March 3, 2024

I, Science

The science magazine of Imperial College

Aliénor Hammer
4th December 2020

Sustainability: a quick introduction

These days, sustainability is all the rage. As someone that’s always been passionate about the environment, I dove head-first into the concept and the practical applications of it. I drastically reduced my meat consumption, boycotted Amazon (the brand of course) and cut out fast fashion. But is this really what sustainability is about? Can we really save the planet by doing this? Sad disclaimer: no. Why? Because everything we can do in the fight for a more sustainable lifestyle has a trade-off. Let me illustrate this point with a specific case study: salmon aquaculture in Scotland and greenhouse gas emissions.

Sustainability itself is a vague term, a word that can be slapped on pretty much anything because it is so broad. A professor I once knew told me that “sustainability” by itself means nothing. When you say “sustainable” you have to refer to something in particular. The sustainability of “salmon aquaculture” means nothing. The sustainability of “salmon aquaculture in reference to greenhouse gas emissions” is better. But still, not perfect. When you mention sustainability, a thing is never inherently sustainable, it always has to be compared to something. Salmon aquaculture is not inherently sustainable, but salmon aquaculture is more sustainable compared to cattle farming. Now that that is established, let’s move on.

Salmon farming – a crash course

Let’s look at salmon farming and its background before we tackle sustainability. Salmon farming is quite a resource intensive activity: baby salmon fish, known as smolt, are reared in big tanks inland (or in lakes, as long as it is freshwater) until they reach 500 grams, and are then transferred into the sea, in open-net pens. There, they are fed and grown until they reach 1.5 kg, and are then harvested and processed. Salmon farming started in Scotland in the late 70s, and grew exponentially. At the beginning, there were small farms. Now, huge aquaculture companies dominate the economic landscape. Salmon farming is not more sustainable than salmon fishing when it comes to greenhouse gas emissions, because the wild population do not need us to input anything to grow, so there is no generation of emissions. However, because so little salmon remains in the wild (mostly on the Pacific coast of North America), farmed salmon is considered more sustainable in terms of giving us the opportunity to sustain the wild fish population.

Salmon aquaculture is heavily criticised by non-governmental organisations because of the damage that it causes to the environment. Pollution is a big one: fish waste drifts to the bottom and creates dead zones, the feed that is not eaten by the fish decomposes in the water, generating toxic algal blooms. The antibiotics and medicine poured into the pens drift with water currents and harm marine wildlife. Overall, there is a lot to criticise about the industry.

Salmon farming and greenhouse gas emissions

The most greenhouse gas intensive step of salmon farming is the production of the feed, and if we were to make salmon aquaculture more sustainable in terms of greenhouse gases, this is what we should tackle to make the most impact. Salmon feed is made up of a mix of marine ingredients (smaller ground up fishes) and crop ingredients (think soy, maize, wheat). You therefore have to catch the fishes and ground them up; therefore relying on diesel for the fish vessels. Crops are also extremely intensive because you have to grow them, use fertilisers, harvest them and then ground them up. And that is not even considering land-use change.

A lot of feeds are dependent on soy because of its protein, and soy is usually grown in Brazil, where tropical forest has been cleared to plant them, and deforestation is one of the most carbon-intensive activity you could possibly find. And finally, feed is made up of micronutrients (similar to vitamins) and astaxanthin. Astaxanthin is a petroleum-based product that you give to salmon to colour its flesh red, because no, farmed fish is not naturally red coloured! Wild salmon is red because it picks up carotenoids when it eats small crustaceans like shrimps, colouring its flesh red. But since farmed salmon just eat pellets, artificial astaxanthin needs to be added, because no one wants to eat white salmon. Recent research suggests that micronutrients and astaxanthin are very energy-intensive to make, another blow to fish feed when it comes to its greenhouse gas emissions.  

Salmon aquaculture: solutions?

So now what? How do we make the fish feed sustainable? There are many options. How about decreasing the amount of emissions intensive ingredients? A lot of feed companies have slowly reduced the fish ingredients in their feeds and substituted with crops. But salmon are mainly carnivores and thrive on protein rich diets, so substituting those fish proteins with crops is not necessarily a good solution. It can lead to the risk of the fish needing more of the same feed to grow as they would with a more fish-based diet, ultimately using more crops. And as I have mentioned, crops are not the most climate-friendly feed. What about micronutrients, since they are energy intensive? Well, same issue here: reducing these means the salmon is not receiving the nutrients it needs to grow, so ultimately they would need more feed, again leading to more greenhouse gas emissions.

More and more, feed companies are looking to novel ingredients to replace certain key ingredients: making algal oil rich in omega 3 (something which the salmon is known for) could substitute fish ingredients. Similarly, using ground up soldier-fly larvae could increase the protein contents of the feed without relying on crops. But these are new technologies, and it is too soon to tell if these can be efficiently scaled up. Preliminary results indicate that these novel ingredients are not much less emissions intensive than the traditional feeds. Not a clear win, then.


So here you have it. There is no clear solution to making salmon feed more climate friendly in just one go. All the solutions come with a problem elsewhere, it is definitely not clear-cut. A good way to make salmon aquaculture more sustainable in terms of its current greenhouse gas emissions is to have more cooperation throughout the supply chain: feed companies should ask their suppliers for better farming standards, which do not rely on deforestation and these kinds of harmful practices.

The same goes for sustainability in our lives: yes, we can cut fast fashion out of our lives or go vegetarian/vegan and boycott big brands, but it is even better to support systemic change because all of our responsible choices can have unforeseen repercussions. And even if they didn’t, they’d only be a drop in the ocean if the system doesn’t start to change.