jelly invasion

A "monoculture of jellyfish" threatens the oceans as we know them. I believe that this development will make the jellyfish one of the most prominent reminders of the repercussions of human activities.

Dr. Anthony Richardson calls this the "jellyfish joyride" and it is already happening in parts of the ocean: diverse fish populations are being replaced by various jellyfish species. "In recent years, jellyfish blooms have been recorded in the Mediterranean, the Gulf of Mexico, the Black and Caspian Seas, the Northeast US coast, and particularly in Far East coastal waters.”

In a new study in Trends in Ecology and Evolution, Richardson and colleagues explore the causes behind the jellyfish infestation and the need for swift, decisive action to stem the jellyfish take-over: Jellyfish explosions are linked directly to human actions, including over-fishing, the input of fertilizer and sewage into the ocean, and climate change.

Overfishing has removed fish from marine ecosystems at astounding rates. According to Richardson this has opened the door for jellyfish to take their place: “this is because small fish (e.g. anchovy, sardine, herring) appear to keep jellyfish in check by predation (on jellyfish when they are very small) and competition (for the same zooplankton food). So, once we remove fish, jellyfish can proliferate.” As an example Richardson points to Namibia where "intense fishing has decimated sardine stocks and jellyfish have replaced them as the dominant species.” Read more on the angers of overfishing here.

Eutrophication is another human-caused change in the ocean that has likely contributed to jellyfish explosions. Eutrophication is an increase of nitrogen and phosphorous in the ocean, largely caused by fertilizer and waste runoff seeping into the oceans (pollution). This leads to algae blooms, which lower oxygen in the marine ecosystem creating so-called ‘dead zones’, which have been increasing dramatically around the world. According to Richardson, these low-oxygen waters give jellyfish the advantage: “fish avoid low oxygen water but jellyfish, having lower oxygen demands, not only survive but can thrive in these conditions as there is less predation and competition from fish.”

Furthermore, Richardson and his colleague speculate that climate change may expand the traditional ranges of jellyfish at the expense of other marine species. “As water warms, tropical species are moving towards the Poles. This has been documented on land and in the sea. Many venomous jellyfish species are tropical (e.g. box jellyfish and irukandji) and…could move south into more densely populated subtropical and temperate regions,” Richardson says. As an example the paper points to box jellyfish and the incredibly small irukandi in Australia. These fatal species often cause beach closures in their native northeast Australia, and there is a concern that as the water warms they will make their way to more populous southern Australia.

CITATION: Anthony J. Richardson, Andrew Bakun, Graeme C. Hays, and Mark J. Gibbons. The jellyfish joyride: causes, consequences and management responses to a more gelatinous future. Trends in Ecology and Evolution. June 2009, Volume 24, Number 6.

Read more: source 1 & picture source

Here is a related previous post concerning the invasion by the immortal jellyfish.

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