Shrinking Mediterranean sardines the 'latest victims of global warming'

French scientists have finally solved the mystery of why sardines in the Mediterranean are shrinking – losing about two-thirds of their body weight – and living far shorter lives than they did 20 years ago.  

Sardines are essential links in the ocean food chain and are among the most world’s most fished food. AFP - RAJESH JANTILAL

Many theories had been doing the rounds among fisherfolk: an increase in hungry predators such as the bluefin tuna and dolphins, overfishing, and epizootic diseases in the Gulf of Lions – the Mediterranean’s richest pasture for sardines, anchovy and mackerel.

However biologists at the French Research Institute for the Exploitation of the Sea (Ifremer) debunked these hypotheses in a report published Monday, concluding the sardines were getting skinnier because their own food source – plankton – was drying up.

The microscopic marine organisms, which are carried along by tides and currents, have depleted by 15 percent while also becoming less nutritious. It’s a consequence of global warming, given the plankton are usually fed by nutrients from cooler, deeper waters.

#MonaLisa 🐟 Les sardines subissent une baisse significative de leur taille depuis plusieurs années dans le golfe du Lion en #Méditerranée. En cause, leur nourriture et leur environnement 🌊
Des résultats inédits signés l'Ifremer 👇 #plancton #océan https://t.co/uWVKetkaWU

— Ifremer (@Ifremer_fr) November 8, 2021

"Satellite images clearly show a drop in the quantity of microalgae in the mid-2000s … at the same time we started to see a decline in the size of sardines," says Jean-Marc Fromentin, a researcher at Ifremer.

Smaller planktonic cells also offer less energy to the fish that consume them – thus once fat sardines measuring 15 centimetres have now shrunk in size to 11 centimetres, while their lifespans have dropped from seven to two years.

Global warming

“This is the result of major regional environmental changes, reflected in a drop in nutrients brought by the Rhone, modifications in atmospheric and oceanic circulation and a global temperature increase of 0.5°C in 30 years on average,” Fromentin said.

Sardines are essential links in the ocean food chain and are among the most world’s most fished food. Smaller, younger sardines, whose weight has plummeted from 30g to 10g, are of little commercial value.

Dubbed the Mona Lisa project, scientists monitored the natural environment while also carrying out controlled tank experiments on 450 one-year-old sardines caught at sea.

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The aim was to test how the size and quantity of food affected the survival, growth and reserves of the fish.

“A sardine given a small amount of food must have a double portion in order to grow like a sardine given a large amount of food,” said Claire Saraux, a former In-fremer researcher who now works at the CNRS, France's National Centre for Scientific Research.

“We were surprised by the very significant effect of the size of the food."

Sardines fed large quantities of larger feed regained a similar size to those caught before 2008.

Ifremer’s work was carried out within the Marbec joint research unit, which brings together a host of organisations and universities on the conservation of marine diversity.

It was financed by the European Maritime and Fisheries Fund, Feamp.