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

Introduction

The world’s fisheries are causing profound ecological changes to the structure and function of marine ecosystems (Read, 2008). For marine mammals, fisheries interactions are widely recognised as being the leading anthropogenic cause of mortality (Read et al., 2006; Moore et al., 2021). One form of fisheries interaction is peracute underwater entrapment (PUE), the process of acute entanglement and forced submersion that occurs following interaction with fishing gear, where an individual is not able to break free or to reach the water’s surface to breathe (Moore et al., 2013). PUE results in a number of negative welfare impacts, including asphyxiation, injuries, physiological and psychological stress, social disruption and, ultimately, death impacting on the behaviour of affected individuals and conspecifics (Soulsbury et al., 2008; Jepson et al., 2013; Dolman and Moore, 2017; Wessels, 2024; Wessels et al., 2024). Death typically takes minutes but might take an hour or more depending on the species involved. Individually and collectively, the welfare implications of these events for marine mammals are severe and raise significant concerns (Butterworth, 2016; Dolman and Brakes, 2018). This review focuses on PUE in cetaceans and pinnipeds, as less is published on the acute impacts of bycatch for other marine mammal species such as polar bears, mustelids and sirenians (Reeves et al, 2013).

Marine mammals are sentient, sapient beings, exhibiting complex cognitive abilities and social dependency (Marino, 2022). Some species live as long as other K-selected vertebrates, including humans, caring for their offspring for many years. There is a moral obligation when determining the way in which human interactions with them are managed (IWC, 2011). In 2011, the International Whaling Commission (IWC) reported on the Whale Welfare and Ethics Workshop, which covered a wide range of anthropogenic threats including bycatch (Moore, 2013) and concluded that ethics and animal welfare science should be taken into account in decisions relating to the management of all human-whale interactions, including bycatch.

Focusing on PUE, this review looks at how marine mammals interact with fisheries globally: the known and hypothetical events associated with entrapment on the affected individual and on conspecifics, including the stress response and pathophysiology; the use of necropsy data to help provide a greater understanding of the events surrounding PUE; and, further research needed to improve our understanding of the welfare implications on individuals and populations. Case studies are provided from the United Kingdom, where the authors are based.

Marine mammal - fisheries interaction: Why does it happen?

Many hundreds of thousands of marine mammals are killed in fishing gear every year (Read et al., 2006). Static nets are passive, fixed in place or left to drift in the water-column, typically hard-wearing, monofilament nets such as gill nets, tangle nets and drift nets, and are responsible for the majority of marine mammal deaths, with at least 11 critically endangered small cetaceans declining as a result (Brownell et al., 2019). Entanglement in static lines, such as longlines, as well as ropes, packing bands, fishing hooks and mobile gears including pair trawls, mid-water and pelagic trawls, purse seines and ring nets can all also trap marine mammals at depth underwater. In mobile fisheries, midwater trawls pose a greater risk to marine mammals than bottom trawls as they are usually large in size and towed at higher speeds; moreover, they target small, schooling species such as squid and herring, which are common prey species for marine mammals (Northridge, 1988). All fishing gear is made to persist in the unforgiving marine environment and possesses enormous breaking strength (Butterworth, 2016). As such, abandoned, lost or discarded fishing gear can still actively entangle and kill marine mammals and other wildlife.

We do not fully understand why marine mammals are prone to entanglement. Species such as harbour porpoise (Phocoena phocoena) are thought to become accidentally trapped and are particularly prone to entrapment in static nets. It is likely partly due to their behaviour but factors such as foraging, inexperience, curiosity, carelessness and distractions (for example predator escape or play behaviour) may be significant (Nielsen et al., 2012; Macauley et al., 2021). A study of post-mortem cases in the Netherlands found that juvenile harbour porpoises were mainly bycaught (IJsseldijk et al., 2022). Juvenile porpoises may be more prone to bycatch because they are inexperienced or curious, but the actual reasons are not known. Nielsen et al. (2012) found that porpoises do not usually actively approach gill nets, concluding that bycatch seems to be the result of individual animals accidentally being caught, due to attention shifts or to auditory masking (compromising capacity to ‘hear’) reducing their ability to detect the nets using echolocation. In addition, echolocation is reduced during sleep (Goley, 1999) which may influence odontocete entanglement, but this is not known.

