This paper was written as part of the 2011 Alaska Oceans Sciences Bowl high school competition. The conclusions in this report are solely those of the student authors.

Jellyfish Blooms and Their Effects in the Sea of Japan


Nathan Feemster
Maggie Herbert
Eileen Audette
Alex Ashford

Team Smack WHAT?!

Seward High School
2001 Swetmann Ave
Seward, AK 99664


Jellyfish blooms can cause extreme problems to both marine ecosystems and human enterprises. In the Sea of Japan, smacks of the jellyfish Nemopilema nomurai have presented challenges because their population has been growing at a rapid rate. These circumstances are affecting many aspects, both within the marine ecosystem and in its dependent economy. Although the Sea of Japan has faced extreme blooms before, they have usually only occurred every 40 years. Since 2002, a bloom has been present almost every year. This paper explores the complexity of the problem and suggests various management ideas.


The Sea of Japan is located to the northwest of Japan, between it and the Asian continent. The lands around the sea create a relatively closed body of water (figure 1). Factors such as pollution and over fishing have a much greater effect in a closed basin than in the open ocean. In recent years, a common jellyfish, Nemopilema nomurai, has experienced population explosions (blooms) that have caused hardships in the fishing industry, affected other ocean reliant businesses, and changed the local ecosystem.

Massive jellyfish blooms are a big problem in the Sea of Japan. Although the N. nomurai jellyfish population has probably been present in the Sea of Japan for many thousands of years, they have rarely been associated with problems. During the twentieth century, there were only a few documented jellyfish blooms. This is drastically different than the current rate of almost one bloom per year since 2002.

This paper will explore the biology and ecology of the N. nomurai. The movement of larval medusae from their breeding grounds in the Yellow Sea to the Sea of Japan will be described. The typical reasons for jellyfish blooms will be examined. The direct effects on commercial fishing and other parts of the ecosystem will be discussed. In order to address this problem, we feel that a management plan is necessary. Without action, this problem is likely to persist and have negative implications for both the environment and Japan's economy.


Jellyfish, or the Cnidaria Scyphozoa Aurelia, are gelatinous opaque looking creatures that can be found almost anywhere on the planet. Living anywhere from tropical to sub-polar waters, these marine organisms feed on fish, fish eggs, marine invertebrates, zooplankton, and, in some cases, even other jellyfish. They are not a threatened species due to the fact they do not have many predators (O'Toole). Some of the jellyfish's few predators include spadefish, sunfish, sea turtles, and humans (Whitaker, King, and Knott). Jellies begin their life as planula larva, later developing into polyps, strobila, and ephyra. Finally, jellies enter their medua stage, their mature stage (figure 2) (Dery). A majority of giant jellyfish, N. nomurai, range in size anywhere from 2 to 40 centimeters, but some species can be as large as two meters in diameter. Jellies are only free-swimming during their medua stage. During this stage of their life, jellies contract and relax the muscles around the edge of their bell to move through the water. The tentacles of jellyfish contain stinging cells used for defense as well as to capture food. Jellyfish lack a central nervous system; instead, they have a simple nervous system comprised of a nerve net that is made up of receptors that detect light and smells.

Usually, the term jellyfish bloom refers to a large number of jellyfish in a small area. However, it can also be an unexpected boom in the population of any particular jellyfish smack (O'Toole).

N. nomurai jellyfish breed in the waters of the Yellow Sea, and grow to larval medusae. Heavy rains in June and July create a low salinity water mass that pushes the young medusae out to sea where they can be picked up and transported by the Tsushima current. In July and August, the medusae first appear in the Strait of Tsushima and then spread to the northern Sea of Japan.

