North Atlantic dolphins are dying younger, new study shows
The common dolphin is the most abundant cetacean in the world. (Credit: Charles J. Sharp/Wikimedia)
Common dolphins are among the ocean鈥檚 most abundant mammals, but they are living shorter lives in the North Atlantic, according to published October 10 in Conservation Letters.

Etienne Rouby
The 兔子先生传媒文化作品-led research team discovered that the longevity of female common dolphins has declined by seven years since 1997, an alarming trend that the authors say threatens not only the species but also the marine ecosystem it helps maintain.
鈥There is an urgent need to manage the population better,鈥 said Etienne Rouby, a postdoctoral researcher in the Institute of Arctic and Alpine Research (INSTAAR). 鈥Otherwise, there is a risk for decline and, ultimately, extinction.鈥
About 6 million common dolphins roam tropical and temperate oceans around the globe. They are the world鈥檚 most common 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 species of fish like anchovies and sardines that dolphins eat. But those same conditions also make it one of Europe鈥檚 fishing hotspots.
While dolphins are not the target of fishing, many of them end up in nets by accident, also known as 鈥渂ycatch鈥. Most dolphins caught as bycatch die. Some studies that in 2021, fishing bycatch was responsible for the death of 6,900 dolphins in the bay, from a winter population of 180,000.
Despite those numbers, traditional abundance monitoring methods previously suggested that the dolphin population in the bay was stable.

The common dolphin is the most abundant cetacean in the world. (Credit: Observatoire Pelagis - France)
Conventionally, scientists have estimated dolphin numbers by tallying individuals spotted from survey ships and planes. Because dolphins are constantly moving in and out of a region, this approach can miss population changes until drastic shifts occur. For long-lived animals that only produce a few offspring in their lifetimes, such as dolphins, recovery may not be possible once population declines are significant enough to show up in general counts, Rouby said.
He and his team set out to reevaluate how well these cetaceans were surviving using a new approach they developed: counting and analyzing deceased dolphins stranded on beaches in the bay.
Dolphins often come onto the shore because they are old, sick, injured or disoriented, and rarely any survives after being stranded. While stranded dolphins represent only about 10% of total dolphin deaths, changes in their mortality patterns over time can reveal broader population trends.
The team studied 759 common dolphins stranded on beaches along the Bay of Biscay between 1997 and 2019.

The Bay of Biscay. (Credit: Wikimedia)
鈥淲e wanted to capture changes in the population鈥檚 survival and fertility rates. These are more sensitive indicators of population health, and they enable us to identify the problems before they become irreversible,鈥 he said.
By analyzing dolphin teeth, the team determined the age at which these animals died. The team found that female dolphins鈥 longevity decreased from 24 years in the late 1990s to just 17 years two decades later. This decline has led to fewer calves born, the study suggests.
The researchers estimated that the dolphin population growth rate has declined by 2.4% from 1997 to 2019. In ideal conditions, a healthy common dolphin population grows ideally at about 4% per year. This means if the population was thriving perfectly in 1997, it was only growing at 1.6% annually by 2019.
鈥淭he numbers are likely to be lower in reality,鈥 Rouby said. If the trend continues, the growth rate could dip below zero, a threshold that would signal population decline, he added.
Since 2024, the French government has been closing the Bay of Biscay to fishing for one month every January to protect the dolphins. While some data has suggested the measure has been helpful, Rouby said a more flexible schedule could work better.
Depending on ocean conditions, dolphins may arrive at the bay earlier or later than the set period in January, so听timing fishing restrictions to match dolphin visits would better protect the animals.
听听Etienne Rouby received the from the International Society for Ecological Modelling for the methology he used in this work in September. He pioneered听the use of age-at-death data to estimate how well an animal population is doing.听
Other cetaceans in the North Atlantic, including Harbor porpoise and bottlenose dolphins may also be experiencing similar declines. 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, Rouby said.
鈥淒olphins are the top predators in the Bay of Biscay, and they play a very important role in the ecosystem. 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,鈥 Rouby said. 鈥淎s 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.鈥澨
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