Behavioural factors in other species such as common (Delphinus delphis) and bottlenose dolphin (Tursiops truncatus) may contribute to their capture as they interact with trawl gear exploiting the feeding opportunity this fishery type provides, accidentally becoming entrapped as they are actively feeding in the net (Jaiteh et al., 2014; Wessels et al., 2024a). Dolphins may recognise characteristic engine noises produced by trawl vessels during different stages of fishing leading them to feed during certain periods such as gear deployment or hauling back onboard (Leatherwood, 1975; Fertl and Leatherwood, 1997; Pace et al., 2003). Individuals may get bycaught as the net forms into a U-shape, due to friction between the headline of the net and the surrounding water, creating spaces that may lead to entrapment (Northridge, 1988). Entrapment may also occur when the trawl changes speed or direction, altering the shape of the headline and the size and shape of the net and its mouth, or when the boat slows or stops to haul in the catch (Fertl and Leatherwood, 1997; SGFEN, 2002; Zollett and Rosenberg, 2007; FAO Technical Guidelines for responsible fisheries, 2021). In contrast to harbour porpoises, common dolphins may be attracted to fishing boats and subsequently become entrapped when static nets are being shot or hauled, rather than whilst they are set/static. This was illustrated by Tregenza et al. (1997), who found that common dolphins were around a boat in two of the three bycatch events they witnessed, either during or within 15 minutes of the deployment of the net. They also reported that one common dolphin was hauled up alive, indicating that they were entrapped during or shortly before hauling.

It is clear therefore that the behaviour of different species influences the risk of entrapment in different fisheries. However, it is important to recognise that the spatial overlap of species and high-risk fishing gears, as well as the behaviour of marine mammals will inevitably influence the risk of entanglement (Chadwick, 2024).

Factoring in the potential effect of the behavioural and physiological differences on the welfare and time to death of different species after PUE is challenging and at this stage it is unlikely that the physiological differences immediately prior to entrapment can be separated from those thereafter. In addition, it is unknown if physical factors once entrapped (e.g. the crushing by target species in the net) would also affect behaviour and/or physiology up to the point of death or release.

Seeking understanding via necropsy: what necropsy can tell us about the behaviour and pathophysiology of events surrounding PUE

Post-mortem studies demonstrate a number of events occurring during PUE in the period between entrapment and death: asphyxiation; physical injuries; events associated with stress; social disruption and suffering of conspecifics. Both the size and behaviour of the individual and the nature of the fishery will impact on the welfare of the marine mammal from the onset of PUE until their death (Wessels et al., 2024a).

When a marine mammal approaches and enters fishing gear, the netting or line may enter their mouth and become entangled on the individual’s teeth or baleen. Fins, tail flukes and flippers can become entangled in the gear as struggling ensues. The majority of small marine mammals that are caught in nets asphyxiate, with many also sustaining multiple and sometimes severe injuries (Duignan and Jones, 2007; Soulsbury et al., 2008; Barnett et al., 2021; Wessels, 2024). Of the whale species, minke whales (Balaenoptera acutorostrata) are more likely to be caught, tethered and perish than larger whales, whereas large cetaceans may sometimes escape, given their size and girth (Song et al., 2010; Leaper et al., 2022). More generally in baleen whales, the younger age classes (calves and juveniles) of minke, North Atlantic right (Eubalaena glacialis) and humpback whales (Megaptera novaeangliae) are more at risk due to their smaller size, with juveniles less than 11 metres being the most susceptible (Cassoff et al., 2011). This is the result of insufficient body weight and strength to break rope lines and move to the surface to breathe. In large species, another important factor was found to be the number of body parts affected by the entanglement. In baleen whales, fishing gear fixed to five or more body parts resulted in increased likelihood of PUE as a result of decreased mobility and likelihood of breakage of fishery gear (Lien, 1994; Cassoff et al., 2011).

