Fisheries interactions are widely recognised as the leading human-caused source of marine mammal mortality worldwide. Static nets, hard-wearing monofilament gill, tangle and drift nets fixed in place or left to drift, account for the majority of deaths, and have been implicated in the declines of at least 11 critically endangered small cetaceans. But mobile gear is not benign: mid-water trawls, towed fast and large, target the same schooling squid and herring that dolphins hunt, and purse seines and ring nets can trap animals at depth. Abandoned, lost or discarded gear keeps killing long after crews leave, because fishing equipment is engineered to survive the ocean and carries enormous breaking strength. Crucially, the review notes, death in these events typically takes minutes, and can take an hour or more depending on the species, a timeframe that sits at the heart of the welfare concern.

Why marine mammals end up in nets in the first place turns out to be a story about behaviour as much as geography. Harbour porpoises, one of the most frequently bycaught species, do not usually approach gill nets deliberately. Studies suggest individuals are caught accidentally when attention shifts, perhaps during predator escape or play, or when auditory masking compromises their ability to detect thin monofilament using echolocation, the primary sensory tool they rely on to map their surroundings. Echolocation is also reduced during sleep, a factor the authors flag as an unexplored influence on entanglement risk. Juvenile porpoises appear disproportionately likely to be bycaught, possibly through inexperience or curiosity, though the true reasons remain unknown. In contrast, common and bottlenose dolphins are often actively feeding around trawls, exploiting the fishery as a foraging opportunity, and may even recognise characteristic engine noises associated with gear deployment or hauling. One witnessed study found common dolphins near a boat during or within 15 minutes of net deployment, and one dolphin hauled up alive, indicating entrapment during or just before hauling.

Because so few PUE events are directly observed, necropsy, the post-mortem examination of retrieved carcasses, has become the primary window into what happens between entrapment and death. The accumulated pathological record is extensive. External findings include net impression marks, cuts and lacerations, subcutaneous haemorrhage, tooth fracture and loss. Internal injuries span muscular haemorrhage, acute peritoneal hernia, fractures of the mandible, ribs, skull, fin bones and vertebrae, ocular hyphema, cervical spinal cord compression and systemic hypoxia. Histology adds a characteristic suite of changes: pulmonary congestion and oedema, intra-alveolar haemorrhage, fragments of striated muscle lodged in the air spaces, alveolar hyperinflation and emphysema, acute degenerative change in the heart muscle, and congestion of major organs. Diagnosis remains harder in pinnipeds than cetaceans because fur rarely records net marks, but the combined pattern, alongside circumstantial evidence such as overlap with known fisheries, now supports increasingly confident diagnosis.

The injuries tell a story about the animal’s own behaviour during entrapment. Most small marine mammals caught in nets asphyxiate, but many also sustain severe trauma, indicating vigorous struggling rather than passive submission. A striking example comes from UK stranding data: common dolphins caught in pair trawls, banned within 12 nautical miles of part of the English Channel coastline in 2004, showed a higher rate of broken beaks than other species, reflecting both the gear type and the violence of their escape attempts. Some dolphins reportedly enter a catatonic state, thought to reflect an accentuated diving bradycardia driven by the stress response, yet the lesion record shows physical activity after entanglement is common. Age matters too: calves and juveniles of minke, North Atlantic right and humpback whales, particularly animals under 11 metres, lack the body weight and strength to break ropes and surface to breathe, and in baleen whales, gear fixed to five or more body parts sharply reduces mobility and the chance of breaking free.

The physiological cascade that follows entrapment is well characterised in outline, even if measuring it in a dead animal is extraordinarily difficult. Fear and anxiety stimulate the limbic system, activating the sympathetic nervous system and the adrenal medulla within milliseconds, releasing adrenaline and noradrenaline; the hypothalamic-pituitary-adrenal axis follows within seconds, with cortisol measurable minutes later. In a marine mammal, this intersects with the dive response, which itself releases catecholamines and slows the heart, complicating interpretation. Catecholamines are released in pulses and cleared rapidly, and their breakdown products are unstable in necropsy material, limiting forensic use. Cardiac histology can reveal excessive catecholamine stimulation, but morphological changes may only appear after six to eight hours, longer than most PUE events last. The authors propose newer tools, including plasma troponin I assessment and immunohistochemistry of cardiac muscle, as promising avenues for quantifying the acute stress response and, with it, welfare impact.

