On the Rise
While several marine organisms are on the decline, jellyfish populations are rapidly growing. Jellyfish are found in every ocean in the world, and they are an indicator species. According to Lisa-Ann Gershwin, the author of Stung! On Jellyfish Blooms and the Future of the Ocean, an abundance of jellyfish is a sign that the environment is out of balance. When jellies flourish in an area, something is seriously wrong and trouble usually follows their arrival. Recent blooms of jellyfish have been recorded in the Mediterranean, the Gulf of Mexico, the Black and Caspian Seas, the Northeast US coast, and in Far East Coastal waters. Some in the scientific community remain skeptical of these population changes, but there are areas that are clearly experiencing an increase in jellyfish. Some areas have also experienced a decrease or fluctuation in population size over decadal periods.
|Swarm of Jellyfish|
Several factors contribute to the increase of jellyfish populations including overfishing, pollution, climate change, and ocean acidification. All of these factors create the perfect environment the jellies need to thrive. Jellies are able to flourish in low oxygen environments that would suffocate other marine organisms. They also have several methods of reproduction, which increases their population size quickly: hermaphroditism, cloning, external fertilization, and self-fertilization. Their polyps settle in layers on hard surfaces and then detach when conditions are right. The polyp stage can live indefinitely by cloning. A polyp colony started in 1935 in a lab in West Virginia is still alive and growing today. Also, jellies have long lifespans. They can actually “de-grow.” They are able to reduce their size while their bodies remain proportionate. When food is abundant they are able to begin growing again.
One factor that is contributing to the boom of jellies is overfishing. For example, at one time, anchovies were abundant in the Black Sea and off the coast of Africa. Overfishing of anchovies, which compete with jellies for food, has led to an abundance of food for the jellies which then take over.
Another factor in the jellyfish population boom is pollution. Trash and garbage polluting the ocean kills many jellyfish predators, such as sea turtles. Man-made objects within the ocean, such as piers and boat hulls, provide perfect nursery sites for jelly polyps. Another form of pollution affecting jelly populations is nutrient run-off, which leads to eutrophication zones. These zones are areas that are oxygen depleted and provide a great area for jellies to thrive.Climate change, the warming of the oceans, is extending jellyfish ranges. Warm water also contains less oxygen, which the jellies like. Also, another impact to consider if ocean acidification. Because jellies don’t have hard parts, they aren’t affected as much by acidification.
Jellyfish consume a huge amount of food and will pretty much eat anything. They have very efficient metabolisms and can put the energy they ingest toward growth. One species (a comb jelly), Mnemiopsis, will gorge and continue to kill and collect prey even though it's no longer hungry. A study showed that this species killed 30% of the copepod population that was available to it each day. They continue to kill until nothing is left.
Jellies can have a large impact on important organisms within an ecosystem and can reduce biodiversity. Mnemiopsis invaded the Black Sea via seawater ballasts and took over by the 1980’s. Anchovies and sturgeon began to disappear in this area due to competition with the jellies for food. The Mnemiopsis jellies made up 95% of the Black Sea’s biomass.
A possible solution to the abundance of jellyfish: eat them. Jellyfish have been part of the human diet for a long time in China. In recent years the global jellyfish harvest has risen to 321,000 tons, and the harvested jellies are mainly consumed in China and Japan.
So much damage has occurred within the oceans that we aren’t sure what the future holds for them. As said by author Lisa-Ann Gershwin, we must “adapt.”
|Giant Nomura's Jellyfish|