Before ending up in your chopsticks or sushi roll, few people are aware that farmed salmon are commonly plagued by parasitic lice on their skin and amoebae on their gills. Fish farmers continuously struggle to control them, and the control methods themselves are often rough on the fish. An innovative new fish farm design has taken a different approach to prevent lice from infecting salmon in the first place.
Lice use light cues to aggregate in the surface layer – this behaviour is their Achilles heel which can be used against them. The new farm design has a deep net roof and a lice-proof ‘snorkel’ tube up to the surface. This surface access tube is needed as salmon use the snorkel to swallop air to re-inflate their buoyancy-controlling swim bladder. If they can’t get to the surface to do this, they are too heavy and keep sinking.
In a new paper published in Preventative Veterinary Medicine, Melbourne University PhD student Daniel Wright and colleagues document how these new snorkel farms are working at industry scale. They clearly reduce lice, but gill amoebae infections were elevated from holding fish in less space. To solve this problem, snorkels were filled with freshwater to remove the freshwater-sensitive amoebae. Danny’s work shows that farmed salmon of the future could be less burdened by these two important parasites using this new method.
Danny finished his PhD in late 2016 and moved to Norway to do a post-doc to further this exciting work. We wish him well.
Of the many factors which limit the growth and survival of farmed salmon, hypoxia (low dissolved oxygen concentration) is among the most complex to monitor and remedy. In a new study, PhD student Tina Oldham and colleagues manipulated dissolved oxygen levels within sea cages at certain depths by the use of a tarpaulin to block the inflow of water. While caged Atlantic salmon behaviour and distribution after DO levels plummeted in this zone were partially explained by the poor oxygen conditions, other environmental factors such as temperature and salinity were far more powerful predictors of what the salmon did. These findings suggest that, in the highly variable marine cage environment, salmon are likely to expose themselves to sub-optimal oxygen conditions even when ideal conditions are available.
Research published by SALTT lab member Tormey Reimer has been picked up by the global press, including this great story in Newsweek.
In short, Tormey and colleauges found that half of the world’s farmed fish have substantial hearing loss due to conditions in their production environments.
The results have implications for how billions of fish are farmed and how fish are produced to re-stock declining populations of wild fish.
When farmed fish escape into the wild, they do all sorts of damage to native populations, such as narrowing the gene pool through inter-breeding and spreading diseases. A new study led by Melbourne Uni marine biologist Tim Dempster shows that trying to recapture escapees around marine fish farms is a bad solution that can do much damage with little good. Instead, the study suggests a radical change in focus to reduce the impacts of escapees in the wild.
There are lots of places in the world where humans put food into the ocean, either accidently or on purpose. It ends up in the mouths of animals, and can have unintended consequences. Aquaculture is a major source of this food, and feeds from aquaculture are a lot different to a natural diet.
Camille White, a UniMelb PhD student has been looking into this issue. In a paper published today, she shows how you can supersize urchins by feeding them the ‘junk food’ that spills into the environment from aquaculture, but if they get too much of a good thing, they can’t reproduce!
Big Fish by the ABC’s Four Corners program took a deep dive into the practices and sustainability of the Tasmanian salmon industry. Tim Dempster was interviewed on the conditions in 2016, the hottest summer on record in Australia for over 100 years and a challenging time for growing salmon.
Our work has recently featured in the September issue of Science News in an article on the problem of escaped fish entering the wild and what to do about them.
SMOOTH CRIMINAL Farmed sea bass and other fish frequently escape from sea cages out into the ocean. Researchers worry that escapees, like this sea bass found off the coast of Tenerife in the Canary Islands, could threaten wild ecosystems.