Study Area

The Nomura's jellyfish is common in the Bohai, Yellow, East China, and Japan seas. Many of the jellyfish enter the ocean system from the Yangtze River Delta during an early stage of development. Japan's western coastline receives a large number of the jellyfish by way of the Tsushima Current, a warm current that flows north along China's coast then between Korea and Japan. Because of this current and the increase in N. nomurai jellyfish populations on China's and Korea's coasts, Japan's waters are gaining more and more jellyfish. The N. nomurai jellyfish have disrupted the fishing-based economy of western Japan. Kokonogi, a port town on the west coast of Japan, has taken an especially hard hit. Kokonogi is located on the Echizen Coast, after which the Echizen family of jellyfish (which includes the N. nomurai jellyfish) was named.

Past Blooms

Ever since the turn of the century, blooms of Nemopilema nomurai have been increasing at a phenomenal rate. The blooms of these massive jellyfish usually only occur every forty years (1920, 1958, and 1995), but in recently the N. nomurai blooms have occurred almost every year since 2002. During the 1990's the jellyfish reproduced so quickly that by 2000 their population increased by about 40 percent.

In 2005, the world saw perhaps the largest bloom of jellyfish in history. The ship Toyoshio Maru found extensive amounts of madusae gathered around Tsushima Island. The ship encountered as many as 23 N. nomurai per 15 square yards. That year it was estimated that 580,000,000 jellyfish passed through the Strait of Tsushima per day during July and August. However, the rate declined rapidly from late August to October. The blooms occurring closer together may be due to several causes that will be addressed later in this paper.

Why Jellyfish Bloom

A jellyfish bloom is a conglomeration of jellyfish that can number among the hundred thousands. There can be as many as 10 jellyfish within one cubic meter. Many different phenomena are capable of triggering a bloom. Warmer waters, over fishing, polluted waters, saltier waters, dead zones, and ocean currents are all known causes of blooms. In recent years, several of these factors have combined to make blooms so huge as to be detrimental to the environment and the human dependence upon it.

Global climate change has caused the ocean to warm rather erratically over the years. This increase in temperature can prolong the polyp stage of jelly life. This accounts for possible "time lags between ongoing increases in water temperatures and resulting appearances of adult jellyfish swarms" (National Science Foundation, 2008). Although this warming has proved to have negative effects for many species, jellyfish enjoy the warmer climate. This means that, while their predators have struggled, jellyfish have, as a rule, thrived in this new environment. In addition to the problems they suffer because of climate change, many animals who regulate jellyfish numbers are victims of over fishing. With fewer natural controllers, jellyfish populations increase. In addition, as the predators are eliminated, there is less competition for plankton, which also fuels the growth of the jelly populace.

Polluted water, that which is not fit for its intended purpose, can go hand in hand with the need for plankton. Polluted waters enhance the growth of the plankton upon which jellies feed. The fertilizers, animal waste, and sewage that enter the water overload the local ecosystem with nutrients, causing eutrification. This overload cultivates algae blooms off which zooplankton feed, the jellyfish can in turn gain nourishment from the plankton. These pollutants also create dead-zones, or areas with extremely low oxygen contents. Because most organisms cannot survive in these areas plagued by eutrophication, jellyfish are subject to even fewer competitors and combatants. The ability to dissolve oxygen into their watery tissues allows jellyfish to carry oxygen supplies into the eutrophication zones, contributing to their amazing adaptability (National Science Foundation, 2009).

In recent years, droughts and dams have restricted the amount of freshwater that has been entering the oceans. Jellyfish prefer to live in waters with higher saline contents. The halocline is the stratification of water due to saline content. Saltier waters are typically denser and fall below layers with a lower saline content. Water near tributaries is also less saline. However, when the freshwater deposits have not been enough to dilute the waters near the coast, jellyfish are able to approach the shore to such a degree that humans are put at risk. In addition to allowing a closer proximity to shore, these droughts and dams allow the increase of iodine in the water (it is found in saltier waters). Many jellyfish need this to transition from polyp form to medusas (Berking et al., 2005).