The paucity of reports/information on observed PUE bycatch events means we are reliant on data from necropsy of retrieved or stranded carcasses to gain an understanding of the behaviour of an entrapped marine mammal. Diagnosing PUE remains challenging, although significant advances have been made. Evidence to support a diagnosis of PUE includes the physical injuries, both external and internal, histological evaluation of tissues and circumstantial evidence (e.g. spatio-temporal overlap with known fisheries). A full list of the reported PUE associated injuries in marine mammals recorded at necropsy has been collated in Table 1. Diagnosis is more difficult with pinnipeds than cetaceans due to their pelage, which means that marks or wounds in skin are rare (Duignan and Jones, 2007; Barnett et al, 2021). Histological changes are often non-specific; however, a ‘suite’ of changes can help provide evidence of PUE. Table 2 outlines the histological changes seen in cases of PUE.

Table 1. Reported PUE associated injuries in marine mammals recorded at necropsy, reproduced from Wessels et al., 2024b.

Table 2. Histological findings associated with PUE (Sierra, 2024; Jepson et al., 2013).

In some cases of entrapment, dolphins enter a catatonic state which is thought to be associated with accentuated bradycardia due to the stress response (Coe and Stunz, 1980; Oliveros and Maldonado, 2002). However, the information on traumatic lesions gathered on necropsy indicates that struggling and physical activity following entanglement are common. During PUE, animals may traumatise themselves as they struggle resulting in abrasions, contusions, fractures or lacerations depending upon the environment and nature of the fishery gear involved (Jepson et al., 2013; Wessels et al., 2024a; Table 1). For example, until banned within 12 nautical miles of the English coastline in part of the English Channel in 2004, the majority of common dolphin deaths were attributable to pair-trawls rather than static nets (Northridge et al., 2007). This was reflected in an analysis undertaken on stranded cetaceans in UK waters between 1999-2005, which showed that common dolphins had a higher rate of broken beaks (Soulsbury et al., 2008), indicating both the fishery gear and the animal’s behavioural response when entrapped impact on the injuries seen. Both the external and internal injuries sustained will almost certainly cause pain (Boys, 2024). The assessment of pain is challenging in any species but particularly so in marine mammals where little to no research has been undertaken. Yet as pain is recognised as an important factor in suffering and welfare (impacting on behaviour), it should be assessed and judged for those injuries sustained in bycatch events (Wessels et al., 2024b) allowing a greater understanding of the impact of these on the individual.

Broadly speaking, marine mammals bycaught in gill nets or trawls are often reportedly healthy individuals in moderate to good body condition, with pathological findings that are usually consistent with PUE (Moore et al., 2013; Puig-Lozano et al., 2020; Barnett et al., 2021; Martino et al., 2025). It has been suggested that underlying diseases such as systemic bacterial infection, morbillivirus and parasitism, which are seen in a small number of bycaught individuals, may be significant predisposing factors (Pietroluongo et al., 2025) but whether or how these diseases affect the behaviour of marine mammals prior to and during entrapment is unknown.

In future it may be possible to utilise material from post-mortem specimens to provide a greater insight into the stress response seen in PUE. Assessment of the acute stress response by evaluation of hormones such as catecholamines may provide additional information on the duration of the response and impact on the animal from which conclusions may be drawn as to behaviour during the immediate period prior to death.

Case study: grey seal suffering peracute underwater entanglement

Estimates of harbour (Phoca vitulina) and grey seal (Halichoerus grypus) bycatch in UK fisheries is in the region of 400–600 dead seals per year (SCOS, 2024). This is considered to be underestimated as non-UK vessels are not included. Spatially, about 70% of grey seal bycatch occurs in the southwest of England in large-mesh tangle or trammel nets and most bycaught seals are young (SCOS, 2024).

