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

Utilizing sophisticated underwater acoustic monitoring technology, researchers deployed an array of hydrophones strategically within the Little Belt to capture the intricate soundscape produced by harbour porpoises. By correlating these acoustic data with precise vessel traffic records over multiple diurnal and seasonal cycles, the study delineates a clear inverse relationship between boat presence and porpoise acoustic activity. During peak vessel transit periods, the incidence of porpoise buzzes declined by as much as 45%, signaling a substantial reduction in natural foraging and social behaviors critical to their ecological fitness.

This research is particularly salient given the unique ecological niche that harbour porpoises occupy in shallow coastal waters. Their high metabolic demands necessitate near-continuous feeding to maintain energy homeostasis. Consequently, any disruption to their ability to engage effectively in feeding jeopardizes their health, growth, and reproductive success. The observed decrease in buzzing behavior thus suggests a tangible stressor imposed by anthropogenic noise and physical disturbance, potentially compromising the population’s long-term viability.

The team’s analysis revealed that the most pronounced behavioral suppression occurred during daylight hours in summer — precisely when vessel traffic intensifies due to both commercial shipping and recreational boating. Conversely, in moments devoid of vessel presence, porpoise acoustic activity markedly increased, highlighting their behavioral plasticity but also underscoring the persistent impact of human activity on their natural rhythms.

Importantly, the study emphasizes that harbour porpoises cannot simply relocate to quieter waters. The Little Belt offers a unique prey density and habitat configuration essential for their survival. Alternate habitats may lack the necessary abundance and diversity of prey species, suggesting that displacement is not a viable mitigation strategy. This ecological constraint accentuates the urgency for targeted conservation measures within this critical habitat.

The study’s co-lead author, Rachel Lennon, whose work was conducted as part of a master’s program in Marine Vertebrate Ecology and Conservation at the University of Exeter, stresses that chronic exposure to maritime traffic represents a sustained challenge to harbour porpoise populations. This persistent disturbance potentially undermines the animals’ ability to fulfill crucial life history functions, including foraging efficiency and social bonding, which collectively underpin population resilience.

Shannon Merkle, a co-lead on the project, contextualizes these findings within a broader ecological framework, drawing on a growing body of evidence linking foraging disruption to declines in cetacean health, body condition, reproductive output, and survival rates. The research argues that the observed behavioral alterations may exacerbate the vulnerability of harbour porpoises to other environmental pressures, including pollution, climate change, and prey variability.

From a conservation perspective, these findings have immediate implications for the management of vessel traffic in highly frequented coastal zones. The study advocates for the implementation of mitigation strategies such as reducing boat speeds, restricting vessel numbers during critical periods, and designating protected marine areas with regulated access. Such tactics aim to minimize noise pollution and physical disturbances, thereby fostering an environment where harbour porpoises can feed and socialize without impairment.

The Little Belt region, forming one of the three narrow straits connecting the Baltic Sea and the North Sea, experiences thousands of large commercial ship passages annually, in addition to countless recreational boats. This intense marine traffic not only introduces noise pollution but also alters the acoustic structure of the habitat, complicating the porpoises’ echolocation and communication abilities that are vital for prey detection and social interaction.

Adding a layer of urgency, the harbour porpoise population in the Belt Sea is currently classified as “Endangered” by the Helsinki Commission (HELCOM), a regional marine environmental protection body. This status underscores the critical need for immediate conservation action, since any additional stressors such as vessel disturbance could further accelerate population decline.

Jonas Teilmann, professor at Aarhus University and co-author of the study, emphasizes how this research enhances our understanding of underwater noise ecology and its biological ramifications. He proposes that integrating these insights into maritime spatial planning could lead to optimized vessel scheduling and speed regulations, greatly benefiting noise-sensitive species like the harbour porpoise while allowing sustainable maritime economic activities.

Carried out in collaboration between Aarhus University and the industry partner Seiche, this comprehensive study bridges academic research and practical conservation efforts. Published in the prestigious journal Marine Mammal Science, the paper titled “Seasonal and diurnal patterns of harbor porpoise (Phocoena phocoena) behavior and the disruptive effects of vessel presence in a high-traffic coastal habitat” represents a pivotal addition to marine ecology literature, catalyzing further research and policy interventions aimed at protecting one of the Baltic’s most vulnerable marine mammals.

Subject of Research: Harbour porpoises’ behavioral response to vessel traffic in the Little Belt, Denmark

Article Title: Seasonal and diurnal patterns of harbor porpoise (Phocoena phocoena) behavior and the disruptive effects of vessel presence in a high-traffic coastal habitat.

News Publication Date: 9-Feb-2026

Web References: http://dx.doi.org/10.1111/mms.70123

Image Credits: Jakob Højer Kristensen, Bionaut

Keywords: Marine mammals, Marine life, Marine ecology, Marine conservation