Of course, ocean currents are the real facilitator of jellyfish blooms. They can push together many small groups of jellies, or separate a large conglomeration. The Sea of Japan is where several of these triggers combine. The ocean currents present in the Sea bring warmer and more saline water from the southern areas to the north. The water in the northern Pacific Ocean has been warming for the last 25 years (graph (d), figure 3). The pollution runoff from China has drifted into closer proximity to Japan. This, at the same time, allows the growth of plankton for jellies to eat and drives out predators as it causes eutrophication. As the predators are driven out or killed, they are also becoming the victims of over fishing. The Japanese fishing industry has been depleting the population of Blue Fin Tuna steadily, one main predator of jellyfish. Lucas Brotz sums up the situation, "These increases in jellyfish should be a warning sign that our oceans are stressed and unhealthy" (Casey, 2009).

Impacts of Blooms

Jellyfish are voracious predators that are well equipped to adapt to any environment. These traits enable jellyfish to have tremendous impacts on the ecosystem that they inhabit. The main issue surrounding jellyfish is their food source. Ichthyoplankton, zooplankton, and fish eggs are the primary source of a jellyfish's diet along with small baitfish (see jellyfish biology for more information). This is a problem because, by consuming the main source of food for other fish, the jellies become a rival competitor. The next problem is that jellyfish eat the eggs and juveniles of their rival species, thus making it more difficult for the species to recover. This is called the "jellyfish spiral" and during a bloom this effect is exacerbated (Uye, 2008).

Japan's west coast is spotted with small villages that have maintained a fishing economy with plenty of success until recent years. N. nomurai jellyfish blooms began in this area in 2005 and have repeated most years, causing many villages to become economically unstable due to losses in fishing. Millions of Japanese yen is lost production show the extent of the damage from the blooms. Since zooplankton and N. nomurai jellyfish populations are somewhat inversely proportional to each other, evidence of the blooms themselves can be found in the decreased concentrations of zooplankton that has been present in the East Asian marginal seas since 2005 (Uye, 2008). Many communities in Japan have filed complaints (over 100,000) regarding the interference of the Nomura's Jellyfish in commercial fishing. With the bloom of N. nomurai in 2005, Japanese fisherman experienced the effect of a full-fledged bloom. While fishing they would have to constantly scoop out large N. nomurai to keep their nets from bursting. Shin- Ichi- Uye, a graduate student from Hiroshima University, stated four reasons why the N. nomurai bloom impacted the fisherman:

  1. The bloom was abnormally abundant
  2. The medusa were smaller and more fragile
  3. The first sighted N. nomurai was a whole month early
  4. Currents drove the blooms toward commercial fishing areas (Uye, 2008)

Many power plants, desalinization plants, and ocean freighters have been distressed because of jellyfish blooms. An article in Power and Energy magazine describes reactors at the Chubu Electric Power Company's plant in Hamaoka having to reduce their output after jellyfish blocked the seawater cooling system in 2006 (Power and Energy, 2006). Similar instances have been reported elsewhere.

Massive jellyfish blooms not only affect the ecosystem, but they affect many people's lives that depend on that system to survive. In 2009, Diasan Shinsho-maru, a ten-ton Japanese trawler was capsized after the three man crew tried to haul up a net loaded with N. nomurai (Ryall, 2009). Billions of yen has been lost in Japans already declining fishing industry due to blooms.

Management Plan

To manage the jellyfish blooms in the Sea of Japan, an adaptive management plan is ideal. This consists of designing several solutions after assessing the problem and choosing the most appropriate plan. After implementation and a monitoring period, the solution is evaluated and adjusted as deemed necessary. This is more flexible than choosing an idea and using it no matter the consequences. Each proposal in this paper is designed to be applied with this management style in mind.

