Image Jellyfish Bloom © L. Gershwin

Capitalising on Chaos

The Jellyfish Double Whammy of Environmental Effects

The spectacle of millions of jellyfish and thousands of dead fish, and the spectre of millions of dollars in losses certainly are jarring. But we believe that the more alarming loss is the impact on native animals that nobody calculates. Like the tree falling in the woods when nobody hears it, we have no idea how much impact these jellyfish blooms are having on the local ecosystem. But it seems highly unlikely that the impact would be neglible. 

Jellyfish eat the eggs and larvae of other species -- fish, crustaceans, mollusks, starfish -- as well as the plankton that those larvae would eat [1]. This 'double whammy' of predation and competition can (and does!) bring an ecosystem to its knees. Even without data from proper ecosystem studies, we can roughly estimate the impact: jellyfish have to eat to reproduce and grow, so the more jellyfish are present, and the faster and larger they grow, is powered by the eggs, larvae, and other plankton they are consuming. So if there's a lot of jellyfish and they are growing fast, you know there's a lot of food that is not available for other species to eat, and a lot of individuals of other species that are being eaten, and therefore not contributing to the functioning of the ecosystem or breeding for the next generation. 

It appears that these dense jellyfish blooms are stimulated by the fish farms' impacted ecosystems. The salmon have the benefits of husbandry, from provided feed to veterinary care, whereas other species in the ecosystem have nothing. 


Polyps and Gill Necrosis

Experiments on jellyfish polyps have demonstrated that they prefer artificial surfaces [7, 8]. The infrastructure associated with salmon farms, therefore, offers an ideal habitat for jellyfish to thrive in.

Hydroid colonies growing on nets may sting fish as they swim by. Lesions on the skin make the fish less saleable, and there is the possibility for toxic contamination or infection of flesh. There is also high potential for clouds of nematocysts or debris to form as fish bump into them; nematocysts and debris have been shown to cause gill disease and necrosis [3]. Hydroid colonies can also impede water circulation, and can add to drag of the nets in currents. Thus, the hydroids must be cleaned away regularly. 

The primary threat from polyp stages comes from debris generated by net cleaning activities. Cleaning is accomplished by high pressure water blasting, or by manual brushing or scraping of the nets. Debris consists of stingy and abrasive components, both of which can lead to gill injuries, amoebic gill disease, necrosis, and mortality [3, 9].


Polyp Seeding of New Colonies

As bad luck would have it, hydroid regrowth is stimulated by the mechanical action of net cleaning [10]. Laboratory experiments have shown that tiny bits of hydroids left on the nets after cleaning are sufficient to regrow entire colonies, and the more frequent they are cleaned, the faster they reproduce.

Likewise, the tiny fragments created by the cleaning process act like seeds, flowing downstream and settling out to become new vigorously growing colonies. Hydroid seeding can be compared to the broom scene in the Disney film Fantasia, where each effort to destroy the brooms simply resulted in more brooms. Downstream hydroid seeding is a serious issue affecting farmed and native species alike. In the short term, it leads to a higher biomass of medusae stinging the salmon and native species, and in the long term, the extra biomass permanently alters the function of the ecosystem.



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The unusual life cycle of jellyfish presents a duality of threats. Unlike most species familiar to us, jellyfish have essentially two adult stages, the more familiar medusa, or free-floating sexual stage, and the less familiar polyp, which is like a tiny sea anemone or coral polyp that sticks to surfaces. When jellyfish reproduce, the young grow up to become polyps, and vice versa. These two different life cycle stages cause very different threats to farmed salmon, other farmed species, native species, and the habitat.

Under certain conditions, jellyfish quickly breed into super-abundances, called blooms or swarms. While blooms occur naturally, they also may be stimulated by changes like warming water, which cause them to bloom more, and by any impacts on their predators and competitors (fish), like pollution, overfishing, and introduced species. In this way, jellyfish blooms are often a visible indicator that something is out of balance in the ecosystem. Moreover, jellyfish can be a threat multiplier for other impacts, adding to the stress that species must cope with.


Jellyfish causing Fish Kills

In large numbers, jellyfish blooms penetrate the salmon pens, either as small jellyfish passing directly through the mesh, or as fragments of larger jellyfish that become stuck on the outside of the net and rip apart. Either way, the jellyfish release large amounts of mucus packed with stinging cells. It is understood that the mechanism of fish kills happens with a combination of mucus and nematocysts (microscopic stinging cells). When jellyfish are stressed, such as when they are caught up in a net or a cageful of frantic fish, they exude copious amounts of mucus, which contains countless nematocysts. Stings to the gills agitate the salmon, so they breathe faster, inhaling more mucus. The mucus coats the surface of the gills, preventing oxygen uptake. Simply, the salmon suffocate. A typical fish kill event is over and done with in a half hour or so, leaving tens of thousands to hundreds of thousands of fish dead [1]. 

For the surviving fish, it’s not over. Nematocysts in the gills cause microscopic injuries, which often lead to gill disease [2, 3]. Moreover, some types of jellyfish are known to carry bacteria associated with gill disease, such that the jellyfish may act as vectors or their stings may act as threat multipliers [4, 5].

These blooms are occurring with increasing frequency and ferocity [2]. For example, the table below gives an overview of the 'really really bad' blooms, and ignores the background thrum of lesser fish kills and environmental impact. There is also an increasing financial burden to these blooms. For example, salmon producer Huon Aquaculture lost 64% of its profit in F/Y 2018-2019 due to jellyfish [6]. These lethal blows are getting worse. 


TABLE. Jellyfish bloom trend in Tasmanian salmon industry.