Sardines are up to two-thirds smaller now than they were 12 years ago as warming waters caused by climate change are killing the plankton they feed on.
Researchers at the French oceanographic institute, Ifremer, have been studying the sizes of sardines in the Mediterranean sea and the Bay of Biscay in the Atlantic.
While the former has seen the largest reduction in average sizes, the team report that Atlantic sardines have also lost around half of their weight around a decade ago.
The changing sardine populations are having knock on-effects, the team also noted, with other species like cod and seabird also suffering.
Researchers from Ifremer have been studying the state of the sardines in the Mediterranean since 2008, after fishermen drew their attention to the shrinkage.
In this time, Mediterranean sardines have shrunk from an average length of 5.1 inches (13 cm) to 3.9 inches (10 cm) last year, while their counterparts in the Bay of Biscay have gone from around 7.1 (18 cm) to 5.5 (14 cm).
The team’s analysis has ruled out predation, overfishing and disease as the cause of these changes in the sardine populations —with climate change the likely culprit.
‘For sardines and anchovies in the Bay of Biscay and in the Gulf of Lion [in the Mediterranean], we are leaning towards environmental factors linked to the rise in temperature and the fall in the quantity of food available,’ said biologist Clara Ulrich.
Sardines feed off of microscopic plankton — a food source which has grown less wholesome with rising sea surface temperatures.
The problem is that the nutrients that feed the plankton are not rising from the cooler deep waters as they once did, the researchers explained.
Alongside reducing their size, the diminishing food stocks also appear to be curtailing the lifespans of the sardines, which has fallen from three years around a decade ago to just one year today.
‘It’s very alarming to see fish so damaged,’ Foundation of the Sea institute president Sabine Roux de Bézieux — who regards the newly-released findings as amounting to a crisis — told Europe 1 radio.
‘It’s a sign of the very bad health of the surrounding environment.’
‘This must alert us to the disaster which is playing out in the oceans because of climate change.’
Experts are also reporting that the shrinking sardines are have a knock-on effect further up their food chain, with their predators — seabirds in particular — no longer receiving the same levels of nourishment they used to enjoy.
The cascading shrinkage is not confirmed to the Mediterranean and the Atlantic, however — similar trends have been see in the Baltic Sea, the Indian Ocean and in the Pacific Ocean off of the coast of South America.
‘The Baltic cod have become so thin that they look like sardines,’ Ms Ulrich said.
Meanwhile, off of the coast of the Galapagos Islands, Nazca booby birds have changed their diet preference over the last ten years from sardines to less nutrient-rich flying fish in response to the falling sardine numbers.
According to the researchers, the fish may have moved into cooler waters — and could therefore vanish completely from around the archipelago within the century if ocean temperatures continue to rise unabated.
There was some good news in the results of Ifremer’s latest survey, however.
Researchers have found that — following drastic restrictions imposed on the catch sizes of fishing boats — red tuna populations in both the Atlantic and the Mediterranean are once again growing healthily.
Despite this, around a quarter of the fish caught by French vessels belong to species which are presently threatened by overfishing — these include sardines in the Bay of Biscay, haddock in the Irish Sea and cod in the northeast Channel and the Irish sea.