In January 2016, England’s Inshore Fisheries and Conservation Agency (IFCA) observed a local fisherman hauling his gill net one mile off the coast of Falmouth, Cornwall. In it were two bycaught male grey seal pups, freshly dead. They were probably around four months old and were in good body condition, their stomach contents indicating that they had been recently feeding. The pups had fresh abrasions around the eyelids and muzzles, presumably inflicted as they tried to push themselves through the mesh of the net which had a knot to knot length, or mesh size, of 13.6 cm, a gap through which they had no chance of escape. One of the pups had been cut from the net because their jaw was clamped around a section of it as they went into rigor. The fore and hind flippers also had fresh abrasions and wounds again most likely obtained whilst they tried to free themselves from the net (Figures 1, 2).

The stress response and the pathophysiology of death by PUE

Entanglement in fishing gear would have two quick and profound impacts that are the source of stress: an inability to move freely and an inability to take in oxygen. Initially, engagement with fishing gear would not be expected to initiate a significant stress response as the animal is not confined or restricted and can show normal behaviour. However, it is highly likely that once entrapped the animal is no longer able to behave normally and a stress response would be initiated. PUE would elicit an acute stress response including a neurologic (behavioural), endocrine and physiological response. The relatively short period between entrapment and death would not elicit an immunological response.

Fear and anxiety during the acute phase stimulate the limbic system with sympathetic nervous system activation targeting the adrenal medulla to release catecholamines. This response takes milliseconds after which adrenaline and noradrenaline are released to initiate their physiological responses. Co-activation of the hypothalamic-pituitary-adrenal axis (HPA), which is a secondary response, is also initiated by the sympathetic nervous system within seconds with a response seen in minutes following the release of cortisol and other hormones from the adrenal cortex (Atkinson and Dierauf, 2018; Curry, 1999). Evaluating the endocrine response in PUE could provide a quantitative measure by which to assess welfare. However, this is challenging due to the short timeframe involved, necropsy samples being invariably the only material available for testing, and the instability of some hormones (and metabolites). Catecholamines are often released in a pulsatile fashion with rapid clearance. In marine mammals there appears to be no circadian rhythm and little work has been done to evaluate baseline values and variation (Atkinson et al., 2015). To complicate matters further, in pinniped species as part of the normal dive response catecholamines are released leading to reduced heart rate which is counterintuitive of the acute stress response (Atkinson et al., 2015; Atkinson and Dierauf, 2018). Assessment in post-mortem material has been undertaken particularly in humans with very variable results as the hormones and their breakdown products are unstable, curtailing their use in estimating the time of death and the duration of the agonal period from necropsy material (Rosato et al., 2021). Histological assessment of cardiac muscle may help in establishing whether an excessive level of catecholamine stimulation has occurred (Turnball and Cowan, 1998; Cowan and Curry, 2008; Camara et al., 2020) but morphologic changes can often only be detected after 6–8 hours on routine histological examination and as such the short periods involved in PUE mean this change may not be detectable and the use of other techniques (i.e. troponin 1 assessment in plasma and immunohistochemistry on cardiac muscle) (Camara et al., 2020) should be investigated further in PUE cases. Although glucocorticoids are more stable their slower release means they are of more value in assessing the welfare impacts of chronic entanglement (Kellar et al., 2015; Lowe et al., 2021).

Assessment of the stress response has the potential to provide information on the welfare effects of bycatch. Any assessment would normally be based on the behavioural and physiological responses. However, PUE presents significant challenges in assessing both of these responses as direct observation of behavioural changes is rarely possible, and the relatively short period between entrapment and death means many of the physiological responses cannot fully develop and therefore be measured accurately.

Physiological stress refers to the body’s physical response to the stress associated with PUE, whilst psychological stress refers to the mind’s response or negative thoughts. The two are inextricably linked and a stressful situation will activate a cascade of stress hormones that produce physiological changes. The stress associated with capture, ante-mortem injuries and asphyxiation is likely to be high (Soulsbury et al., 2008; Dolman and Brakes, 2018). Once entrapped the animal cannot behave normally, feeling fear and distress and eliciting an acute stress response. Physical factors such as crushing by target species in the net, exhaustion from attempts to escape and disorientation, along with physical injuries, are just some of the stress initiators seen in cases of PUE. For bycaught animals that may be trapped underwater for prolonged periods especially whilst struggling to escape a net, the inability to replenish oxygen stores could lead to the experience of some form of breathlessness, which could further lead to dizziness, fatigue and confusion (Boys, 2024). Fatigue, elevated pain from injuries and/or acute stress levels experienced between entrapment and death all contribute to a reduction in welfare (Butterworth, 2016; de Vere et al., 2018).

