Victims of accidental captures, are the dolphins of the French coasts really threatened with extinction? – Nature

How many common dolphins are there off the French Atlantic coast?

Census campaigns carried out in recent years have resulted in a relatively constant number which is a consensus, even if uncertainties remain as these marine mammals can be on the move. “This population is estimated at more than 200,000 in winter and 400,000 in summer,” says Étienne Rouby, researcher at the Pelagis observatory in La Rochelle, and author of a thesis, published in 2022, on the evolution of common dolphin populations in the North-East Atlantic. This summer increase could be due, in part, to migratory movements related to reproduction.

What are the signs that show that common dolphins could disappear from the Bay of Biscay?

At least three factors suggest common dolphins in the northeast Atlantic are in bad shape, according to scientists at the Pelagis Observatory. “We have quantified a life expectancy at birth which has dropped by seven years in a few years, it has gone from around 24 to 17 years, it is a strong signal”, notes Étienne Rouby. The other two red flags are an advanced sexual maturity of two years and a declining population growth rate.

Are these worrying signals the consequence of accidental catches made by professional fishermen?

Étienne Rouby remains cautious on this point. “We cannot establish a causal relationship. We can however say that we observe, in an environment where there is a high level of accidental captures, a decrease in these three factors: population growth rate, life expectancy and age of sexual maturity. »

It has also been established that the theoretical maximum number of deaths by accidental capture compatible with a favorable conservation status of small cetaceans in the North-East Atlantic is included in a wide range of 1,000 to 5,000 individuals per year, recalled the state Council. However, the real number would be much higher. In a notice published on January 24, 2023, the International Council for the Exploration of the Sea (ICMS) estimated that the level of accidental capture deaths was around 9,000 per year.

“As long as there can be external influxes of dolphins, the system will be balanced but it could, one day, quickly collapse. »

Is it contradictory to have, on the one hand, a constant population of common dolphins, and on the other, elements which suggest that they are on the verge of extinction?

“The common dolphin and the harbor porpoise face a serious danger of extinction, at least regionally”, pointed out the Council of State, in its decision of March 20, based on reports from Ciem and on data from the National Inventory of Natural Heritage. However, the population level of common dolphins in the Bay of Biscay has not changed in recent years, according to the censuses carried out.

“This observation may seem paradoxical but it is not aberrant, if we analyze the situation on a larger scale. We think, in fact, that immigration comes into play: the population of dolphins in the Bay of Biscay receives individuals from elsewhere, perhaps during the summer, when there is a strong mixing, “suggests Étienne Rouby.

When could the common dolphin population approach zero?

The curves produced in Étienne Rouby’s thesis seem to show an extinction of the species in the next 40 to 50 years. Values ​​to be taken with a grain of salt, according to the scientist: “To arrive at something very precise, it would be necessary to carry out much more complex modeling analyzes by adding the impact of immigration and also to have more information on tracking individuals. The researcher nevertheless warns: “As long as there can be external contributions of dolphins, the system will be balanced but it could, one day, quickly collapse”.

What would be the consequences of the disappearance of common dolphins?

The common dolphin, like many species at the top of the food chain, regulates populations in its ecosystem. If, tomorrow, it disappeared, an imbalance could be created. “The phenomenon has already been observed in the Black Sea, where the disappearance of a predatory species has left too much room for the one it was hunting, leading to a collapse in diversity”, illustrates Étienne Rouby. The longer-term economic impact could be catastrophic. One of the darkest hypotheses would result in fishermen finding themselves catching more jellyfish than fish…