North Atlantic dolphins are dying younger, study shows

Common dolphins are among the ocean's most abundant mammals, but they are less abundant in the North Atlantic, according to a new study. published V Letters of conservation.

A research team led by the University of Colorado Boulder found that the life expectancy of female common dolphins has decreased by seven years since 1997. This alarming trend, the authors say, threatens not only the species, but also the marine ecosystem it helps support.

“There is an urgent need to better manage the population,” said Etienne Ruby, a researcher at the Institute of Arctic and Alpine Research (INSTAAR). “Otherwise there is a risk of decline and eventual extinction.”

About 6 million common dolphins roam tropical and temperate oceans around the world. They are the world's most widespread cetaceans, a group that includes all whales, dolphins and porpoises.

The Bay of Biscay in the Atlantic Ocean off the French coast is a popular destination for dolphins in winter because its warmer, nutrient-rich waters attract smaller fish species such as anchovies and sardines, which the dolphins feed on. But those same conditions make it one of Europe's fishing hotspots.

Although dolphins are not fished, many are accidentally caught in nets, also known as bycatch. Most dolphins caught as bycatch die. Some studies estimate that fishery bycatch was responsible for the deaths of 6,900 dolphins in the Gulf in 2021, out of a winter population of 180,000.

Despite these numbers, traditional population monitoring methods previously assumed that the bay's dolphin population was stable.

Traditionally, scientists estimate dolphin numbers by counting individuals spotted from research ships and aircraft. Because dolphins are constantly moving in and out of the region, this approach may miss changes in the population until radical shifts occur.

For long-lived animals that produce only a few offspring in their lives, such as dolphins, recovery may not be possible if population declines become significant enough to be reflected in overall counts, Ruby said.

He and his team decided to reassess how well these cetaceans survive using a new approach they developed: counting and analyzing dead dolphins that ended up on beaches in the bay.

Dolphins often come ashore because they are old, sick, injured or disoriented, and rarely do any survive being stranded. Although dolphin strandings account for only about 10% of total dolphin deaths, changes in their mortality patterns over time can reveal broader population trends.

The team studied 759 dolphins stranded on the Bay of Biscay between 1997 and 2019.

“We wanted to track changes in the survival and birth rates of the population. These are more sensitive indicators of population health and allow us to identify problems before they become irreversible,” he said.

By analyzing the dolphins' teeth, the team determined the age at which the animals died. The team found that female dolphins' life expectancy had dropped from 24 years in the late 1990s to just 17 years two decades later. This decline has led to fewer calf births, the study said.

Researchers estimate that the growth rate of the dolphin population decreased by 2.4% from 1997 to 2019. Under ideal conditions, a healthy common dolphin population ideally grows at about 4% per year. This means that while the population was thriving in 1997, by 2019 it was growing at only 1.6% per year.

“The actual numbers are likely to be lower,” Ruby said. If the trend continues, growth rates could fall below zero, a threshold that would signal population decline, he added.

Since 2024, the French government has closed the Bay of Biscay to fishing every January for one month to protect dolphins. While some evidence suggests the measure has been helpful, Ruby said more flexible scheduling might work better.

Depending on ocean conditions, dolphins may arrive in the bay earlier or later than expected in January, so fishing restrictions that match the timing of dolphin visits will better protect the animals.

Other North Atlantic cetaceans, including the harbor porpoise and bottlenose dolphinsmay also experience a similar decline. Studying how well their populations are doing could improve current conservation policies, including the US Marine Mammal Protection Act and the European Marine Strategy Framework Directive, Ruby said.

“Dolphins are the top predators in the Bay of Biscay and play a very important role in the ecosystem. Without these predators fish populations can get out of control, and they in turn will consume too much plankton and vegetation until the system collapses,” Ruby said.

“As humans, we must make conscious decisions to protect the living and nonliving things around us. Faced with signs of loss of vitality, we need to act before it is too late.”