While jellyfish populations in the Gulf of Mexico have been increasing for over a decade, unusually large numbers of adult jellyfish were present in the northern Gulf of Mexico during the late summer of 2000. The native jellies, such as the moon jelly, Aurelia aurita, and the sea nettle, Chrysaora quinquecirrha, experienced population explosions this year. Of particular concern was the appearance of invasive jellyfish species. The Australian spotted jellyfish, Phyllorhiza punctata, was seen in tremendous concentrations as was another invasive jelly, Drymonema dalmatina. Both of these invasive species appear to have come into the Gulf from Caribbean waters.
These unusually high jellyfish densities raise a number of concerns. Fishermen have experienced damage to equipment due to large numbers of jellies in their nets. The jellyfish are very efficient filter feeders and their primary food source is plankton. The native moon jelly concentrations were observed in the same areas that king mackerel and red snapper spawn and the Australian spotted jelly was concentrated in the passes between the barrier islands that separate Mississippi Sound from the Gulf. The jellyfish concentrations were in prime locations to feed on the planktonic larvae and eggs of shrimp, crabs and many important fish species that spawn offshore as they drifted on the currents to inshore nursery areas of the Sound. The coincidence of high jellyfish concentrations in such locations with the breeding seasons of many commercially important fisheries species could have an impact on next year's recruits. It has been speculated that should this situation occur in successive years, commercially important fisheries could be affected in the long-term as well.
The life cycle of jellyfish includes several stages. The most obvious one is the adult medusa stage and it is this stage which was visible in the Gulf during the summer of 2000. The medusae produce larvae that attach to hard substrate and become hydra-like polyps during the winter. In the spring, the polyps release tiny medusa-like ephyrae which grow up into adult medusae.
Nutrient runoff, hypoxia, harvest of competing fish, and offshore platforms are several factors which may have contributed to the large numbers of jellyfish. The proliferation of oil and gas platforms and artificial reefs in the Gulf of Mexico has increased the hard substrate available for all types of attached organisms, including jellyfish. This study will investigate the potential role that offshore platforms play during the polyp stage of the jellyfish life cycle, with emphasis on the Australian spotted jellyfish, Phyllorhiza punctata.
The objectives of this study are:
- to determine the areal extent of the sessile polyp stage of the jellyfish;
- to determine the proportions of Australian spotted jellyfish recruits with respect to other jellyfish species and other attached organisms on offshore platforms, other hard substrates and the bottom of the Gulf.