Semi-enclosed Mediterranean: concentrations do not disperse. Rising water temperatures, overfishing, eutrophication. Pelagia noctiluca and Velella now year-round. Salps and ctenophores interfere with fishing and power plants.
Invasions of Portuguese man o' war off Galicia, jellyfish blooms in the Mar Menor, and sightings of salps or ctenophores are recurring topics among divers. The community links the proliferation of these species to ocean warming, overfishing of their natural competitors, and coastal eutrophication. The debate includes whether some blooms are natural or signs of deep ecological imbalance.
The Mediterranean is particularly vulnerable because, being a semi-enclosed sea, local concentrations do not disperse. The main factors are water warming, overfishing that eliminates key predators such as tuna, ocean sunfish, and turtles, and coastal eutrophication. Species such as Pelagia noctiluca have altered their seasonal cycle and are now detected virtually year-round in areas where they previously only appeared seasonally. Gelatinous organisms beyond jellyfish also interfere with trawl fishing and cooling water intakes of power plants.