Jellyfish population surge in Marmara Sea highlights pollution, overfishing issues
The surface of the water in the shores of Sariyer and Besiktas along the Bosphorus in Istanbul was covered with jellyfish, Türkiye. (AA Photo)
In Istanbul, coastal areas in districts such as Sariyer, Besiktas, Fatih, Zeytinburnu, and Uskudar have been covered by jellyfish. Fishermen using both rods and shoreline techniques have raised concerns about the rise in jellyfish and pollution.
Jellyfish populations in the Sea of Marmara and the Istanbul Strait have surged, primarily due to increasing pollution and excessive fishing, experts have warned.
Professor Firdes Saadet Karakulak, Head of the Department of Fisheries Technology at Istanbul University’s (IU) Faculty of Aquatic Sciences, noted that while jellyfish are usually seasonal in spring and autumn, they have increasingly become a year-round issue because of pollution and other environmental factors.
“The Sea of Marmara is a body of water where pollution levels have risen, mucilage events are becoming more frequent, and the nitrogen and phosphorus load continues to increase,” said Karakulak. “In such conditions, opportunistic species like jellyfish thrive. As oxygen levels drop, fish and other marine life leave, while jellyfish populations grow.”
Impact of jellyfish on fisheries
Karakulak also highlighted how the increase in jellyfish negatively impacts fishing, with longer stays in the sea, higher fuel consumption, and lower catch rates, all becoming major challenges. She explained that jellyfish feed on fish eggs and larvae, particularly targeting anchovy eggs, which may contribute to a decline in fish populations.
“We are currently conducting research to confirm this,” she said. “The decline in fish populations may be linked to the growth of jellyfish. It is not just one factor at play—oxygen depletion in Marmara has caused oxygen-loving fish to abandon the region. We no longer see tuna or swordfish in Marmara, species that would typically eat jellyfish. Their absence is allowing jellyfish populations to flourish.”
Karakulak added that these changes are causing disruptions in the food chain and that overfishing exacerbates the issue. “Opportunistic species like jellyfish take advantage of these imbalances,” she stated.
Overfishing and pollution accelerate ecological damage
Ahsen Yuksek, a faculty member at the IU Institute of Marine Sciences and Management, pointed to scientific studies indicating that an increase in jellyfish is a sign of severely disturbed ecosystems.
“Domestic pollution leads to a rise in plankton, and when the fish population is reduced due to overfishing, jellyfish, rather than fish, end up consuming the excess plankton, causing jellyfish numbers to spike,” Yuksek explained. “Additionally, rising sea temperatures allow new species to enter the ecosystem, putting pressure on native species or extending the breeding periods of existing ones.”
Environmental pressures in Marmara Sea
Yuksek noted Marmara is experiencing significant ecological pressures, with climate change showing its most damaging effects in already disrupted systems.
He pointed out that the region’s high population density and the abundance of plankton are key factors in the current ecological crisis.
“The excess food that should be used to support fish populations is instead being wiped out by overfishing, leaving behind only an abundance of plankton and jellyfish,” Yuksek said.
He also drew a connection between jellyfish blooms and the formation of mucilage, noting that in previous years, such blooms were observed just before mucilage outbreaks in 1994, 2007, and 2020-21.
Jellyfish blooms cause environmental havoc
Yuksek further explained the consequences of jellyfish overpopulation on marine life. “When fish populations decline, we often see jellyfish numbers spike. This usually coincides with the end of the fishing season, when plankton growth and jellyfish breeding periods align, leading to sudden increases in their numbers.”
“Excessive growth of one species can lead to mass deaths,” he continued. “In these cases, the organic matter broken down by bacteria leads to poorer water quality, lower oxygen levels, a rise in pathogenic bacteria, and habitat loss. Such explosions in jellyfish populations and subsequent mass die-offs are undesirable events for ecosystems.”
