Decline in Dolphin Lifespan and Its Ecological Impact

Researchers from the University of Colorado Boulder have revealed that female common dolphins now live an average of seven years less than they did in 1997. The team warns that this decline threatens not only the species but also the ocean ecosystems that these dolphins help support.

The Importance of the Bay of Biscay for Dolphins

Approximately 6 million common dolphins inhabit tropical and temperate waters worldwide, making them one of the most numerous members of the cetacean family, which includes whales and porpoises. The Bay of Biscay, off the coast of France, is one of their most important winter gathering areas, attracting nutrient-rich waters filled with anchovies, sardines, and other small fish that constitute the dolphins’ diet.

However, this same area is one of the busiest fishing regions in Europe. Although dolphins are not the target catch, many fall victim to incidental capture during fishing operations. This phenomenon, known as “bycatch,” results in the death of thousands of dolphins each year. In 2021 alone, bycatch was estimated to have killed about 6,900 dolphins out of the 180,000 winter population in the bay.

A New Approach to Monitoring Decline

Traditional monitoring methods rely on counting dolphins observed from ships or planes. Due to the animals’ constant movement, these surveys can overlook population changes until they become severe. For species like dolphins, which reproduce slowly and live for decades, this delay can make recovery extremely difficult once decline becomes visible.

To better track survival trends, the research team adopted a different approach by examining stranded dolphins that washed ashore in the Bay of Biscay. Dolphins typically strand when they are sick, injured, or disoriented, and most do not survive. Although stranded animals represent only about 10% of total deaths, their condition over time can reveal important patterns in population health.

The researchers analyzed 759 stranded common dolphins collected between 1997 and 2019.

Evidence in the Teeth

By studying growth layers in dolphin teeth, scientists determined the ages at which these animals died. Their findings showed that the average age of female dolphins in the Bay of Biscay dropped from 24 years in the late 1990s to just 17 years by 2019. This decrease also led to fewer births, indicating a broader decline in fertility.

The study found that population growth slowed by 2.4% since 1997. Under ideal conditions, common dolphin numbers could increase by about 4% annually, meaning that growth in 2019 was likely only around 1.6%.

Policy Changes and Future Actions

Since 2024, the French government has implemented an annual one-month fishing ban in the Bay of Biscay every January to protect dolphins. While initial reports suggest this measure may help, researcher Roby proposed that adjusting the timing of the ban based on dolphin migration patterns could make it more effective. Since dolphins do not always arrive at the same time each year, aligning the closure with their movement would provide better protection.

Conclusion

Dolphins are key predators in the Bay of Biscay, and their role in the ecosystem is crucial. Without them, fish populations could spiral out of control, consuming too much plankton and plants, leading to ecosystem collapse. This challenge highlights the need for conscious and swift action to protect marine environments and biodiversity. Understanding these patterns of decline can help strengthen marine policies like the U.S. Marine Mammal Protection Act and the European Marine Strategy Framework Directive.