Jellyfish are one of the biggest problems for salmon farming... and unfortunately for everything else in their wake. Whereas we typically think of jellyfish as drifting organisms, most species of jellyfish have an alternate phase of their life cycle that is a tiny polyp -- a hydroid -- which weirdly enough grows preferentially on artificial surfaces. Salmon cages offer opportunistic pest species like hydroids a perfect platform on which to grow.
Hydroids rapidly grow into thickets on salmon cages, where they sting the fish and interfere with water flow. They must be cleaned off frequently. But this is an unimaginably bad thing to do, for several reasons.
Net-cleaning bad outcome 1: Gill disease in the salmon
The mechanical action of cleaning off the hydroids breaks them into squillions of tiny fragments. These particles act like sand grains in the gills of the salmon, irritating and injuring the delicate gill tissue . And plus they sting. The same way a tiny pebble in your shoe on a hike can lead to a blister or an infection, so too hydroid fragments open the door to gill disease in salmon. We have been told by veterinarians working in the Tasmanian salmon farming industry that they experience spikes in losses and gill disease after each cleaning, which during the summer may be as frequently as every two weeks.
Net-cleaning bad outcome 2: Impacts on other species
Salmon experience a spike in acute deaths and chronic illnesses with net cleaning, but they have veterinarians and antibiotics to do what they can to nurse the fish back to health after a gill assault. But what about the effects on native species that have no such access to care - the mussels, clams, oysters, scallops, and other fish?
Think it sounds too speculative? Think again. Salmon farms are limited to a 35m zone of impact around the leases, but fouling organisms and net wash tend to be very badly behaved. A mussel farm 150m down-current from a Tassal farm in Dover was driven out of business by the damage to his mussels from salmon net cleaning . This is not only a story of a small operator squashed by a large business, it is also evidence of the net-wash affecting other organisms. But again, what about the countless species and organisms that nobody fights for?
Net-cleaning bad outcome 3: Stimulates rapid re-growth
We generally think of weeds as being plants. But weediness is a set of biological and behavioural traits, a lifestyle. Hydroids are among the world's weediest of weeds.
One of the most unexpected things that hydroids have in their bag of tricks is the way they respond to being cleaned off surfaces, for example, salmon pens. The mechanical action of the cleaning stimulates them to grow faster and more vigorously . The more often you clean, the more they grow. The harder you scrub, the more they grow.
Net-cleaning bad outcome 4: Seeds even more hydroid colonies
In addition to growing more vigorously, hydroids are like some nightmarish version of the Sorcerer's Apprentice in Disney's Fantasia, where the mops keep multiplying. Hydroid biology possesses an unfortunate inevitability: even the tiniest fragments can (and typically do) seed a whole new colony. Once liberated from their attachment, hydroid fragments may drift anywhere and produce more offspring - vigorously.
A Diabolical Cycle
Image Jellyfish Bloom, copyright L. Gershwin