The authors of a study partially funded by NCCOS theorize that the recent global increase in jellyfish blooms is due to the proliferation of man-made structures such as sunken ship hulls, underwater aquaculture pens, and artificially hardened shorelines. Through observations and experimental evidence, the researchers learned that jellyfish larvae settle in larger numbers on these structures than they would normally on natural substrate alone, which leads to favorable conditions for dense local concentrations.
For those needing some jellyfish 101, the blooming "jelly" species in this study have a two-phase life history: a juvenile stage that requires something hard to attach itself to on the seafloor, and a free-swimming adult. Once the young, called polyps, detach from the substrate, they're known as medusae. Swarms of thousands or millions of jellies can foul power plant or ship intake pipes, fishing nets, aquaculture enclosures, and beaches. Some species are armed with painful stingers that can kill fish and injure people.