The city of Venice and its surrounding lagoon are irreversibly affected by climate change.
Rising sea levels threaten to submerge the island in the coming decades as the seawall system that currently protects it from storm surges becomes obsolete.
But this is only half the picture. Venice lives in symbiosis with the surrounding brackish waters, and its health is becoming increasingly fragile.
A new study highlights how warming oceans are introducing invasive species that threaten lagoon ecosystems and the livelihoods of local fishermen.
Cannibalistic jelly invades Venice lagoon
The latest invader in Venice’s lagoons is the cannibalistic comb jelly, considered one of the world’s 100 most harmful invasive species.
Warty comb jellies are gelatinous invertebrate ctenophores that are known to eat their own offspring.
This tree, also known as sea walnut, has been living in the Adriatic Sea for almost a decade.
A new study by researchers from the University of Padua and the National Oceanographic Geophysics Institute (OGS) has found that climate change has recently created particularly favorable conditions for the growth of the warty comb jelly in the waters around Venice.
“This could increase their presence in large populations, thereby increasing the risk of seriously impacting the functioning of the entire lagoon ecosystem,” says OGS researcher Valentina Tirelli.
This study identified a seasonal pattern characterized by peaks in reproductive flowering during late spring and late summer to early fall. These blooms can be affected by increasing temperatures and optimal salinity levels.
This species abundance suggests that the species can survive under a wide range of temperatures and salinities, but very high temperatures or low salinities can have a major impact on its survival, the scientists say.
Invasive species threaten fishing villages
The warty comb jellyfish poses a serious threat to Venice’s thriving lagoon ecosystem.
“This species is a voracious prey on zooplankton, which explains its high reproductive rate,” explains Tirelli. Zooplankton is an essential food for many fish.
“These ctenophores have also been shown to prey on the eggs and larval stages of ecologically and economically important species, such as fish and bivalves, which can further compromise recruitment and ecosystem stability,” she says.
This poses a significant challenge for fishermen who are seeing their catches plummet and their nets clogged by the slimy critters.
“Our results show that overall catches of key target species have declined by more than 40 percent since the arrival of the invaders,” Tirelli says. “The most affected species include cuttlefish and gobies, both of which are culturally and economically important products for the Venetian lagoon.
In the 1990s, Black Sea fishermen blamed the spread of ctenophores for the collapse of fish stocks and its devastating economic impact.
Blue crabs drastically reduce fish catches in the Adriatic Sea
Fishing communities in the northern Adriatic Sea are already battling another deadly predator.
The population of giant blue crabs has increased explosively in recent years. This crustacean is not endemic anywhere along the Italian coastline. It is thought that it probably came from the coasts of North and South America on cargo ships in ballast water in the late 1940s.
Although the existence of this crab is not new, the population of this rapidly reproducing crab has soared to a critical point, especially since it has no natural predators in Italian waters.
The suspected cause is climate change. “With rising sea temperatures, crabs are becoming more active and voracious,” one fisherman told Euronews Green. As water temperatures drop, crabs eat less and reproduce less, but recently the opposite has been happening.
Enrica Franchi, a marine biologist at the University of Siena, told The Associated Press: “Normally this crab does not survive well during certain times of the year when the water temperature is below 10 degrees Celsius, but now it has ideal temperatures for 12 months of the year.”
Blue crabs love to feed on local seafood and have powerful claws that can tear through fishing nets, making them seemingly unstoppable. Clams, mussels, oysters and, in Venice, shellless crabs known as moeches are all at risk.
Officials and fishing groups are scrambling to find ways to use and dispose of the shellfish, including shipping container loads to the United States, where they are considered a delicacy.
But Italy’s agricultural lobby group Coldiretti proposed adopting the American diet and adding blue crab to the menu.
Blue crabs are already available at fish markets and supermarkets for around 8 to 10 euros per kilogram.
However, the “if you can’t win, eat” plan comes with great risks. Investing resources in harvesting blue crabs as a food source undermines the Adriatic’s fishing and culinary traditions.
Skills in breeding, fishing and processing native species such as clams, as well as recipes and dishes that are part of the region’s gastronomic heritage, could be lost.