Based on the known pathological consequences, we understand that the affective states being experienced by animals undergoing PUE are likely to be negative and we can begin to consider the quality of potential mental experiences that entrapped marine mammals may have (Boys, 2024). Affective states refer to the feelings and emotions experienced. Suffering results from the interactions of physical stimuli, including pain and psychological well-being (Brakes, 2004). As such it may be considered a combination of subjective feelings and lack of control (Broom, 1991; Boys, 2024). As sentient and sapient species, we can expect marine mammals to experience feelings of distress (Butterworth, 2016). Pain, anxiety, fear, panic, discomfort, fatigue, exhaustion, lethargy, confusion, breathlessness, dizziness and other marine mammal-specific mental states are relevant during PUE (Wessels et al., 2024a).

Pathological data indicate that, following prolonged submersion, the majority of bycaught marine mammals asphyxiate in the net, rather than drown, after having been trapped underwater (Duignan and Jones, 2007; Soulsbury et al., 2008; Wessels et al., 2024a). This is considered the most important welfare aspect associated with PUE as it is likely to be a protracted process and is considered to be extremely stressful (Soulsbury et al., 2008; Wessels et al., 2024a).

There are significant differences between death following immersion in water between terrestrial and aquatic mammals as, unlike terrestrial animals, seals and cetaceans asphyxiate rather than drown. Asphyxia refers to death by rapid cerebral anoxia and hypoxia, which is a sudden and severe lack of oxygen to the brain. In terrestrial animals, asphyxia results in immediate dyspnoea, convulsions, bradycardia and apnoea, isoelectric electroencephalogram, agonal respiratory movements and cardiac arrest (McEwen and Gerdin, 2016; McEwen, 2016). Struggling occurs due to severe ‘air hunger’ before there is a loss of consciousness. Little or no research has been carried out on asphyxia in cetaceans and seals, and as such comparative physiology/pathology taken from terrestrial species has to be used with some assumptions to provide a suggested outline of the path to death in PUE. The time to asphyxia from entrapment is difficult to determine and will depend on a variety of factors, including the amount of struggling which utilises more oxygen, thus hastening the process and when in the dive sequence entanglement occurs (Wessels et al., 2024a). It has been estimated that this is likely to be from minutes for delphinids to more than an hour for deep-diving specialists (Leaper et al., 2006; Soulsbury et al., 2008). In seals we would expect a similar time range to asphyxia related to the ecology and dive behaviour of the species concerned (Ponganis and Williams, 2016).

Behavioural responses associated with entrapment

Very little information is available on the behavioural responses associated with PUE. As such, certain assumptions need to be made based upon other similar scenarios and those described in terrestrial species (Proulx et al., 2022). However, occasional events have been described and are useful in providing some understanding of the behaviour of marine mammals experiencing PUE.

One recorded harbour porpoise entanglement case in a static net in the English North Sea detected during a passive acoustic monitoring experiment demonstrated significant activity following entrapment (Macaulay et al, 2026). Initially, a pair of porpoises were recorded acoustically, making foraging buzzes close to a static net, for about 5 minutes. One porpoise became entangled, immediately resulting in a change in acoustic behaviour from buzzes to communication calls. Whilst entangled and alive, the net was displaced upwards by about seven metres over about four minutes before the porpoise asphyxiated. After the bycaught porpoise died, the non-entangled porpoise produced only communication calls for a period of time and then continued to click and call towards the net for at least 59 minutes and possibly several hours following the death of the entangled porpoise. At 1.15 m, the entangled porpoise was likely a sub-adult, so the pair were possibly mother and calf, and the bycatch event indicated a stress-mediated social response (Macauley et al., 2026). This case may be the first known acoustic recording of a surviving porpoise individual staying at the scene of entrapment and calling repeatedly at the loss of their companion.

