Atlantic dolphins are dying much younger. Scientists sound the alarm

Researchers from the University of Colorado Boulder found that female common dolphins are living an average of seven years less than they did in 1997. The team warns that this decline threatens not just the species but also the ocean ecosystems they help sustain.

"There is an urgent need to manage the population better," said Etienne Rouby, a postdoctoral researcher at the Institute of Arctic and Alpine Research (INSTAAR). "Otherwise, there is a risk for decline and, ultimately, extinction."

Dolphins of the Bay of Biscay

Roughly 6 million common dolphins inhabit tropical and temperate waters worldwide, making them the most numerous members of the cetacean family, which includes whales and porpoises. One of their key winter gathering spots is the Bay of Biscay, off the coast of France, where nutrient-rich waters attract anchovies, sardines, and other small fish that serve as food for the dolphins.

However, this same region is also one of Europe's busiest fishing zones. While dolphins are not the intended catch, many become accidental victims of fishing operations. Known as "bycatch," this phenomenon causes thousands of dolphin deaths each year. In 2021 alone, bycatch was estimated to have killed about 6,900 dolphins out of the bay's winter population of 180,000.

Despite such figures, past surveys suggested that dolphin numbers in the area were stable.

A New Way to Measure Decline

Traditional monitoring methods rely on counting dolphins spotted from ships or aircraft. Because the animals are constantly moving, these surveys can overlook population changes until they become severe. For species like dolphins, which reproduce slowly and live for decades, that delay can make recovery extremely difficult once the decline becomes visible, Rouby explained.

To better track survival trends, the research team took a different approach by examining stranded dolphins that had washed ashore along the Bay of Biscay. Dolphins usually beach themselves when 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.

"We wanted to capture changes in the population's survival and fertility rates. These are more sensitive indicators of population health, and they enable us to identify the problems before they become irreversible," Rouby said.

Evidence in the Teeth

By studying the growth layers in dolphin teeth, the scientists determined the ages at which the animals died. Their findings showed that the average lifespan of female dolphins in the Bay of Biscay fell from 24 years in the late 1990s to only 17 years by 2019. This decrease has also resulted in fewer calves being born, signaling a broader reproductive decline.

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

"The numbers are likely to be lower in reality," Rouby said. He warned that if this trend continues, growth could eventually turn negative, marking the beginning of an overall population decline.

Policy Changes and Future Action

Since 2024, the French government has implemented a one-month annual fishing ban in the Bay of Biscay each January to protect dolphins. Although initial reports show the measure may be helping, Rouby suggested that adjusting the ban's timing based on dolphin migration patterns could make it more effective. Because dolphins do not always arrive at the same time each year, matching the closure to their movements would offer better protection.

Other North Atlantic cetaceans, including harbor porpoises and bottlenose dolphins, may be facing similar pressures. Understanding these patterns could help strengthen marine protection policies such as the US Marine Mammal Protection Act and the European Marine Strategy Framework Directive.

Why Dolphins Matter

"Dolphins are the top predators in the Bay of Biscay, and they play a very important role in the ecosystem," Rouby explained. "Without these predators, fish populations could become out of control, and they would in turn consume too much plankton and vegetation until the system collapses."

He emphasized the need for swift, informed action. "As humans, we should make conscious decisions to protect the living and non-living things around us. Facing evidence of viability loss, we need to act before it is too late."