Over-fishing in the Sea of Japan has greatly contributed to the jellyfish blooms. Because adult fish are the main predators of jellyfish, their rapid disappearance has allowed the jellyfish population to skyrocket. Increasing and enforcing fishing regulations in the area would allow the fish populations to build themselves back up. With the predator population on the rise, it would be much easier to control the jellyfish blooms. The increased fishing regulations would help to restore the natural balance between species. Although jellyfish feed on juvenile fish, adult fish feed on jellyfish polyps. Without outside interference, this process balances itself out almost perfectly. To re-establish balance to the jellyfish and fish scale, and help the declining Japanese fish market, it would be viable to establish sustainable fish hatcheries along the Changjian River in China. This is the breeding ground for N. nomurai jellyfish polyps. The release of fish in this area would enable the fish to feed on the polyps before they reach the adult stage. Then, the well-fed fish would likely migrate with the Tsushima current, bringing them into the Sea of Japan, and in range of commercial fisherman. In the American pacific northwest, "Salmon are extinct in almost 40 percent of the rivers where they were known to exist" (Extinction, 2008). "Excessive commercial fishing impacted [fish] runs...Poor hatchery practices also contributed to the decline of [fish] runs" (Extinction, 2008). This is exactly what needs to be avoided in the Sea of Japan.

Another cause of jellyfish blooms is the warming of the waters in which they live. This is due to global climate change. Any action that can be taken to fight and prevent further global warming is also an action taken to manage the jellyfish blooms. Countries of the world need to work together to solve this global problem.

Further control of massive jellyfish blooms may be achieved by taking some key preventive measures. One main factor that contributes to jellyfish blooms is pollution. The chemicals and runoff from pesticides and cleaning products, that are dumped into the ocean enables large algae blooms in a process called eutrification. The alga, in turn, attracts zooplankton, which is a main food source for jellyfish. Jellies thrive in acidic, polluted, and/or saline waters (see Why Jellyfish Bloom section). By reducing pollution and illegalizing the dumping of chemicals and other hazardous materials into the ocean, we can reduce the likelihood of jelly blooms. In order to be effective, the action taken against pollution must be universal. Stricter laws regarding pollution in the Changjian River should be set to help control the bloom sizes. Regulations should also be put into place concerning dam output to ensure that the saline content estuaries are kept balanced. With these plans implemented, control of jellyfish blooms would be well on its way. However, additional in-depth research is mandatory for a flawless and permanent solution.


The recent jellyfish blooms in the Sea of Japan are unlike any that have previously plagued the area, both in numbers and duration. Because of these differences, these recent blooms are much more alarming. For instance, these blooms have caused a shockingly large amount of money to be lost through various enterprises because of their interference. They have affected the commercial fishing industry as well as other businesses that rely on the ocean in one form or another. Jellyfish have also started to dominate their environment. They will eat larvae, small fish, and algae, both shrinking a population and preventing others from having an adequate source of food. The jellyfish blooms in the Sea of Japan are a problem that needs to be addressed with long-term goals in mind.

Over fishing, pollution, overall ocean health, and predator populations are all factors in the blooms. There are many ways to attack the problem. Stricter regulations on fishing and pollution, greater thought before executing ocean related projects, aiding predator populations, and directly controlling the jellyfish population would all, if implemented with an adaptive management style plan, help to manage the jellyfish blooms.

These blooms are a serious problem. If left alone, the western Japanese fishing industry could collapse, businesses reliant upon the ocean could fail, along with countless other implications. This paper explores the difficulties in the Sea of Japan with Nemopilema nomurai blooms. Further research into the problem and possible solutions is necessary to ensure fully understand and correct the blooms.


map of the sea of japan

Figure 1. The Sea of Japan
Source: Northwest Power & Conservation Council

life cycle of a jellyfish

Figure 2. The life cycle of a jellyfish from larva to medua.
Source: Dery

six graphs showing a warming trend in six different sections of the Pacific Ocean

Figure 3. Yearly mean SS TAs (°C) (dots) averaged in the (a) global ocean, (b) tropical Indian (30°S–30°N), (c) tropical Pacific (30°S–30°N), (d) North Pacific (120°E–110°W, 30°N–60°N), (e) tropical Atlantic (30°S–30°N), (e) North Atlantic (80°W–0°, 30°N–60°N) in 1950—2009. Black lines are the five-year running means of the yearly mean SS TAs. SS Ts are the Extended Reconstructed SS T version 3b of Smith et al. (2008). All anomalies are defined as departures from the 1971–2000 climatology.
Source: NCDC and NOAA