Similarly, a few incidents of acoustic recording of other cetaceans being bycaught have been captured. The entanglement of three female bottlenose dolphins, two adults and a sub-adult, in a commercial mid-water pair trawl net occurred in the Adriatic Sea, where the novel change in whistle acoustic parameters recorded conveyed to conspecifics the distress experienced due to entrapment (Corrias et al., 2021). In the Ionian Sea, one of 11 bottlenose dolphins, a sub-adult male, became entangled in a static net and died. The authors suggested that some members of the dolphin pod produced an unusual number of vocalisations in response to the high levels of stress and agitation linked to the distress calls emitted by the entangled dolphin (Terranova et al., 2022). Similarly, in southern Brazil, intense vocalisations suggesting the presence of multiple Franciscana dolphin (Pontoporia blainvillei) were recorded around a 3000-metre bottom-set gill net. The authors suggested that a high proportion of burst-pulses indicated potential social communication and epimeletic behaviour which in this case continued for almost two hours, because of the bycatch of their pod member (Bearzi and Reggente, 2018; Arrial et al., 2025). Later a dead, entangled Franciscana dolphin was hauled up by the fisherman. Whilst this behaviour is not surprising, given what is known about odontocete close relationships and society generally, these novel acoustic insights provide a visual image about bycatch that can otherwise seem abstract.

Direct observation of trawl bycatch has been recorded in Western Australia involving bottlenose dolphins (Jaiteh et al., 2014). The study observed two dolphins entrapped in nets showing a ‘startle’ response with repeated attempts to push against the upper surface of the net. These animals showed this behavioural response for ‘several minutes’ before becoming motionless, presumed unconscious, or dead.

Bycatch scenarios in eastern spinner dolphins (Stenella longirostris) and northeastern offshore spotted dolphins (Stenella attenuata) were extensively studied at the end of last century in the eastern tropical Pacific tuna fishery in light of the high mortality rates experienced (Curry, 1999). Purse seine capture of dolphins, which frequently associate with tuna, occurs in this fishery and follows a period of pursuit prior to capture. Animals would experience different physiological stressors associated with this type of bycatch compared to that in trawl and static net fisheries leading to different behavioural responses. Once entrapped within the net, dolphins showed a variety of behavioural changes including agitation, high levels of activity, diving deeper into the net (spinner dolphins) and ‘sleeping’, sinking and rafting (northeastern offshore spotted dolphins). These behaviours are all thought to be associated with fishery interaction induced stress. The likely and novel stressors associated with this behaviour include social separation (especially mother/calf separation), isolation and restraint and social aggression.

A variety of tools are being developed to provide more information on cetacean interaction with fishery gear (Wessels et al., 2024b) however until these are in regular and widespread use it is unlikely we will understand in any detail what the behavioural responses associated with PUE are. In light of this, assessing stress physiology through other means should be considered.

What we know about behaviour and post-release/escape

Once an animal has become entrapped two scenarios are possible: death following hypoxia/asphyxiation or release/escape with return to the surface to breath. Both of these have significant welfare impacts on the individual and although death may be regarded as the greatest impact, injuries and other pathologies (i.e. hypoxic brain injury) may lead to long-term welfare issues should the animal survive. There is little information available for those animals which escape or are released following entrapment, although survival should not be assumed. A retrospective analysis of dead stranded cetaceans in the Canary Islands found that 19% of sampled cetaceans were released alive but died because of the fishery interaction (Puig-Lozano et al., 2020).

Post-entrapment survival could lead to changes in behaviour including avoidance of fishing gear, changes in the acute stress response should future engagement occur, learned behavioural changes that may be communicated to conspecifics and changes in foraging activity if these previously involved fishing gear. These could all potentially lead to both positive and negative survivability however no information is available about this.

