Twenty to 30 new jellyfish species have been detected in the Marmara Sea in the last 20 years, posing threats to fish stocks and marine livelihoods.

Speaking to Anadolu Agency (AA), professor Melek Işibili Okyar, dean of Istanbul University's Faculty of Aquatic Sciences, stated that jellyfish, which have survived on the planet for 550 million years without undergoing any morphological changes, are extremely sturdy organisms.

Emphasizing that jellyfish have a high tolerance for changes such as temperature and seawater salinity, Okyar said, "As they feed on plankton and zooplankton, they have an intense food supply."

"There are also creatures that feed on jellyfish, the most well-known are sea turtles. Apart from that, over 120 fish species in the ecosystem feed on jellyfish, which includes sardines, swordfish and sprats. While dolphins feed on jellyfish to train their young, the numbers of these fish species and sea turtles have dwindled significantly," she explained.

Stressing on the impact of climate change, Okyar said: "On one hand, with dams built on rivers, seas are losing their regeneration abilities with rivers; and the input of domestic, industrial and agricultural wastes such as nitrogen and phosphorus into the sea has increased. Due to global warming, the marine temperature has been expanding to the north as creatures accustomed to warmer temperatures begin migrating north. On the other hand, overfishing has been a factor. We have industrial fisheries with very strong equipment and they hunt fish that feed on jellyfish during the fishing season. Man-made structures made of wood and concrete on shores prevent water circulation and allow jellyfish larvae to affix to the hard surfaces, thereby boosting their numbers."

Effect of plastic

Okyar pointed out that plastic pollution at sea is one of the reasons for an increase in jellyfish numbers. Stating that plastics create a hard area where jellyfish fix themselves and play an active role in their migration to places where they are not normally found, Okyar said: "You normally do not see an increase in jellyfish in the heart of the Marmara Sea, but a large increase has been seen in coastal areas. The ecosystem on the coast is more suitable for the species to attach themselves to plastic and spread to deeper areas as well."

The increase in jellyfish affects other fish species in terms of reproduction, as they feed on the eggs and larvae of several species. Their dominance over plankton, fish eggs and feeding on their larvae prevents future fish stocks from forming. Predicting that many fish species may disappear in the future, Okyar said, "You will order your child, 'Eat the jellyfish on your plate' instead of nutritional fish."

Okyar noted that jellyfish are not only increasing in Türkiye but globally as well. This includes regions such as the seas of Japan, China, Israel, France, Spain and the Mediterranean part of Italy that have witnessed these problems.

"It is not just the result of the past few years, but decades of how we have mercilessly utilized the Marmara. Local governments and ministries must work together and establish treatment facilities to protect the fish that will form the future stock at sea," Okyar added.