What is a jellyfish bloom?
A bloom (also smack or swarm) is a large increase in the number of jellyfish in a short period of time. This can happen for ecological reasons (wind or current patterns) or behavioral reasons (aggregation or reproductive surge). Jellyfish blooms are increasing across the globe , though there is a lack of long-term data in many areas . Despite a lack of long-term datasets, jellyfish blooms are occurring now and causing problems now, so from a management perspective, blooms and their consequences need to be addressed . Jellyfish blooms are one of the prominent ways the general public becomes aware of them because they can negatively affect human activities. Beaches close, power plants shut down, ships are incapacitated, as huge numbers of jellyfish can interfere with operations, potentially causing millions of dollars in damages.
WikiCommons - Miketsukunibito (CC BY-SA 3.0)
Jellyfish blooms are in response to stress (food, salinity changes, pH changes, etc.) from their environment . There is evidence that human impacts upon the environment contribute to the creation of jellyfish blooms , though there is debate about the exact level of impact that can be attributed to them. Over-fishing, eutrophication (resulting from nutrients being added into the water), the addition of hard substrate providing breeding platforms, and even climate change all contribute to a world in which jellyfish are better adapted to out-compete other marine species [4,6]. The effects of climate change do not directly cause more jellyfish in all ecosystems by warming waters. There is a complex interaction of many biophysical factors that influence jellyfish abundance . Jellyfish traits (fast growth, tolerance for hypoxia, wide diet) make it possible for them to live through unfavorable environmental conditions, and then take full advantage of favorable conditions to reproduce in large numbers [1,6].
Even though jellyfish blooms are causing problems, it should be noted that the conditions that give rise to blooms of jellyfish are the problem!
Image from (Richardson et al., 2009)
Contributing factors to jellyfish blooms.
Condon (2012) found there was evidence that shallow-water jellyfish blooms have 20 year periodic cycles, though the reasons are not entirely clear. This adds temporal nuance to the idea that jellyfish are increasing worldwide when other scientists have cautioned against drawing too broad a conclusion based on the available evidence . Difficulties in counting, capturing and assessing jellyfish mean that many studies based on abundance are possibly unreliable .
What can be done about blooms?
There are multiple possible responses to jellyfish blooms, but it is unlikely that any single response will work without addressing the underlying causes of the blooms and the environmental conditions that gave rise to them . As jellyfish abundance increases, there are thresholds at which humans can no longer cope, but must adapt to a jellyfish-dominated ecosystem, or transform their ways interacting with those systems 
Cutting them up with nets has been proposed, however there is a high risk of bycatch and some jellyfish may be able to survive the cutting process due to their regenerative abilities.
Killing them in large numbers can cause declines in ecosystem functions and services due to increased oxygen demand, increased dissolved organic nitrogen and acidification of the water .
Enforcing regulations on ballast water (jellyfish transport) and overfishing have regulatory barriers across international boundaries and enforcement issues .
Fishing for jellyfish is also not necessarily a long-term solution, because of a lack of scale and its dependence on connecting markets .
Blooms have many potential causes and local conditions apply, so each bloom needs to be dealt with in a different manner .
It is also possible that some jellyfish "blooms" (rapid population growth) are a matter of perception, wherein changes in proliferation, aggregation or distribution lead human observers to conclude a bloom that is not necessarily happening . If the density of a population increases or is redistributed, it can seem like a bloom has occurred when none has .
Three jellyfish species, Aurelia aurita, Cyanea nozakii and Nemopilema nomurai, form the majority of recent large blooms in Chinese seas , though only two out of the three species are recognized by the Hong Kong Agriculture Fisheries and Conservation Department as being present in Hong Kong's waters. Since developing a reporting network can be an easily replicated, cost-effective way to collect abundance and distribution data , the Hong Kong Jellyfish Project was created to address this gap. Citizen science is a contributor to this endeavor! When you are out on the water, swimming, kayaking or fishing, and see jellyfish, take a picture and report your sighting here!
You can also find information about jellyfish research on the Resources page.
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