The survival of any individual is highly dependent upon the nature of the injuries sustained during entrapment. Overt lesions such as fractured rostrum and deep lacerations are easier to evaluate and hence understand their impact however microscopic lesions may be equally compromising on survivability. For example, post-mortem analyses of odontocetes caught and trapped in trawl gear illustrated the potential long-term effects of capture myopathies and cardiac damage sustained during the acute stress of entanglement (Soulsbury et al., 2008).

Hypoxia constitutes the most important process in PUE resulting in unconsciousness and death of cetaceans (Soulsbury et al., 2008) and other marine mammals. Their unique physiology and metabolism mean the assessment and evaluation of any hypoxic insult is challenging. Evidence of asphyxial death is mainly provided through circumstantial evidence and necropsy findings in cetaceans (Moore et al., 2013). To date no detailed studies have been undertaken to gain an understanding of the process in cetaceans. Reduced oxygen availability (hypoxia) impacts most significantly upon cardiac and central nervous system function. Cetacean physiology has adapted to periods of hypoxia through changes in metabolism of peripheral tissues (switching to anaerobic respiration) and alterations in blood flow to ensure critical organs receive adequate oxygen concentrations for aerobic respiration (Ponganis and Williams, 2016). It is unknown whether cetaceans surviving a period of hypoxia as a result of PUE show long term brain injury/pathology. The brain is not homogenous in its response to hypoxia with areas varying in susceptibility (Barranco et al., 2021). These have been well defined in humans but similar work has not been undertaken for any marine mammal species although it is likely similar areas/cell types would be involved (frontal lobe, hippocampus, Purkinje cells and globus pallidus). Hypoxic injury to these areas would affect behaviour and survivability but until further research is done it remains an unknown.

Case study: common dolphin live stranding and death following bycatch

In Cornwall, England, bycatch is the most common cause of death in necropsied stranded common dolphins, accounting for 39% of all necropsies of the species carried out between 2018 and 2024 (Barnett et al., in press).

In December 2020, an adult male common dolphin live stranded in one of the tributaries of the Helford River, a deep tidal ria in the southwest of the county, but died. The dolphin was necropsied and this revealed that the animal was already somewhat compromised prior to stranding, being in suboptimal nutritional state and having parasitic bronchopneumonia. In addition to this, there were clear monofilament net associated wounds on the rostrum, dorsal fin and one tail fluke. Histopathological examination of the tail fluke and mandible wounds indicated that these had occurred at least 6 to 8 hours prior to death. This is a rare example of an animal known to have survived after being bycaught and the likely effect of bycatch on their survival, which proved to be short lived in this case. The location of the site of live stranding was approximately four miles from the open sea and, despite presumably being only going through a brief period of enforced immersion as no histopathological changes indicative of hypoxia were seen, it seems likely that the subsequent disorientation, confusion and distress experienced by this compromised animal after being bycaught had contributed to their subsequent live stranding and death. For illustration purposes, Figure 3 shows a common dolphin that had stranded with monofilament net associated wounds.

Effects on conspecifics

Throughout this paper the effects of PUE bycatch events on the behaviour of conspecifics have been mentioned, however there are so few reports in the literature it is not possible to provide any detailed information about this important aspect of marine mammal behaviour. Reggente et al. (2018) reviewed the social relationships and death related behaviours in aquatic mammals, highlighting differences between cetacean and non-cetacean species in relation to death events which appear to be linked to certain social characteristics. These behaviours occur due to a mix of ecological, taxonomical, cultural and abiotic factors. It is highly likely that the events surrounding PUE would impact upon the behaviour of conspecifics but further research in this field is required. In addition, how survivors interact with others in their social environment is unknown.

Due to the highly social nature of many odontocetes, survival and reproductive success can depend on social cohesion and organisation, and the effects of social disruption caused by bycatch mortalities may go beyond the dynamics of individual removals and impede population recovery (Wade et al., 2012; Dolman and Brakes, 2018).

