Trauma linked to fishing nets and accidental capture of thresher sharks
The thresher shark (Alopias vulpinus) is a pelagic species recognizable by its very long upper caudal fin, used to stun its prey. This particular morphology and its lifestyle in the open sea make it particularly vulnerable to fishing gear,...
What is it?
The thresher shark (Alopias vulpinus) is a pelagic species recognizable by its very long upper caudal fin, used to stun its prey. This particular morphology and its lifestyle in the open sea make it particularly vulnerable to fishing gear, notably drifting gillnets and longlines. Trauma linked to nets and accidental captures, called bycatch or “bycatch”, constitute one of the main threats to the survival of this species today. It is not a disease in the classic sense, but a set of physical injuries and serious physiological consequences resulting from entanglement or involuntary capture in fishing gear.
Key point
The thresher shark is a species classified as vulnerable, whose populations are in decline on a global scale, in particular because of accidental captures; each individual counts for the conservation of the species.
Commonly observed signs
These signs are neither exhaustive nor specific. Only a veterinarian can establish a diagnosis.
Related signs — each one links to its page:
A thresher shark caught in a net or line often has wrap marks around the body, caudal fin, or pectoral fins. The skin may be cut, abraded or lacerated at points of contact with ropes or mesh.
The very long tail of this species is particularly exposed: it can be severed, deeply cut or remain permanently tangled.
An animal captured and then released may swim in a disorganized manner, have difficulty maintaining its position in the water column, or exhibit abnormally rapid or slowed breathing, a sign of exhaustion.
The mucous membranes (visible at the gills or mouth) may appear pale in cases of significant physiological stress or blood loss.
In the most severe cases, the animal remains immobile or is no longer able to swim normally, which seriously compromises its survival once released.
When to consult without delay
This list does not replace veterinary advice. It gathers situations that justify a prompt consultation.
Causes and risk factors
Entanglement in gillnets, driftnets, longlines or trolling lines is the direct cause. The thresher shark's long caudal fin, which it uses by whipping to stun its prey, is easily caught in mesh or cables, increasing the risk of entanglement compared to other shark species.
Capture can be accidental, when the species is not the target sought by fishermen, or occur during unregulated targeted fishing.
Areas of high pelagic fishing activity, particularly where driftnets or surface longlines are deployed, greatly increase the risk.
The migration and breeding areas of the thresher shark, where the species concentrates, are particularly at risk of repeated accidental captures.
The hunting behavior of the species, which causes it to evolve near schools of fish that are often themselves targeted by fishing, increases its exposure to gear.
Young individuals and pregnant females seem particularly vulnerable during certain periods of grouping.
Diagnosis and care
Diagnosis is the veterinarian's responsibility, based on clinical examination and laboratory tests. Care is decided case by case.
A veterinarian specializing in marine wildlife or a marine biologist assesses the animal's condition by direct observation: extent of skin lesions, presence of bleeding, swimming behavior, color of mucous membranes and breathing.
As part of scientific studies or conservation programs, additional examinations (blood samples, biopsies) can be carried out to assess the level of physiological stress of the animal before and after release.
Post-release observation, when possible via satellite tagging, makes it possible to judge the actual survival of the animal in the medium term.
Treatment
Management is primarily based on rapid and careful removal of the net or line, minimizing handling time out of the water.
Cleaning superficial wounds can be considered in a supervised rescue framework, with antiseptic solutions adapted to the marine environment.
In cases of deep wounds or damage to the caudal fin, no curative treatment at sea is possible; the prognosis depends on the severity of the lesion.
No classic drug treatment (antibiotic, anti-inflammatory) is applicable in natural conditions; these options remain reserved for specialized marine wildlife care structures, which are extremely rare for this species.
Treatment depends on the cause and the animal's condition: it must be determined by a veterinarian. No self-medication.
Prevention
Reducing the use of drift gillnets and adopting selective fishing gear limits the risk of accidental capture.
Bycatch exclusion or reduction devices, as well as hook or line modifications, can reduce unintentional catches.
Training fishermen in good practices for handling and quickly releasing individuals caught alive improves their chances of survival.
The regulation of fishing zones and periods in the reproduction or concentration sectors of the species helps to reduce exposure.
Raising awareness among crews about the identification of the species, often confused with other pelagic sharks, helps to better document and limit catches.
Possible complications
Deep lesions of the caudal fin can permanently compromise the animal's swimming and hunting ability.
Significant hemorrhage or prolonged physiological stress during capture can result in severe, sometimes fatal, exhaustion, even after apparently successful release.
Unhealed wounds expose the animal to secondary infections in the marine environment.
At the population level, the repetition of accidental captures threatens the demographics of the species, whose reproduction rate is naturally low.
When to see a veterinarian
A thresher shark found entangled or accidentally captured should be reported immediately to the relevant maritime authorities or conservation agencies.
Any observation of active bleeding or deep lesion of the caudal fin requires the advice of a marine wildlife specialist before any handling.
An animal that remains immobile or unable to swim after release must be reported for possible follow-up.
Any capture, even accidental, in the professional context must be declared in accordance with the regulations in force on protected species.
Prognosis
The prognosis strongly depends on the severity of the lesions and the speed of return to water. Brief entanglement without deep injury may allow normal survival.
On the other hand, severe damage to the caudal fin or prolonged time out of water greatly reduces the chances of medium-term survival.
At the species level, the demographic prognosis remains worrying due to the combination of accidental fishing pressure and the low rate of natural population renewal.
Frequently asked questions
Are thresher sharks often caught accidentally?
Yes, it is one of the main known causes of mortality in this species, due to its long caudal fin which easily becomes entangled in nets and fishing lines.
Does a thresher shark released after capture always survive?
Not always. Survival depends on severity of injury and time spent out of water or entangled; some individuals die despite apparent release into the water.
What to do if you find a thresher shark caught in a net?
You should quickly contact maritime authorities or a competent conservation organization, and avoid handling alone if you are not trained, for the safety of the animal and the person.
Can you treat an injured thresher shark like a pet?
No, traditional veterinary care does not apply at sea; Treatment is generally limited to removal of the mesh and, in rare cases, to intervention supervised by specialized structures.
References
WOAH — Organisation mondiale de la santé animale (ex-OIE)
FAO — Organisation des Nations unies pour l'alimentation et l'agriculture
Content reviewed by the TunisieVet editorial team · Updated 24 September 2026
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