When you think about tropical paradise, Hawaii is often at the top of the list. Waikiki is one of the most iconic Hawaiian beaches on Oahu and is a popular swimming and surfing spot. However, it is also a popular stop for the box jellyfish, one of the most venomous animals in the world. Once a month, about 8 to 12 days after the full moon, the shallow waters of Waikiki beach are temporarily flooded with box jellyfish. They are not coming in for a mai tai under the waning moon; rather, scientists believe that jellyfish reproduce in these waters. This monthly influx creates a hazard to swimmers due to the jellyfish’s painful—and even lethal—stings.
The environmental factors that affect these influxes are not well understood, and learning more about them may help us predict and mitigate the risk that box jellyfish pose to swimmers. Several scientists from Hawaiian institutions published the first long-term (14-year) assessment of the environmental conditions that potentially correlate with box jellyfish population changes in the North Pacific Sub-tropical Gyre.
The researchers surveyed a 400-m section of Waikiki beach during the days jellyfish were present. They counted more than 66,000 jellyfish over 14 years and compared the data to 3 measures of how the climate changes over time, called climate indices; 13 physical and biological variables, such as sea surface temperature and plankton; and seven weather measurements, including wind speed, air temperature, and rainfall.
They confirmed that box jellyfish arrive at Waikiki monthly after each full moon and stay for 2- 4 days. They counted on average 400 jellyfish each month, but the range was quite wide at 5-2,365 individuals. Rather than seeing a net population change over 14 years, researchers observed approximately 4-year periods of increased population count followed by 4-year periods of decreased population count, which coincided with fluctuations in three main environmental factors: oceanic changes in salinity and nutrient availability, called the North Pacific Gyre Oscillation, small organisms’ ability to access nutrients, called primary production, and abundance of small zooplankton.
The researchers suggest that the relationship between environmental fluctuations and jellyfish population changes at Waikiki may result from changes in the availability of food for jellyfish in the ocean around Hawaii, brought about by the North Pacific Gyre Oscillation. During an increase in nutrient availability, phytoplankton populations also increase, meaning more food for jellyfish, allowing them to grow faster and increase their rate of reproduction.
Previous studies have shown that jellyfish populations change due to human-caused disturbances, but this is one of the first long-term studies showing that large-scale climate patterns may also impact box jellyfish populations. Understanding long-term climate and oceanic trends and their effects on jellyfish populations may provide information to develop strategies for avoiding mass stinging events and beach closures at Waikiki and other popular recreation sites in the Pacific.
Citation: Chiaverano LM, Holland BS, Crow GL, Blair L, Yanagihara AA (2013) Long-Term Fluctuations in Circalunar Beach Aggregations of the Box Jellyfish Alatina moseri in Hawaii, with Links to Environmental Variability. PLoS ONE 8(10): e77039. doi:10.1371/journal.pone.0077039
Image Credit: Jellyfish by James Brennan Molokai Hawaii