The previously mentioned eastern tropical Pacific purse-seine tuna fishery is the longest running study documenting sub-lethal and conspecific impacts resulting from fisheries interactions. Despite a dramatic decrease in the number of spotted and spinner dolphins bycaught, from more than six million to fewer than 1000 dolphins per year, the populations are not showing signs of recovery (Wade et al., 2002; Gerrodette and Forcada, 2005), with calf production declining since the 1980s (Cramer et al., 2008). Permanently separated dependent calves may represent unobserved mortality events meaning that unweaned calves may die of starvation following orphaning. Further, there is some evidence that setting nets on dolphins can result in miscarriage in pregnant females (Gilman, 2011).

Application of the Welfare Assessment Tool for Wild Cetaceans model to PUE

If the Welfare Assessment Tool for Wild Cetaceans (WATWC) model (Nicol et al., 2020) is applied to PUE, the following areas of welfare concern can be identified (Table 3; Wessels et al., 2024; Wessels, 2024). For each domain deemed relevant, brief discussion is given highlighting the problems in assessing the welfare impact and potential methods that can be used to improve the accuracy of any assessment.

Table 3. Applying the welfare assessment tool for wild cetaceans 5 domains approach to PUE of an individual, adapted from Wessels, 2024.

Reviewing the separate factors of the domains clearly identifies large gaps in our current knowledge and presents difficulties in applying an accurate WATWC score to a PUE event. At present the development and refinement of a trauma score such as the Cetacean Bycatch Injury Impact Score (Wessels et al., 2024) would go some way to quantify the impact of PUE on an individual and it can be used by welfare experts to provide more robust answers to welfare associated issues in bycatch.

Conclusions and recommendations

The evidence provided in this article and elsewhere demonstrate that marine mammals experiencing PUE, and their conspecifics, can suffer considerably and this is a common process resulting from bycatch, impacting hundreds of thousands of marine mammals every year. As a result, the implementation of measures to prevent bycatch are warranted on welfare as well as conservation grounds.

Observations of behavioural changes can be helpful in assessing the welfare of an affected individual. The problem being PUE is rarely observed. Increased knowledge of the behavioural effects of PUE would help provide a better understanding of the stress these animals experience and from this we could assess the effect on welfare. It is therefore important to obtain a better understanding of marine mammal behaviour and fisheries interaction to help mitigate bycatch and entanglement events. Development of programmes to study and evaluate behavioural changes associated with PUE via direct and indirect observation of individuals/populations would provide welfare experts with much needed information. The challenges of collecting and interpreting information are vast. However, the use of equipment such as remote data loggers, hydrophones to monitor acoustic behaviour and 3D movement of nets, and direct observation of trawl bycatch using net cameras needs to be investigated and more widely applied. New and novel methods will need to be developed to forward this area of research.

Hypoxia constitutes the most important process in PUE, resulting in unconsciousness and death of cetaceans. Research into hypoxic injury would help to refine the diagnosis of equivocal cases of PUE and in future be able to provide information on the duration of hypoxia/hypoxic interval which in turn would be useful in the assessment of welfare in these circumstances. Future work might include development of a ‘pathological fingerprint’ of change that allows confirmation of asphyxia associated with PUE and a hypoxic index to help assess welfare, with the identification of ‘watershed’ areas in the brain critical in hypoxic injury using histological techniques and metabolomics. Potentially important information could then be obtained on post-release survivability.

Further priorities would also include research into the acute stress response in marine mammals to understand how the pathophysiology impacts on behaviour. It is also important to document and age bycatch lesions systematically, particularly where other potential causes of death are also present, to better assess the chronology of events. This may lead to the identification of animals that have been bycatch survivors which have subsequently died and this may help expand our knowledge on what happens to animals which survive the bycatch event.

Generative AI statement

The author(s) declared that generative AI was not used in the creation of this manuscript.

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Footnotes

Edited by: Yann Henaut, ECOSUR Conservation and Biodiversity, Mexico

Reviewed by: Alick Simmons, Independent Researcher, Ilminster, United Kingdom

Michaella Pereira Andrade, University of São Paulo, Brazil