How acoustic research is helping scientists understand why harbour porpoises become entangled in fishing nets — and what can be done to prevent it.
Harbour porpoises (Phocoena phocoena) face numerous pressures in UK waters, but one of the most serious is bycatch — the accidental capture of animals in fishing gear.
Unlike fish, porpoises are mammals that must return to the surface to breathe. When they become entangled in a fishing net, they may be unable to surface and can drown.
Bycatch is recognised as a major conservation concern for harbour porpoises, particularly in fisheries using static nets.
Tangle nets and bottom-set gillnets
Two types of fishing gear associated with porpoise bycatch are bottom-set gillnets and tangle nets.
Bottom-set gillnets are anchored to the seabed and held upright by floats. They are designed to catch fish as they swim into the mesh, becoming caught around their gills or body.
Tangle nets are generally more loosely hung, allowing fish to become entangled rather than simply caught by their gills. They are used to target species such as monkfish and other bottom-dwelling fish. Tangle nets are themselves often bottom-set, so the two categories overlap.
Both types can present a risk to porpoises moving and feeding close to the seabed.
Although porpoises possess highly sophisticated echolocation, allowing them to detect objects underwater, this does not guarantee that they will avoid fishing nets.
Understanding why an animal capable of detecting a net nevertheless becomes entangled is an important scientific challenge.
Listening to porpoises around fishing nets
Clyde Porpoise was pleased to assist researchers at the University of St Andrews’ Sea Mammal Research Unit (SMRU) in testing specialised acoustic recording equipment developed to investigate this problem.
The research, led by Dr Jamie Macaulay, was published in 2022 in Methods in Ecology and Evolution.
The equipment uses multiple hydrophones to detect porpoise echolocation clicks and reconstruct the animals’ movements in three dimensions around fishing nets.
During four deployments in UK waters, researchers recorded 12 sufficiently detailed harbour porpoise encounters. The results revealed animals actively feeding close to nets, sometimes approaching within just a few metres without becoming entangled.
These observations suggested that bycatch involves more complex behavioural interactions than simply failing to detect an obstacle.
Clyde Porpoise’s contribution to testing the equipment was acknowledged in the published research.
A remarkable insight into an actual bycatch event
In August 2026, the SMRU researchers published a further study describing the acoustic behaviour of two harbour porpoises during an actual bycatch event in a static net off Cornwall.
The recordings showed both animals foraging near the net before one became entangled.
The trapped animal struggled sufficiently to lift part of the net by approximately seven metres before dying. The second porpoise remained nearby and produced unusual click sequences interpreted as possible communication behaviour.
The researchers suggested that the animals may have been socially associated, potentially a mother and older calf.
This rare recording provided valuable insight into what happens immediately before, during and after a porpoise becomes entangled.
It also raised important questions about how feeding behaviour, social interactions and responses to fishing gear might influence bycatch risk.
What does this mean for the Clyde?
The Clyde supports harbour porpoises alongside a range of commercial and recreational fishing activities.
Understanding where porpoises feed, how they move through the environment and where their habitat overlaps with static fishing gear is important when assessing potential bycatch risks.
However, evidence from fisheries elsewhere in the UK cannot simply be used to estimate how many porpoises are caught in the Clyde. That requires local information on fishing activity, net types and bycatch incidents.
Clyde Porpoise’s long-term acoustic surveys can help identify areas of regular porpoise activity and provide information relevant to assessing potential interactions with fishing gear.
Reducing the risk
Potential solutions include acoustic warning devices known as pingers, modifications that make nets easier for porpoises to detect, changes to fishing practices and targeted management in areas of particular risk.
Each approach requires careful assessment. Acoustic deterrents, for example, can reduce bycatch but may also introduce additional underwater noise or displace animals from feeding habitats.
Working closely with fishers will be essential to developing practical measures that protect porpoises while supporting sustainable fishing livelihoods.
The more we understand about how porpoises behave around fishing nets, the better equipped we will be to prevent accidental entanglement.
For Clyde Porpoise, contributing to research that improves this understanding is an important part of our wider commitment to studying and conserving harbour porpoises in Scottish waters.
Further reading
Macaulay, J. et al. (2022). Passive acoustic tracking of the three-dimensional movements and acoustic behaviour of toothed whales in close proximity to static nets. Methods in Ecology and Evolution, 13, 1250–1264.
Macaulay, J. et al. (2026). First detailed insights into harbour porpoise behaviour during a bycatch event. Royal Society Open Science, 13(8), 260565.
Scottish Government (2026). Scotland’s Future Catching Policy: Selectivity Proposals Consultation. Includes information on gillnets, tangle nets and measures to reduce sensitive-species bycatch.
Read Scottish Government information
Royal Society (2026). Working with Fishers to Understand Porpoise Bycatch. An accessible account of the research, including an animation of the recorded bycatch event.