It might be hard to imagine how a few little escaped captive fish here and there could be a pesty problem. But indeed they are! It's not just a few here and there. It's a systemic problem. Government authorities have estimated that 2%-3% of stocked fish are lost to low level leakage in Tasmania, along with occasional large-scale (>1000 individuals) losses. For example, just in the last few years: 120,000 escaped in May 2018, 50,000 escaped in Nov 2019, 52,000 in Nov 2020, and another 130,000 in December 2020. The industry says these escapes wouldn't survive, but independent studies indicate that around 15% do survive. Given the number of escapes, this is a lot of fish of a species that is not native here.
Image Salp Bloom © L. Gershwin
Capitalising on Chaos
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Strange jellyfish-like creatures called salps are emerging as an unnerving link in bioaccumulation of toxic algae and biomagnification of heavy metals. The potential implications of this ecological pathway for toxic contamination in our food is frightening and must be researched with urgency.
Wondrous nightime displays of bioluminescence draw tourists and local photographers, but many do not realise that the species responsible is an introduced pest responding to a banquet of food stimulated by nutrients. Nutrients (a polite way of saying "too much excrement") emanating from the salmon farms dwarfs our urban effluent. Not all algal blooms are toxic, but many are. Besides poisons, algae suck the oxygen out of the water and suffocate other species, and they get into the gills and their gritty bodies cause mechanical injury. Algae are also a fundamental link in a process whereby nutrients lead to a cascade of effects, leading to jellyfish as the top predator.
Flaw 4: Pests
The Tasmanian salmon farming industry is a huge disruptor of the environment in which it operates. This unnatural situation invites pests of all kinds that throw the ecosystem even more critically out of balance. Toxic algae are stimulated by the enormous quantity of excrement emanating from the farms, which in turn stimulate uncontrollable jellyfish blooms. The large concentration of salmon in one place is a resounding dinner bell for hungry seals, which will do absolutely anything for the rich feed that a packed salmon pen promises. And the salmon themselves are just as voracious, and the tens of thousands that escape every year consume and compete with native species.
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.
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. The mucus coats the surface of the gills, suffocating the salmon. Typically, tens of thousands of fish, or more, are dead in a half hour. For the surviving fish, it’s not over. Sting injuries to the gills often lead to gill disease, which often kills the fish slowly. These blooms are occurring with increasing frequency and ferocity.
The spectacle of millions of jellyfish and tons of dead salmon, 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. Jellyfish eat the eggs and larvae of other species, as well as the plankton that those larvae would eat. This double whammy of predation and competition can and often does rapidly crash an ecosystem.