What makes this review resonate far beyond pathology labs is the emerging evidence that entrapment is a social event, not just an individual tragedy. In the English North Sea, a passive acoustic monitor recorded two harbour porpoises making foraging buzzes near a static net before one became entangled. The trapped animal immediately switched from feeding buzzes to communication calls, displaced the net upward by about seven metres over four minutes while struggling, then went silent. The surviving porpoise, possibly a mother given the sub-adult size of the victim, produced only communication calls for a period and then continued clicking and calling toward the net for at least 59 minutes, possibly for hours. The researchers suggest this may be the first acoustic recording of a porpoise staying at the scene and calling repeatedly for a lost companion.

Similar patterns appear elsewhere. Bottlenose dolphins entrapped in an Adriatic mid-water pair trawl showed novel changes in whistle parameters, effectively broadcasting distress to pod mates. In the Ionian Sea, after a sub-adult male bottlenose dolphin died entangled in a static net, members of his pod produced an unusual number of vocalisations, apparently in response to his distress calls. Off southern Brazil, intense vocalisations with a high proportion of burst-pulses, suggestive of social communication and care-giving behaviour, continued for almost two hours around a 3,000-metre bottom-set gill net before a dead, entangled Franciscana dolphin was hauled up. Direct video observation in Western Australia captured two bottlenose dolphins in a trawl net showing a startle response and repeatedly pushing against the net’s upper surface for several minutes before becoming motionless. In the eastern tropical Pacific tuna purse-seine fishery, spinner dolphins dived deeper into nets while spotted dolphins showed agitation or a passive ‘sleeping’ behaviour, both interpreted as stress responses involving social separation, restraint and isolation.

The consequences ripple outward. Despite bycatch of spotted and spinner dolphins in the tuna fishery falling from more than six million per year to fewer than 1,000, those populations show no signs of recovery, and calf production has declined since the 1980s. Permanently separated dependent calves may starve after orphaning, unweaned and uncounted, and there is evidence that setting nets on dolphins can trigger miscarriage in pregnant females. Because many odontocetes depend on social cohesion for survival and reproductive success, the review argues that bycatch mortality may disrupt populations in ways that individual removal counts cannot capture, impeding recovery even when numbers of deaths fall. For the survivors, escape is not a clean ending: a Canary Islands study found 19% of sampled stranded cetaceans had been released alive but died from the interaction, and post-mortem work reveals capture myopathies and cardiac damage that can compromise animals long after they leave the net.

Case studies from UK waters ground the science in individual deaths. In January 2016, inspectors in Cornwall recovered two four-month-old grey seal pups from a gill net off Falmouth, freshly dead, recently fed and in good condition, with abrasions around their eyes, muzzles and flippers where they had pushed hopelessly against a 13.6-centimetre mesh. In December 2020, an adult male common dolphin live-stranded in the Helford River and died; necropsy showed clear monofilament net wounds on its rostrum, dorsal fin and fluke, with histology dating the injuries at least six to eight hours before death, a rare documented case of short-lived post-bycatch survival. UK seal bycatch is estimated at 400 to 600 animals annually, concentrated in southwest tangle and trammel nets, and bycatch accounts for 39% of necropsied stranded common dolphins in Cornwall between 2018 and 2024. The authors argue the evidence warrants bycatch prevention on welfare as well as conservation grounds, and call for wider deployment of hydrophones, net cameras, remote data loggers and a proposed pathological ‘fingerprint’ of asphyxia, alongside a hypoxic index and trauma scoring, so that the invisible minutes inside a net can finally be measured, and ultimately prevented.

Subject of Research: The welfare, behaviour and pathophysiology of marine mammals experiencing peracute underwater entrapment in fishing gear

Article Title: The impacts of peracute underwater entrapment in fishing nets on marine mammal behaviour and welfare

Article References: Dolman, S., Barnett, J., & Wessels, M. (2026). The impacts of peracute underwater entrapment in fishing nets on marine mammal behaviour and welfare. Frontiers in Ethology, 5, Article 1882616. https://doi.org/10.3389/fetho.2026.1882616

Image Credits: AI Generated

DOI: 10.3389/fetho.2026.1882616

Keywords: marine mammals, bycatch, peracute underwater entrapment, cetaceans, pinnipeds, fishing gear, asphyxiation, stress response, necropsy, animal welfare, harbour porpoise, common dolphin