The jellyfish sea A myth buster on the recent jellyfish bloom by a marine biologist

I was one of the thousands of people who flocked to the North Coast of Egypt for some beach time over the Eid holiday a couple of weeks ago, only to be kept out of the water by a huge swarm of large stinging jellyfish. Their pale, semi-transparent bodies dotted the turquoise waves densely enough to be visible from shore, and many of them were washed up by the surf, to be stepped on and buried in the sand. At the same time, reports came from as far away as Kuwait and Wales of unusually large jellyfish swarms.

Throughout my trip, I heard many speculations, myths, accusations and rumors regarding the causes of such a phenomenon, and, as a local marine biologist, I was the target of many questions: Is this a “natural” phenomenon? Why are there so many of them this year? Does the expansion of the Suez Canal really have anything to do with it? And many more.

Rapid increases in jellyfish populations (known as jellyfish blooms) occur naturally from time to time in various locations around the world for various reasons, and they are usually short-lived, lasting no longer than a few weeks. It is often impossible, especially in data-poor regions such as the Egyptian North Coast, to pinpoint the exact cause of a single bloom without collecting large amounts of data before, during, and after the bloom — a very labor and time-intensive endeavor that is costly, even for countries with ample funding for research. However, there are a few things we do know.

Some of the causes of natural or seasonal jellyfish blooms include: Seasonally warm temperatures boosting the reproduction of jellyfish, changes in ocean currents pushing blooms from the open ocean toward coastlines and seasonal blooms of plankton (very small, floating marine organisms) on which jellyfish feed, among others. However, not all jellyfish blooms are natural phenomena occurring without the influence of irresponsible human behavior, and a global increase in jellyfish blooms over the last two decades is believed to be an indicator of the degradation of marine habitats.

The actual occurrence of a jellyfish bloom in the Egyptian Mediterranean this year is not the troubling part of the story, but rather, it is its unprecedented size and geographic spread. As far as we know, a jellyfish bloom of this magnitude has never been reported on this part of the coast before, and this is why it is a legitimate cause for concern. This is also true for Kuwait and the UK, where blooms occur seasonally, but not in such large numbers. These unprecedented blooms are indicators that uncontrolled and unsustainable human activities may be altering natural processes in unfavorable ways. Here is a list of some of the ways in which irresponsible and unsustainable human behavior can cause jellyfish blooms:

  1. Climate Change: The warming of our oceans, caused mainly by the excessive burning of fossil fuels and production of greenhouse gases, favors jellyfish by speeding up their reproduction and extending their reproductive seasons. Scientists have already predicted that jellyfish blooms will increase as global warming continues and that jellyfish blooms will start to show up in places where they did not previously occur. The Sea of Japan, the East China Sea, the Black Sea and the Baltic Sea, among others, have all seen definite increases in jellyfish blooms since the 1990s.
  2. Overfishing: When we fish too much without taking the long-term sustainability of fish populations into account, we severely deplete the numbers of fish that may be feeding on jellyfish larvae in the earliest phases of their lifecycles.
  3. Inadequate protection of sea turtles: Turtles are the main predators of adult jellyfish, and they help keep their numbers in check. Fishermen and poachers often directly target turtles and turtle eggs, and large commercial fishing vessels accidentally trap and kill them in their nets. Many turtle populations around the world have already been decimated, and several species are threatened by extinction.
  4. Trawling: Large commercial trawlers drag their enormous nets along the bottom of the sea, clearing bare spaces on the seafloor for jellyfish larvae to attach and reproduce. (You can read more about the typical lifecycle of jellyfish here.)
  5. Coastal development and habitat modification: Similar to trawling, altering the coastline and seafloor in order to build resorts, marinas, piers, artificial reefs and oil platforms clears previously-inaccessible spaces for jellyfish larvae, leading to increased reproduction. Coastal development has also taken away habitats that were previously used by sea turtles to lay their eggs.
  6. Chemical pollution: The dumping of sewage, agricultural waste and other chemicals into the sea increases the amount of nutrients present in the water (a process known as eutrophication). This leads to plankton blooms, rapidly increasing the amount of food available for jellyfish, and thus increasing their survival rates. It also favors jellyfish by lowering oxygen levels, which they tolerate better than their competitors.
  7. Solid waste pollution: Plastic pollution in particular leads to even higher death rates for the turtles who feed on jellyfish. Many turtles cannot distinguish between plastic bags and jellyfish, and ingesting too much plastic eventually kills them.
  8. Translocation: Cargo ships can take up or discharge water at any time from their ballast tanks to help balance and stabilize them. Fish, plankton and other marine life (including jellyfish) can be picked up in one ocean and dropped into another, and, if they successfully settle in a new location, they become known as “invasive species” in that location. Sometimes, invasive species end up in locations where they drastically outcompete the local species, thus rapidly increasing in numbers and severely altering the ecosystem.


