Europe 1 with AFP 8:15 p.m., November 08, 2021

An Ifremer study published on Monday ensures that the decrease in the size of sardines is linked to a "drop in the quantity of microalgae in the mid-2000s".

In question, of "significant regional environmental changes", with less nutrients.

Sardines are getting smaller and smaller due to changes in their food due to changes in the environment.

This was revealed on Monday by the French Research Institute for the Exploitation of the Sea (Ifremer) after a study carried out in the Mediterranean.

As essential links in the food chain in the ocean, sardines are among the most widely caught fish in the world.

But since the mid-2000s, their size has sharply decreased, dropping in the Mediterranean from 15 to 11 cm on average, according to the institute whose research has shown that neither fishing, nor natural predators, nor a virus were at the origin of these changes, but rather their diet.

A responsible temperature increase?

"The satellite images clearly show a drop in the quantity of microalgae in the mid-2000s, going up to 15%", reports in a press release Jean-Marc Fromentin, researcher at Ifremer in Sète, further reporting a decrease in the size of planktonic cells.

"These modifications would result from significant regional environmental changes, resulting in a decrease in the nutrients brought by the Rhône, changes in atmospheric and oceanic circulation, and an overall increase in temperature of 0.5 ° C in 30 years on average in link with climate change, "says Ifremer.

Food size, a key variable

An experiment unprecedented by its scale in a controlled environment was also carried out within the framework of this study. A total of 450 sardines were distributed in eight tanks to test the effect of size and amount of food on their survival, growth and reserves. "We were surprised by the very significant effect of the size of the feed", notes Claire Saraux, former researcher at Ifremer and now at the CNRS, explaining that "a sardine receiving small-sized feed must have some. a double portion to grow like a sardine with large foods ".

With food of small size, the sardine consumes its prey by "filtration", through its gills, which implies a sustained swimming for a rather long period of time.

With large foods, the sardines gobble up their prey one by one, which requires a much shorter sustained swimming time and therefore less energy expenditure.

Sardines fed in large quantities with larger feeds returned to a size similar to those caught before 2008.