Japanese fishermen working the Sea of Japan are preparing for what has been called "a huge jellyfish typhoon." Swarms of the giant Nomura sea jelly, which can reach six feet in width, have been sighted. They first appeared in large numbers in 2005, poisoning the fish being caught as they were hauled up together in nets. The problem was so severe that salmon fishermen would not go out to sea and lost up to 80% of their income.
While sea jellies can congregate in large numbers normally, the increasing size and frequency of these aggregations is of major concern to fishermen, public officials (public safety), and researchers.
Researchers at the University of Queensland, Australia, reporting in Trends in Ecology and Environment, have theorized that these aggregations can well be due to human activities. Fertilizer runoff can add excessive nutrients which negatively impact many fish species but can allow sea jellies to flourish because of the associated increase in phytoplankton (a primary food source). Then the die off of the phytoplankton blooms produce decomposition that increases bacteria, causing a reduction in oxygen - again impacting fish but sea jellies are better able to survive in low oxygen environments.
Overfishing also can lead to increased sea jelly aggregations. There have been documented cases of sea jelly blooms when fish such as sardines and anchovies, which feed on the same food sources as sea jellies, are overfished, thereby upsetting the balance of food source competition that normally controls species populations. This has occurred in as disparate areas as off the coast of Nambia in Africa to the Black and Caspian Seas.
And finally, sea jelly larvae can be transported in the ballast water of large ships and, as an invasive species, can take hold in new marine ecosystems where plankton-eating fish are being commercially caught in large numbers, providing the sea jelly with a foothold to grow in numbers.
Increasing sea jelly populations can reach a "tipping point" where they overwhelm the ability of predators to control their numbers, while at the same time feeding on the fish eggs and larvae of those very same predators. When that happens, human management of the situation is made extremely difficult. Researchers are recommending that prevention - controlling runoff, avoiding overfishing, better methods for controlling invasive species - would be a better long-term strategy than eradication.