Ebb and Flow

The apparent jellyfish explosion in recent years may simply be part of a repeating boom-and-bust cycle that has been going on since 1940, scientists say in Proceedings of the National Academy of Sciences.

Over the last decade, researchers have reported unusually high numbers of jellyfish blooms in places such as the Bering Sea and Black Sea. People have taken these reports as a sign that jellyfish are on the rise around the world, and the spike “has been portrayed as a symptom of degraded oceans,” the study authors note. But no one had assessed global jellyfish numbers over a long period of time.

The team assembled 37 datasets of jellyfish abundance taken from 1790 to 2011, mostly in the northern hemisphere. The observations suggested that jellyfish numbers have risen and fallen on a roughly 20-year cycle for the last seven decades. For example, populations spiked in 1957, 1985, and 2004 and bottomed out in 1951, 1971, and 1993. Jellyfish blooms were more common from 1971 to 1985 and from 1993 to 2004.

The latter increase in blooms, which supported the idea that jellyfish numbers were rising worldwide, “may therefore be best interpreted as part of an oscillation,” the authors write. The team also found a slight increase in jellyfish after 1970, but researchers will need to track the next “bust” to see whether populations drop back to their previous levels. Roberta Kwok | 1 January 2013

Source: Condon, R.H. et al. 2012. Recurrent jellyfish blooms are a consequence of global oscillations. Proceedings of the National Academy of Sciences. doi: 10.1073/pnas.1210920110.

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