web References: https://doi.org/10.1098/rsos.260565

References: Royal Society Open Science, “First detailed insights into harbour porpoise behaviour during a bycatch event,” DOI: 10.1098/rsos.260565

Image Credits: Sea Mammal Research Institute/University of St Andrews

Keywords: harbour porpoise, bycatch, fishing nets, passive acoustic monitoring, marine mammals, echolocation, distress calls, marine conservation, gill nets, cetacean behaviour

Beaked whales are infamous for their cryptic lifestyles, spending most of their time far from human sight in offshore, deep waters. Unlike the more populous and recognizable whales such as blue whales or orcas, beaked whales emerge only fleetingly at the ocean surface, rendering traditional observation techniques less effective. Their inconspicuous surfacing patterns are believed to be driven partly by predation pressures, as avoiding detection by predators is crucial for survival. These whales also hold the distinction of being the deepest divers among mammals, plunging to depths of around 3,000 meters for durations exceeding two hours – a feat that continues to fascinate marine biologists and physiologists alike.

A collaborative team of researchers from Brazilian institutions, including Instituto Aqualie and Juiz de Fora Federal University, embarked on an ambitious study beginning in 2022 to tackle the challenges associated with studying these elusive creatures. Their approach centered on combining visual observations with passive acoustic monitoring techniques. Using hydrophones and autonomous recording devices capable of functioning at ultra-high frequencies between 192 and 384 kilohertz, the team successfully captured detailed sound recordings of beaked whales, enabling them to correlate acoustic data with visual sightings gathered concurrently in the Foz do Amazonas Basin.

This dual approach yielded nine distinct audio recordings alongside four confirmed visual encounters. Upon rigorous analysis of these acoustical signatures, the researchers concluded that at least three separate beaked whale species were present in the recorded samples. This finding marks a significant advancement in our understanding of the species composition and acoustic characteristics of beaked whales inhabiting Brazilian waters – a region that until now had been sporadically studied with limited data available.

One of the focal points of the study involves the acoustic emissions produced by beaked whales. Unlike other toothed whales that use echolocation clicks abundantly at the surface, beaked whales emit their echolocation pulses primarily during deep dive phases, which complicates efforts to attach specific sounds to accurately identified species. The researchers’ high-frequency acoustic recordings provided unprecedented detail about these pulses, laying the foundation for more comprehensive species classification via remote sensing techniques.

Raphael Barbosa Machado, lead author of the study, emphasized the importance of their work in expanding cetacean biodiversity knowledge in Brazilian maritime zones. He explained that acoustic monitoring presents a promising tool to unveil the behaviors and distribution of these cryptic animals, which are otherwise difficult to track visually. This method leverages advances in bioacoustics and underwater technology to fill critical gaps in scientific understanding, thereby offering avenues for improved conservation strategies.

The researchers highlighted that their findings are instrumental for both ecological research and the formulation of public policies. Effective management and conservation efforts for beaked whales depend heavily on accurate data regarding their habitat use, population dynamics, and responses to environmental pressures. The complex acoustic environment of the deep Atlantic, coupled with the elusive habits of these whales, makes the establishment of reliable monitoring systems a top priority for marine biologists.

Supporting these endeavors, the study advocates for continued and expanded acoustic surveillance in the western South Atlantic Ocean, an area still considered underexplored in marine mammal research. Increasing the frequency of simultaneous visual and acoustic records will enhance species-specific acoustic profile databases, thereby refining species identification methodologies. Such iterative progress bolsters the scientific community’s ability to document marine biodiversity effortlessly and non-invasively.

Moreover, by capturing the first truly detailed acoustic parameters of beaked whales in Brazilian waters, Machado and his colleagues set a precedent for future studies worldwide. Their methodology demonstrates that even the most elusive marine mammals can be studied effectively through innovative monitoring techniques, thus offering hope for uncovering unknown populations and behaviors that remain hidden beneath the ocean’s surface.

Beyond the immediate scientific implications, this research underscores the ecological significance of the Foz do Amazonas Basin itself. Positioned at the confluence of major ocean currents and rich in biodiversity, this basin serves as a natural laboratory for studying marine life adapted to extreme conditions. The presence of multiple beaked whale species highlights its role as critical habitat and raises awareness about the need to protect such vulnerable ecosystems against mounting anthropogenic pressures.

Looking ahead, the integration of bioacoustic monitoring with other emerging data collection frameworks such as satellite tracking and environmental DNA sampling holds great promise. These multidisciplinary approaches are poised to revolutionize the way scientists study deep-diving cetaceans, offering unprecedented resolution in understanding their population structures, migration patterns, and ecological roles, thereby facilitating informed conservation decision-making.

In summary, the 2025 study spearheaded by Machado et al. represents a major milestone in marine mammal research. By combining visual and acoustic evidence, the team has opened new frontiers for exploring one of the ocean’s most secretive inhabitants. The insights gleaned not only deepen our appreciation of beaked whale biology but also underscore the vital importance of advancing marine acoustic technologies to illuminate the mysterious depths where these magnificent creatures reside.

Subject of Research: Acoustic and visual documentation of beaked whales in the Foz do Amazonas Basin, focusing on deep-diving cetaceans and their bioacoustic characteristics.

Article Title: Finding beaked whales in the Foz do Amazonas Basin: Visual and acoustic records of a deep diving cetacean

News Publication Date: September 9, 2025

Web References:
https://doi.org/10.1121/10.0038973

References:
Machado, R.B., Mura, J.P., Ferreira, G.A., de Castro, F.R., Rodrigues-Soares, N.S., Kascher, L.K.L., da Silva, B.S., Rodrigues, G.M., Alencar, L., Viana, Y., de Godoy, D.F., de Castilho, P.V., & Andriol, A. (2025). Finding beaked whales in the Foz do Amazonas Basin: Visual and acoustic records of a deep diving cetacean. The Journal of the Acoustical Society of America. DOI: 10.1121/10.0038973.

Image Credits: Machado et al.

Keywords: Whales; Cetaceans; Marine mammals; Acoustics; Animal sounds