A shocking study reveals common dolphins’ lifespans are shrinking in the North Atlantic. Learn how fishing bycatch is causing this decline and what it means for marine conservation.
Common dolphins, one of the most common and widely distributed marine mammals, are living shorter lives in the North Atlantic compared to a few decades ago. A recently published study in Conservation Letters on October 10 highlights a significant decrease in their lifespan, creating concerns not only for the dolphins but also for the whole ocean environment.
Researchers from the University of Colorado Boulder discovered that female common dolphins now live about seven years less than they did in 1997. The scientists say that if this trend continues, it could pose a threat to the population’s survival.
Dolphins of the Bay of Biscay
There are approximately six million common dolphins living in tropical and temperate waters globally, making them the most numerous member of the cetacean family, which also includes whales and porpoises. One major winter habitat is the Bay of Biscay, located off the French coast. The area’s nutrient-rich waters attract small fish like anchovies and sardines, which are the dolphins’ main food source.
However, this area is also one of Europe’s busiest fishing zones. Dolphins are not the intended target of fishing activities, but many die accidentally in fishing nets, a situation referred to as bycatch. Each year, thousands of dolphins die from this cause. In 2021 alone, around 6,900 dolphins are estimated to have died in the bay’s winter population of 180,000.
Despite these losses, earlier surveys suggested that dolphin numbers remained stable, making it difficult to notice a decline until it became significant.
New Method to Assess Population Health
Traditional methods for monitoring dolphin populations often involve counting them from boats or aircraft. Since these animals are constantly moving, this approach may fail to detect early signs of population decline. For species with slow reproduction rates like dolphins, the delay in identifying problems can make recovery quite challenging.
To get a more detailed understanding, the research team used a different method: they studied stranded dolphins, those that wash ashore. Dolphins often beach themselves when they are sick, injured, or disoriented, and most do not survive. Even though stranded animals represent only about 10% of all deaths, they provide important insights into the overall health of the population. The scientists analyzed 759 common dolphins that stranded between 1997 and 2019.
Clues in the Teeth
By analyzing the growth layers in the dolphins’ teeth, the researchers were able to determine their age at death. They found that female dolphins in the Bay of Biscay now live around 17 years on average, compared to 24 years in the late 1990s. This decrease in lifespan has also resulted in fewer calves being born, indicating a decline in reproduction.
The study estimates that population growth has slowed by 2.4% since 1997. Under optimal conditions, common dolphin populations can grow by approximately 4% annually. By 2019, the growth rate may have been only about 1.6%. Rouby cautioned that if the trend continues, the population could start to decrease.
New Policies and Future Steps
Since 2024, the French government has implemented a one-month fishing ban in the Bay of Biscay every January to protect dolphins. Initial reports suggest this measure could be beneficial, but Rouby pointed out that aligning the timing of the ban with dolphin migration patterns might make it more effective, as dolphins do not always arrive at the same time each year.
Other North Atlantic cetaceans, such as harbour porpoises and bottlenose dolphins, may also face similar threats. Gaining a better understanding of these trends can help shape stronger policies for marine conservation, such as the US Marine Mammal Protection Act and the European Marine Strategy Framework Directive.