Salmon are anadromous: their lives begin in rivers, they grow to adulthood in the seas and then return to freshwater to reproduce. The salmon makes this 2,000-mile journey only a few times in its lifetime. In recent years numbers have depleted; in 2018 the average number of salmon caught in Scotland was just 37,000, the lowest number since records began. As salmon is one of the UK’s largest exports, one industry is being blamed by environmentalists.
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Aquaculture, or seafood farming, accounted for 52 per cent of fish for human consumption in 2018, according to the UN’s Food and Agriculture Organisation. From 1990 and 2018, global aquaculture production increased by 527 per cent. Fish farming could provide a simple solution for feeding the growing population, expected to reach 8.5bn by 2030, without further depleting the already overfished seas, but this rapidly growing industry is causing trouble for wild salmon.
“One of the main concerns [about aquaculture] is the impact on wild Atlantic salmon stocks,” said Dr Sam Collin, living seas manager at the Scottish Wildlife Trust. Sea lice are a kind of crustacean and have evolved over millions of years to attach themselves to salmon, feeding on their blood to survive. Although these underwater parasites exist naturally in the environment, outbreaks in farmed salmon pens occur and can grow very quickly.
This becomes a problem because the farmed and wild salmon share the same water. “When wild salmon pass by the farms, they get hit by lots of sea lice larvae, and the wild fish, suddenly, have unnaturally high numbers of sea lice on them as well,” said Collin. “This can be particularly problematic for juvenile salmon, as they’re unable to handle such a large number of sea lice, and is generally recognised that this is one of the factors contributing to the decline in wild salmon populations.”
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And the problems don’t stop there. Farmed salmon are bred to grow fast in their specific environment, relying on medicines and food to be provided for them. But occasionally farmed salmon escape and breed with wild salmon, leading to fish populations that aren’t adapted to their own environment. Collin said scientists are now trying to understand the extent to which wild salmon along the west coast of Scotland are being changed by their farmed cousin.
In the wild, salmon eat other fish such as capelin or herring. When farmed, their feed consists of fishmeal and fish oil, along with soya, wheat and pea protein, with the fish products coming from wild fish. The controversy around the farming system is that it comes down to fishing to feed the fish, which in terms of nutrition is largely inefficient. Some research suggests that farmed salmon is not a net producer of protein, meaning that more protein goes into feeding the salmon with plants and wild fish than comes out from eating it afterwards.
Research is taking place to find sustainable alternatives for feeding salmon, be it increasing the use of by-products and trimmings from fish caught for human consumption, or using insects such as black soldier fly larvae or termites.
One approach is to farm different species together to reduce the impact of another, known as integrated multi-trophic aquaculture. For example, if mussels are cultivated to surround a salmon farm, the mussels could eat sea lice larvae and potentially reduce the levels of sea lice in the water, or salmon farmers could cultivate sea cucumbers that would feed on the waste from the salmon farm.
These alternative farming approaches would not make a full ecosystem but it is an improvement, with working examples spread across the Yellow Sea, said Andrew Hudson, head of the water and ocean governance programme at the UN Development Programme.
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“More importantly, it moves away from the monoculture approach of most aquaculture, which is often highly polluting and can have issues with genetic introduction of invasive species which can get in the gene pool of the endemic species. It’s a much more circular approach where many of the organisms consume the waste products of the other organisms and can create at least a partially circular system,” said Hudson.
Ironically, fish farming originally aimed to tackle the depleting numbers of wild fish. “One of the reasons aquaculture started as an industry was to try to replace the fish protein lost through overfishing,” said Ruth Westcott, coordinator of the Sustainable Fish Cities project at the food and farming alliance Sustain. “The aim was to try to have a sustainable source of fish protein without having to further endanger the fish stocks.”
Bivalve farming, such as that of mussels and oysters, is believed to be one of the most sustainable examples of aquaculture, explained Westcott: “Mussels are in fact one of the most sustainable sources of animal protein.
“They filter the water and deposit heavy metals in their shells. So they actually clean ecosystems up and it delivers really good jobs in remote communities.”
Mussel farming simply involves putting ropes in the water, where mussels settle; no chemicals or medicines are required. In terms of the sustainable use of the oceans, also referred to as the blue economy, this appears to be an ideal business.
But to protect wild salmon even larger problems remain. Oceans are currently absorbing about a third of carbon emissions, which are mainly produced by fossil fuels. This is leading to the acidification of the seas, which has contributed to the disappearance of half of the world’s shallow-water coral reefs. Reefs support 25 per cent of all marine species on the planet.
This also affects the world’s salmon, said Mark Kurlansky, a journalist and the author of Salmon: A Fish, the Earth and the History of Their Common Fate. After speaking to river managers, Kurlansky learned that, particularly in the Atlantic, large numbers of fries – young salmon – go into the sea but don’t return to the river to reproduce at the same rate they used to.
The Atlantic’s carrying capacity – its ability to sustain the species in it, among them, young salmon – is changing. The oceans can no longer feed the animals that live in them as they used to due to the effects of climate change driven by human activity.
The world’s population is increasing and 8.9 per cent of people globally were undernourished in 2019, according to the UN. Aquaculture – in some cases with a few necessary tweaks – could help feed everyone. But one thing is clear, sustainability and rethinking food systems go hand-in-hand.
“There is nothing written that said that a job has to destroy the planet or that economic activity has to be destructive”, said Kurlansky. Especially not when the destruction of our ecosystems is at stake.