Jellyfish swarms, tourists, and the Christ-child

One of the most remarkable sights in the Western Pacific is a perennial swarm of 1.5 million golden medusae (Mastigias sp.) crowded into a land-locked marine lake in Palau, Micronesia. This 'Jellyfish Lake' became a popular off-gassing stopover for SCUBA divers and a destination in its own right for non-diving tourists in the mid-1980s. Since then, tourism in Palau has boomed, increasing 500% between 1986 and 1997. However, in December 1998, the golden-medusae disappeared. Apart from patchy occurrences between December 1998 and April 1999, the medusae have since been absent from the lake. Field measurements, including temperature and salinity depth profiles, Mastigias medusae population sizes, and the distribution of scyphistomae, in 'Jellyfish Lake' between 1979 and 1999 were integrated with laboratory-based experiments on the effects of salinity, temperature, sunscreen and zooxanthellae enrichment on Mastigias scyphistomae or medusae. These studies indicated that the disappearance of medusae was due to physical changes in lake structure, including a substantial increase in temperature, initiated by the 1997-98 El Nino. Here, we describe these studies, the changes in Jellyfish Lake and their probable influence on the Mastigias. We further elucidate the changes in Jellyfish Lake by reference to coincident changes in three other 'jellyfish lakes' in Palau: Big Jellyfish Lake, Clear Lake and Goby Lake.