If you're a fan of sardines, you're a member of an international dining club including whales, sea birds and turtles. ‘The fish are preyed upon by seabirds, but also by larger fish’, says Elisa Thoral from the Claude Bernard University Lyon 1, France, adding that the silvery fish (Sardina pilchardus) dine, in turn, on swarms of plankton teeming in the oceans. However, sardines cannot depend on a reliable food supply year-round; plankton populations rise and decline seasonally. ‘In winter, plankton productivity is limited’, Thoral explains, adding that this naturally restricts the sardines’ diet during the colder months. However, the Mediterranean Sea is warming 20% faster than the rest of the planet, placing its summer plankton populations at risk too. What impact might summer plankton losses have on shoals of sardines along the coasts of southern Europe, North Africa and the Levant? Thoral teamed up with researchers from the Universities of Lyon and Montpellier, Centre National de la Recherche Scientifique (CNRS) and the French Research Institute for Exploitation of the Sea (Ifremer), France, to find out how the fish fare on meagre rations in warm summer and cooler winter waters.
‘With the help of fishermen, we developed a way of fishing sardines without taking them out of the water to ensure their survival by tightening the purse seiner net, coaxing them gently into buckets then transferring them immediately into tanks of oxygenated seawater on board’, says Claire Saraux from Ifremer, Sète, France. The team then transported the fish back to large tanks at Ifremer, Palavas-les-Flots, France. There, they weaned the fish off their live plankton diet onto aquaculture pellets, with some being transferred to water at a summer temperature of 20°C, while the rest remained in cold winter water (12°C). Once the fish had adjusted to their new home, the team cut the rations of the winter and the summer fish, initially for 30 days and then for a further 30 days, checking how the fish's metabolism adapted to going without food during the summer and winter.
Unfortunately, the summer fish seemed to struggle when their food supply dried up. Although the fish needed to reduce their metabolic rate to conserve energy at the higher temperature – and their energy-producing mitochondria did reduce their energy expenditure – the animals’ metabolic rates remained high at the exact time when they needed to scale back to conserve energy at the higher temperatures. They also lost muscle. And, when the team tried to revive the fish by resuming feeding, the summer sardines were unable to recover from the interruption to their food supply. ‘Long-term food deprivation in a warm environment may cause irreversible deterioration of the ability to assimilate nutrients’, says Thoral, suggesting that summer famines could prove disastrous for sardine survival.
In contrast, the winter fish dealt better with the loss of their food supply, maintaining their muscle and mitochondrial energy production to emerge from the winter shortages. ‘As resources are naturally scarce during winter, sardines may have adapted to a lack of food in the cold season’, says Thoral.
Summer heatwaves could pose a genuine threat to sardine populations, removing a key element from the diets of larger fish, aquatic mammals and humans too. So maybe it's time we all started eating less meat, insulating our homes better and walking and cycling more to protect sardines from future summer heatwaves.
