Jellyfish populations around the world have been increasing in recent years, and several very large jellyfish blooms have been reported since the early 2000s. The cause of these, and the general population increase, has remained somewhat unclear until now though. Is it simply observation bias? Cyclic population change? Warming waters? Changing currents?
But now, thanks to new research from the Institut de Recherche pour le Développement (IRD), the causes have become clear. The primary cause is overfishing, and with it the decline of many ecologically important species. Many significant predators of jellyfish, such as tuna and sea turtles, have seen their numbers plummet in recent years as a result of overfishing. And with their decline, jellyfish have begun to see their populations grow. But perhaps far more important than that decline, though, is the overfishing of small pelagic fish, such as sardines and herring, which are the main competitors of jellyfish.