This semester, I have taken a course on the topic of aquatic food webs. It is fascinating to learn how the increase (or decrease) in the abundance of one species can impact an entire food chain. In one of the lectures, the professor explained that we, humans, might be promoting the frequent blooms in jellyfish observed in our oceans worldwide. More and more frequent outbreaks of jellyfish have been reported worldwide, with serious financial consequences for the fishing and tourism industry. I couldn’t see how humans might influence the life of a jellyfish, so I looked into the subject a little more and found an interesting paper written by prof. Anthony J. Richardson and colleagues. It seems like humans are indeed a big part of the problem, and the increase in jellyfish is a bigger problem than you might think.
In this article, I will talk about:
Based upon: Richardson, Anthony & Bakun, Andrew & Hays, Graeme & Gibbons, Mark. (2009). The jellyfish joyride: Causes, consequences and management responses to a more gelatinous future. Trends in ecology & evolution. 24. 312-22. https://doi.org/10.1016/j.tree.2009.01.010
The problem with jellyfish blooms
Why is it such an issue that these jellyfish outbreaks are becoming more frequent? Certain human-induced stresses (which I will talk about a little further in the article) promote jellyfish blooms to the detriment of other marine organisms, usually species important to fisheries. Diverse fish communities are replaced by a massive monoculture of jellyfish, having enormous ecological, economic, and social impacts. The presence of a high abundance of these gelatinous creatures might lead to big losses in tourist revenue through beach closures or the death of bathers. Furthermore, the reduction in the amount of commercial fish, the killing of farmed fish, the clogging of fishing nets and the contamination of the catches, as a consequence of the high abundance of jellyfish, cost the fishing industry tons of money.
Once jellyfish have the overhand in an ecosystem, they don’t like to give up their dominant position. These animals are very good at surviving in harsh environments due to their broad diet, fast growth rates, the ability to shrink when starved, the capacity to fragment and regenerate and the ability to tolerate hypoxia (conditions of low oxygen availability).
Did you know that the Nomura’s jellyfish (Nemopilema nomurai), which can be found off the coasts of Japan and China, can measure up to two metres across and weigh around 200 kg? [2]
How humans are helping jellyfish
How are we part of the reason jellyfish have been so successful in recent years? Five potential pathways might affect jellyfish populations: overfishing, eutrophication, climate change, species translocations, and the creation of new habitats.
Overfishing
Probably, you are already well aware of how humans strip the ocean from a lot of marine life. The fishing industry takes too much fish from our oceans. The decline in fish stocks we are seeing might be beneficial for one particular group of marine animals: the jellyfish.
Fish compete with jellyfish for the same prey, namely zooplankton. Thus, less fish means less competition and more food for the jellyfish. Some fish are predators of jellyfish, controlling their number. So when fish stocks crash, jellyfish bloom.
Eutrophication
Eutrophication is the process by which a body of water becomes enriched in dissolved nutrients, promoting the growth of aquatic plant life. When excessive nutrients from fertiliser runoff and sewage run into coastal waters this encourages phytoplankton (microscopic marine algae) to bloom. In addition, these added nutrients, rich in nitrogen and phosphorus and poor in silica, promote a change in the phytoplankton community. This change at the base of the food chain is more favourable for jellyfish than fish, further promoting a decrease in fish abundance. By now, we know what it means if fish populations are getting smaller in size…
Another consequence of eutrophication is the increasing number of dead zones. Dead zones, or oxygen-depleted zones, are the result of the bacterial breakdown of the phytoplankton bloom. At some point, these microscopic algae die off, sink to the bottom of the water column, and there, bacteria throw a party in which they start to decompose all of the dead material or detritus. During this process, bacteria use oxygen, and the environment becomes limited in oxygen. Jellyfish thrive in these lower oxygen conditions because of their lower oxygen and food demands compared with those of fish and shellfish.
Climate change
The warming of our oceans due to climate change impacts species worldwide. Just like it can become too hot for us, jellyfish prefer a certain temperature range. A change in climate makes their current habitat a little too hot, so they move towards subtropical and temperature latitudes.
You might think why we should care about jellyfish scattering to new areas. In Australia, there is a growing concern that box jellyfish, really venomous creatures, might expand southwards to the more populated areas. You can probably imagine the severe consequences for the tourist industry.
Translocations
A warming climate might cause a big translocation of our gelatinous friends, but humans are also directly responsible for their spread into new areas. Humans can unintentionally assist in the movement of marine species through the exchange of a ship’s ballast water between regions. Some animals also attach themselves to the hulls of ships (biofouling) and hitch a ride. Polyps of Cnidaria (including jellyfish, sea anemones, corals etc.) are known to be frequent stowaways.
An example of such a translocation facilitated by the shipping industry was the introduction of the spotted jellyfish Phyllorhiza punctata from the Pacific Ocean into the Gulf of Mexico. From May to September 2000, an estimated total of 5,000,000 jellyfish over 150 km2, equivalent to 40,000 tonnes (40,000,000 kg) wet weight were discovered in Mexico. This jellyfish bloom contributed to millions of dollars of economic losses due to their clogging of shrimp nets.
Habitat modification
The last thing we humans do to help jellyfish is the (obviously unintentional) creation of new habitat. To protect ourselves from the future rise in sea level, coastal defences are built along the coast. These new infrastructures are the ideal habitat for polyps, the “babies” of jellyfish. At the beginning of their life, polyps need a solid substrate to attach themselves to and start growing. More suitable habitats for polyps means that these juvenile jellyfish will be more successful in reaching the adult stage in life.
Recommendations for management
According to Richardson and colleagues, “ecosystems that have rapidly switched to being dominated by jellyfish because of anthropogenic stressors might not return to a pre-jellyfish state without significant intervention”. This sounds pretty serious, so what needs to be done?
A start would be to harvest more jellyfish for human consumption, but this alone would not be enough. Restocking the affected ecosystems with the jellyfish’s natural predator (in most cases another fish) can help to control their abundance. This type of management is called top-down control in ecology and it could be accomplished by reducing the fishing pressure on the jellyfish’s natural enemies. Finally, reducing the use of fertilizers, thereby reducing nutrient run-off, can also be part of the solution. Other potential ways to manage this problem are explained in the paper by prof. Anthony Richardson and colleagues.
Sources:
[1] Richardson, Anthony & Bakun, Andrew & Hays, Graeme & Gibbons, Mark. (2009). The jellyfish joyride: Causes, consequences and management responses to a more gelatinous future. Trends in ecology & evolution. 24. 312-22. https://doi.org/10.1016/j.tree.2009.01.010
[2] Marine Biological Association. (2017, February 22). Fact Sheet: Jellyfish. Retrieved January 23, 2022, from https://www.mba.ac.uk/fact-sheet-jellyfis
I hope you enjoyed reading this article. It is the first article I wrote of this “scientific” type. I believe that knowledge is a strength. So, I hope to share some knowledge I gain while researching the main problems occurring in the natural world. As I learn more about different topics in nature conservation and ecology, I will share my insights, so you will be on top of the latest findings.
If you have a request for a certain topic, feel free to reach out to me.
Would you like to continue reading? Check out these articles:
The Wandering Biologist 