Around the world, vast aggregations of jellyfish and comb jellies seem to be more common. These aggregations are known as "jellyfish blooms" or "jellyfish outbreaks," which can cause a wide array of problems. Too many jellies in the water can be a danger to swimmers, forcing towns to close their beaches. Jellies have clogged up machinery at coastal power plants, causing power outages. They can interfere with fisheries by eating fish larvae, and fisherman catch jellies instead of the fish they want. Where they occur, blooms of jellyfish even change seawater chemistry.
Why are jellies becoming more common around the world? It seems likely that their spread is human-caused, although some scientists have argued that the blooms are part of a natural cycle. If the blooms are human-caused, there are several probable culprits.
OVERFISHING Over the past two decades, between 100 and 120 million tons of marine life have been removed from the ocean by fisheries each year on average. A lot of these marine species, including fish and invertebrates such as squid, eat some of the same food that jellies do: mainly, zooplankton. As these other predators of plankton are fished from the sea, jellies have less competition for food, and are able to grow and reproduce with fewer limits.
NUTRIENTS When fertilizers runoff from the rivers to the seas, they can create dead zones: areas of ocean where little life survives. The nitrogen and phosphorus in fertilizer helps phytoplankton grow very quickly, and there can be so many of these single-celled plant-like animals that they deplete oxygen from the water. Most animals can't survive in these conditions, but many jellies can better tolerate low-oxygen environments.
CLIMATE CHANGE The ocean is warming, and this might give some jellies a boost. The warmer water could help jelly embryos and larvae develop more quickly, allowing their populations to grow more quickly. And jellies that prefer warmer water will have more area to live in. However, this could also hurt some species as cold-water jelly species see their habitat shrink.
SUBMARINE SPRAWL Many industries, such as shipping, drilling and aquaculture, build docks, oil platforms and other structures in the water—sometimes referred to as “ocean sprawl"—which can serve as nurseries for jellyfish. To undergo their polyp stage, jellyfish need solid surfaces to settle upon. It’s much easier for jellyfish polyps to attach to man-made structures made of wood, brick and concrete than sand. Ocean sprawl provides more and better habitat for jellyfish to reproduce and complete their lifecycles.