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Jellyfish Population Decreasing at Jellyfish Lake Due to Drought

90 percent of the jellyfish have disappeared since November 2015

The numbers of golden jellies in Jellyfish Lake has been declining rapidly since November 2015, from an average of 8 million to the most recent March population estimate of only about 600,000 jellies. Coral Reef Research Foundation (CRRF) continues to monitor Jellyfish Lake at this critical period during El Niño.

The population of endemic golden jellies, Mastigias papua etpisoni, in Jellyfish Lake is rapidly declining in numbers, probably due to the drought and associated El Niño conditions. (CRRF photo)

Known as Mastigias papua etpisoni to the scientific world, this jellyfish disappeared from the lake in 1999 following the strong El Niño/La Niña in 1997/1998. Anecdotal evidence also suggests the same happened during the 1982/1983 El Niño/La Niña event.

Conditions in the lake have been changing in the past few months, probably related to the drought and El Niño conditions. Monthly measurements of physical conditions with depth show that the water temperature is cooler than average, as it currently is in the lagoon. The average salinity (saltiness) of the lake, however, is the highest recorded since CRRF started doing monthly monitoring in Dec 1998. This is almost certainly due to a lack of rainfall, which normally reduces surface salinity. At the same time, the microscopic food (zooplankton) the golden jellies eat (necessary for good health of the population) appears to be less abundant than normal in the lake and may play a role.

The exact cause of the decline in the population of golden jellies is not yet understood, but it is clear at present that young medusae are not surviving very long after release by the bottom dwelling polyp stage (the source of all medusae). Older large medusae are still present, but only live less than a year and have to be replaced to maintain the population. At this time there are very small and very large jellyfish in the lake, but the middle size range of them is now absent in the lake which may lead to an unsustainable population.

The golden jelly population could be on the verge of crashing, to the point where there are no more medusae swimming around the lake. This happened in 1999, but fortunately the benthic polyp stage survived and was able to renew the medusae population within about 18 months of the crash. This time around the situation is uncertain, as no one knows how this El Niño/La Niña scenario is going to play out. For more information about Jellyfish Lake please contact Coral Reef Research Foundation at 488-5255. (press release)

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