Unusually large and unseasonable blooms are problematic for many reasons besides simply inconveniencing tourists. Of course, affecting tourism may seriously impact the economies of host countries, but there are many other problems that jellyfish blooms cause in the environments they occur in and for the people who depend on healthy marine life for their survival and livelihoods. For example, jellyfish blooms can block sunlight from algae or coral, which may lead to their death. This means that fish and other organisms who feed on algae or coral may also die and drop in numbers, sending chaos up the marine food chain (a phenomenon known as a trophic cascade, which can also be caused by overfishing and other human activities). Also, jellyfish feed on plankton, including fish larvae, protozoans and other small organisms. In very large numbers, jellyfish could seriously deplete the numbers of fish larvae in the plankton, leading to fewer numbers of adult fish the following season. This could be a major problem for fishermen, especially if fish stocks are already depleted by overfishing. The jellyfish will also outcompete other marine creatures who feed on plankton, leading to a vicious self-reinforcing cycle that keeps jellyfish in a dominant position ecologically and reduces overall biodiversity.

But what about this particular bloom that hit the Egyptian coast? What is its most likely cause?

Many people rushed to blame the recent expansion of the Suez Canal for this recent bloom, and this is an issue that requires some clarification. The particular species making up most of the bloom in the Egyptian Mediterranean is the Nomad Jellyfish (Latin name: Rhopilema nomadica). Indeed, this is originally a Red Sea species that could have only entered the Mediterranean through the Suez Canal. However, the presence of this invasive species in the Mediterranean is a separate issue from the excessive increase in its numbers this year. The Nomad Jellyfish is reported to have existed in the Mediterranean since the 1970s, but, as far as we know, its existence was relatively non-disruptive in Egypt until this year.

Research suggests overfishing is the most likely driver of this bloom and remains the Mediterranean Sea’s biggest threat. 

Some people speculated that the expansion of the Suez Canal removed a salinity barrier that was formed by the extremely salty Bitter Lakes. While there is indeed some evidence that the salinity of the Bitter Lakes acts as a selective barrier to the migration of species between the Red Sea and the Mediterranean, it is just that: selective. Hundreds of species have successfully migrated from the Red Sea to become invasive species in the eastern Mediterranean (which is known as Lessepsian Migration, named after Ferdinand de Lesseps), and, to my knowledge, there is no research showing whether the salinity barrier was ever effective against the Nomad Jellyfish in particular. In addition, water from both the Mediterranean and the Red Sea has been flowing openly into and out of the Bitter Lakes, and their salinity has declined in recent years even before the recent expansion.

It is also important to remember that the migration of species between the two seas (which occurs both ways, but mostly toward the north) is not dependent only on water flow or active swimming, but can also occur via translocation in the ballast waters of cargo ships passing through. This leaves us with the simple fact that we do not have enough evidence to claim that the expansion of the Suez Canal is responsible for the Nomad Jellyfish bloom. Of course, the expansion of the Suez Canal may be harmful to the environment in other ways, but these are outside the scope of this article.

Knowing the exact cause is not as important as focusing our energy and resources on protecting our seas from the threats we already know exist.

Research suggests that overfishing is most likely the driver of this bloom and remains the Mediterranean Sea’s biggest threat, even more than pollution and climate change. In fact, Dr. Enric Sala, and many of his colleagues, predicted in a 2014 study that, unless fishing in the Mediterranean is properly regulated and large parts of the sea completely protected from fishing, this beautiful and productive ecosystem may turn into a “soup of microbes and jellyfish,” with very little edible marine life. Their impressive large-scale study was published in the journal Plos One and is accessible to the public. Even though it only sampled the northern Mediterranean countries and did not include the African coast, the sea is interconnected and does not abide by political boundaries. Also, despite the limited publicly-accessible information regarding Egypt’s fisheries, Dr. Mohamed Samy-Kamal published evidence in 2015, indicating that Egyptian fisheries are in decline both in the Mediterranean and the Red Sea, which is usually a sign of overfishing. Even though some fishing regulations in Egypt exist on paper, compliance and enforcement are often lacking. The other potential local factor in Egypt is agricultural runoff flowing from the Nile Delta into the Mediterranean, leading to excessive eutrophication. However, there is no information to suggest that agricultural runoff was excessive this year.

As mentioned earlier, it is not possible to pinpoint the direct cause of this particular jellyfish bloom. However, I would argue that knowing the exact cause is not as important as focusing our energy and resources on protecting our seas from the threats we already know exist.

Egypt and most other countries lining the Mediterranean are long overdue efforts to put in place necessary regulations for protecting this precious marine ecosystem and for making sure these regulations are followed. Controlling an issue such as climate change requires global collaborative efforts and may be outside immediate local control, but controlling overfishing, poaching, trawling and pollution is not. Each country has direct control over the activities taking place in its waters, and failing to take immediate responsibility for our natural resources will hurt both our generation and, to a much greater extent, all future generations.

Maha T. Khalil