What causes strandings?

Cetacean strandings arise from a tangle of biological, environmental and human factors. They are sentinel events, revealing the pressures acting on marine populations. Understanding their causes is essential if conservation policy is to be steered and the health of ecosystems measured.

Two broad families of causes

Strandings are generally classified according to two main origins:

  • Natural causes: these arise from biological processes (old age, reproduction, disease) or from the hazards of the environment (predation, navigational errors).
  • Anthropogenic causes: these result from human activities (fishing gear, vessels, pollution, noise, habitat alteration).

In most cases several factors coexist. It is often difficult to identify a single cause.

Natural causes

  • Old age and physiological weakening

Animals that are old or weakened by age are more vulnerable. They can lose the ability to orient themselves, to feed, or to keep up with their group, which leads to stranding.

  • Vulnerability of the young

Young cetaceans, particularly around weaning, face a high risk of stranding because of their inexperience, their dependence on the mother, or difficulties in learning to navigate.

  • Reproductive complications

Pregnant females, or females that have just given birth, may strand if complications arise (difficult birth, abortion, physiological stress).

  • Viral and bacterial disease

Morbilliviruses, responsible for deadly epidemics (such as the one of 1990-1992 in the Mediterranean), cause pneumonia, encephalitis and immunosuppression. Brucella, Leptospira and Erysipelothrix are also identified in strandings.

  • Lung parasites

Nematodes such as Halocercus can cause haemorrhage or pneumothorax. Their proliferation may be linked to immunosuppression or to a change in the ecosystem.

  • Predation and competition

Attacks by killer whales or sharks can cause injuries, or flight behaviour that leads to stranding. Intraspecific competition for food also weakens the animals that lose out.

Anthropogenic causes

  • Bycatch in fishing gear

Bycatch is today the leading anthropogenic cause of stranding. Common dolphins are the most affected, particularly in the Bay of Biscay. In 2023, more than 11,000 animals are thought to have been caught. The gear involved includes pelagic trawls, gillnets and trammel nets.

  • Ship strikes

Large species such as rorquals and sperm whales are frequently struck by vessels, particularly in the Mediterranean, where one stranded whale in five bears impact marks. Rising shipping traffic, vessel speed and vessel power all increase the risk.

  • Chemical, plastic and noise pollution

Cetaceans accumulate organochlorine pollutants (PCBs, pesticides) and heavy metals, which cause immune, neurological and reproductive disorders.

Plastic causes blockages, entanglement and, through microplastics, disturbance of the food web.

Anthropogenic noise (military sonar, drilling, traffic) seriously disrupts echolocation, causing disorientation, stress and even internal injury.

  • Overfishing and the scarcity of resources

The decline of fish stocks pushes cetaceans closer to the coast to feed, increasing their exposure to fishing gear, vessels and pollution.

  • Climate change

Ocean warming alters the distribution of prey and food chains, and brings emerging diseases. Marine heatwaves can also force migrations and cause strandings.

Causes of mass strandings

  • Strong social cohesion

Some gregarious species (pilot whales, for example) may follow a sick companion all the way to stranding, out of social instinct.

  • Navigational errors

Certain coastal configurations (sandy bottoms, enclosed bays) can disturb acoustic signals. Falling tides, or hook-shaped bays, trap the animals.

  • Magnetic disturbance

Anomalies in the Earth’s magnetic field, particularly during solar storms, could interfere with the magnetic sense of orientation of cetaceans.

Aggravating factors and complex interactions

  • Chemical immunosuppression

Pollution weakens natural defences, favouring the emergence of pathogens that are normally harmless.

  • Pathogens of terrestrial origin

Agents such as Toxoplasma gondii, from urban or agricultural run-off, reach coastal waters and infect cetaceans.

  • Harmful combinations
  • Pollution + infection = increased vulnerability
  • Overfishing + climate change = scarcer food
  • Noise + stress = disorientation and stranding

Diagnostic limits and scientific challenges

Only 8% of animals that die at sea come ashore. Decomposition, the condition of the carcasses, the delay before recovery and the multifactorial origin all limit what analysis can establish.

Even so, every documented stranding makes it possible:

  • To identify epidemiological trends
  • To assess the effectiveness of conservation policy
  • To detect health crises early
  • To strengthen observation and response networks

The rise in strandings in France (+340% between 1990 and 2021) reflects an intensification of human pressures on the marine environment. Natural causes have always existed, but it is the anthropogenic causes (bycatch, ship strikes, pollution, noise) that dominate today.

Studying strandings is not merely an exercise in veterinary pathology: it is a way into understanding the health of the oceans. Acting on their causes means protecting cetaceans… and also the resilience of marine ecosystems as a whole.