It was a phenomenon that was not explained very well until now. Scientists have just understood why sardines are shrinking in the Mediterranean.
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The measurements are clear: over the past dozen years, the size of sardines in the Gulf of Lion in the Mediterranean has increased from 15 to 11 cm and their weight has been divided by three, from 30 to 10 grams. Another hitherto unexplained finding: in the Mediterranean, sardines over two years old have disappeared.
Among the avenues mentioned to explain this situation, there was that of fishing, that of natural predators: tuna or dolphins in particular, or that of a virus or bacteria. But it is ultimately none of this: it is the plankton that is involved. Sardines feed on it, and the quantity of this microalgae has decreased by 15% for ten years in the Gulf of Lions. Satellite images attest to this.
On the other hand, the size of these microalgae has also been reduced, and small plankton feed sardines less well. This is what Ifremer researchers have just confirmed, through experiments in basins.
450 sardines were fed for seven months with small or large plankton depending on the group. Result: for the same amount of food, the sardines grow and get bigger better with large plankton, and in this case they regain their size from ten years ago, explains Jean-Marc Fromentin, Ifremer researcher in Sète. On the other hand, the “little” plankton is visibly less nourishing, they need to swallow a double portion to meet their needs and as in addition sardines spend more energy to absorb it, they lose weight and their life expectancy decreases. . Less well fed, these sardines actually die after two years instead of the usual five or six years.
The evolution of the size of the plankton is due to environmental changes which affect the quality of the plankton: in particular the modification of the quantity of nutrients brought by the Rhône (which flows into the Mediterranean) or the increase in the temperature of water or changes in ocean circulation. The bad news is that these latest developments being linked to global warming, they are therefore difficult to reverse.
However, this shrinking phenomenon does not concern Atlantic sardines, which continue to display a size of 20 cm and live three times longer.