The Truth Behind Global Jellyfish Swarms

West Coast sea nettle.

Large swarms of jellyfish and other gelatinous animals--sometimes covering hundreds of square miles of ocean--have recently been reported in many of the world's prime vacation and fishing destinations.

In this interview with Lily Whiteman of the National Science Foundation, renowned "jellyologist" Monty Graham of the Dauphin Island Sea Lab in Alabama discusses the origins and impacts of such swarms. (Note: Jellyfish and other gelatinous animals are called "jellies" here.)

1. What types of damage have jelly swarms caused?
"Damage" can be seen as either economic or environmental. Recent examples of jelly swarms that have created such problems include:

• Tourism in Australia where deadly box jellies live and in the Mediterranean where stinging jellyfish closed down many beaches during the summer of 2008.
• Many important fisheries, including the Gulf of Mexico, where jellies regularly clog nets, fowl fishing gear or prey on eggs and larvae of fish.
• Aquaculture operations are often affected. One particular example was in 2007 when an extraordinarily large swarm of jellyfish virtually wiped out a salmon farm off northern Ireland.
• Seafloor diamond operations off the coast of Namibia, where jellies have clogged vacuum pipes.
• Nuclear power plants in many locations. Recently, in November 2008, a jelly swarm clogged intake pipes at the Diablo Canyon power plant in California, and thereby caused a temporary plant shut-down.

2. How densely are jellies packed together in swarms?
In most cases, a dense swarm would be a few animals in one cubic meter of water. However, in extreme cases, there can be more jellies than water. Such swarms may cover a hundred miles of more of coastline at a time.

3. Are jelly swarms natural phenomena?
Yes. Jellies have likely swum and swarmed in our seas for over 600 million years. When conditions are right, jelly swarms can form quickly. They appear to do this for sexual reproduction. Since males need to release gametes (sperm) into the water, they need to be very close to females. Therefore swarming behavior is just a way for them to be close to each other.

4. Are jelly populations increasing?
In some locations, jelly populations are increasing. But such increases are regional in nature; we do not have evidence that there are "global" increases due to global influences, but evidence is mounting that climate change may have an effect.

5. Are human-caused environmental problems promoting population explosions of jellies?
Various types of environmental problems may promote the formation of jelly swarms. These problems include pollution, the overharvesting of fish, the introduction of non-native jelly species into new habitats, the addition of artificial substrate (like fishing reefs, and various offshore platforms) in the ocean and climate change.

6. How may climate change promote population explosions of jellies?
Higher water temperatures may speed jelly reproduction as well as extend the growing season for jellies; a longer reproduction season could result in more jellies. Climate change may also increase the amount of food available for jellies.

Also, climate change may change ocean currents. Changed currents may transport jellies--which drift with currents--into new habitats.

Because climate change appears to be a world-wide phenomenon, climate change may have worldwide impacts on jellies. But we really don’t know yet what the details regarding interactions between climate change and jelly populations.

7. In light of the abundance of jellies, why don’t we know more about them?
Marine ecologists have traditionally regarded jellies as mere nuisances that interfered with studies of truly important creatures. Because of such attitudes, ecologists have traditionally gone to great lengths to avoid jellies, rather than to study them. In fact, in the past, when scientists accidentally caught jellies in their research nets, they often did horrific things to jellies - like pouring bleach over them in order to dissolve them away without destroying the hard critters that they really wanted to study.

Because jellies have traditionally been understudied, we don’t have many accurate long-term records of their population sizes. Therefore, in many places, we can’t yet say for sure whether or how much jelly population sizes may be changing.