The Earth's oceans have an abundance of jellyfish. However, it appears that they are becoming even more abundant. In the past 10 or so years, fishermen, scientists and even nuclear power plant workers have been noticing dense blooms of jellyfish that can number in the billions. Some species also seem to be venturing further from their natural waters. This is adding up to several dangers and there is no consensus regarding the potential consequences of these blooms.
Large jellyfish blooms can hinder or even destroy certain industries. In 2007, it was impossible to fish Irish salmon, as jellyfish were actively attacking the cages that were meant for the fish. Fishing has been a problem in other areas of the world thanks to this as well. Because the blooms are capable of clogging the inlets leading into nuclear power plants, which are necessary to their proper functioning, they have closed several power plants in various countries.
Researchers, laymen and scientists have posited several possible explanations for the increase in jellyfish populations. As usual, global warming is one possible explanation. Overfishing creatures like turtles that eat the same food as jellyfish could also be the problem as jellyfish move to fill in the gap in the food chain. Jellyfish are more resilient to unbalanced water than some food fish, which is another possible explanation or one of a few reasons.
While it seems that this could lead to a problem and possibly a jellyfish filled ocean such as has not been seen in millions of years, some experts think it is no problem at all. The populations of jellyfish, like many other things, ebbs and flows. The population may correct itself, as competing species are replenished, normal temperatures return, predators move in to take advantage of the abundance, etc. It is either that or we'll be swimming in tentacles.