Farming The Food Chain | Low Trophic Aquaculture

If I said ‘seafood’ to you, what springs to mind? Chances are ‘low trophic’ species like seaweed, sea squirts or shellfish weren’t first on your list - but they should be. Here’s how low-trophic level species could change how we farm for the better.

Trophic levels are an organism's position in the food web. Think of a hierarchical pyramid with apex predators (like sharks and tuna) at the top and primary producers (like plants and algae) at the bottom. The ground floor of this pyramid lays the foundation for the entire food web - providing the first sources of energy (or food) for all other organisms. But despite their modest position relative to the rest of the food chain, low trophic species might just hold the key to sustainable seafood farming.

Getting more for our money

To first understand why trophic levels matter for sustainability, we need to take a look at how energy moves through the food chain. As primary producers like seagrass and algae (level 1) become food for first-order consumers like shellfish (level 2), who are then consumed by intermediate predators (level 3), and so on… the energy needed to support the same weight of an organism gradually increases as you go up each trophic level.¹ The reason for this is that when an organism consumes food from a trophic level below itself, much of the energy and biomass from the food source is lost during the transfer from one organism to another. In fact, most estimates indicate that less than 10% of an organism's biomass is transferred when consumed by the level above it. In a marine setting, this means almost 1,000kg of seaweed (at level 1) is needed to support a single kilogram of salmon (at level 4). 

Early American chemist G. Tyler Miller Jr. conceptualised this process by imagining that  ‘about three hundred trout are needed to support one man for a year. The trout, in turn, must consume 90,000 frogs, that must consume 27 million grasshoppers that live off of 1,000 tons (or 1 million kg) of grass’.²

In the wild, eating these top predators is almost always more costly for the environment and our wallets than eating species from lower trophic levels. When it comes to farming, however, it gets a little more complicated. Let’s take salmon as an example - a naturally high trophic species. Today, farmed salmon are fed a man-made and largely vegetarian diet, with around 70% of feed ingredients typically coming from plant-based sources.³ In theory, this largely plant-based diet reduces the trophic level of farmed salmon from a high trophic level (4) to a lower one (say, level 2 or 3) based on the fact that they’re mostly consuming level 1 species - like plants - directly. 

But despite this artificially lowered trophic level, higher-level species like salmon are still naturally built to require higher levels of energy than lower trophic-level species. This means they still need more food per kilogram of fish, regardless of whether that energy comes from animal-based or plant-based feed.⁴ While developments in aquaculture feed have seen improved feed conversion ratios (meaning less food is needed to produce more fish), fish feed still often accounts for over half of the total aquaculture production costs and further contributes to land clearing to produce this feed.⁵ On the other hand, low-trophic species require only a fraction of the energy input to grow into a product we can eat - cutting both costs and the resources needed to grow our food.

Zero input, high value

While lower-trophic species generally need much less energy to grow compared with traditionally farmed species like salmon, farming some lower-trophic species can come with additional benefits. If designed correctly, both bivalve and algae farms can actually positively impact the environment around them – all while producing nutritious foods.⁶

Bivalves (shellfish)

Shellfish are non-discriminate filter feeders - meaning they’ll happily filter up to 100L of water per day, picking out excess nutrients, organic matter and naturally purifying their surroundings while removing CO2 from the environment.⁷ On top of their potential to sequester and store carbon from the atmosphere, shellfish aquaculture also provides fish and other marine organisms with cleaner water and important structural habitats that can boost biodiversity in the local area.⁸ 

Macroalgae (seaweed) 

Turning trash into treasure – a multi-trophic approach

Following the aphorism, “Nothing is created, nothing is lost, everything is transformed”, low-trophic species offer aquaculture producers new ways to turn waste back into useful materials. Integrated-Multi-Trophic Aquaculture or IMTA - a concept dating back to 2100 BCE in China -  involves the joint farming of species from different trophic levels that are ‘ecologically complementary’.¹¹ These bio-integrated systems allow one species’ uneaten feed, waste, nutrients and by-products to be recaptured and translated into feed, fertiliser, or energy for other species in that system. 

By harnessing ecological connections in this way, producers can actively transform lost energy - once considered waste - into new and profitable products while offsetting other negative impacts of the farm system. For example, seaweed and shellfish could be grown mutually alongside salmon farms, feeding off nutrients produced by the salmon. While doing so, they would be filtering the surrounding water to provide a cleaner environment for salmon to thrive in and adding to biodiversity in the area - a true win-win.

Learn more about vertical ocean farming

More diversity, less risk

So then, why aren’t more large-scale commercial producers focussing on farming low-trophic species or switching to IMTA? Well, that’s largely because farming in these alternative ways requires a significant investment of money, time, and expertise to create a new farming model that is both economically profitable and achievable at a large scale. With decades of knowledge, expertise and technology already created around the farming of just a few key species, the majority of commercial aquaculture producers have opted to use the knowledge and infrastructure that is already tried and tested. Unfortunately, this means many producers have followed the same linear blueprint as large-scale land-based producers: one species – one process – one product. 

But these monoculture food production systems - whether on land or at sea - are a high-risk, high-reward business. Much like investing everything in one stock, having all your eggs in one basket - or one species - leaves producers incredibly vulnerable when things go wrong. And as recent decades have shown us throughout our agroecological systems, this method of cultivating has left single-species producers in an increasingly vulnerable position as uncertainty around climate change continues to mount. 

Integrating low trophic options offers producers a potential way to buffer that uncertainty. By increasing the diversity of their products and investing in more environmentally sustainable options, producers won’t have to rely solely on a single - often high-maintenance - species or product to keep themselves afloat when times are tough. Like a healthy ecosystem, resilience to change comes through greater system diversity. For marine farmers, these same rules could apply - both environmentally and economically. 

The catch

Before we pop the champagne, not every aspect of low-trophic aquaculture works so well. Firstly, marine environments are hugely dynamic and ecologically complex, which makes regulating new uses of marine space incredibly difficult. Second, there is the inevitable issue of low consumer demand for unknown products (would you eat a sea squirt…?), meaning the markets are simply not there for many low-trophic species to support profitable farms.

For IMTA, there are also barriers to using integrated approaches on a larger scale. In many regions, regulations for single-species production are already lacking, so the more complex regulatory frameworks and industry expertise needed to support these more dynamic multi-species systems will take time to carefully develop and implement. Luckily, things are looking up - as part of the EU’s Horizon 2020 programme, a new research and innovation project called AquaVitae is now compiling one of the world’s largest and most comprehensive multi-national low-trophic task forces. Their mission is to introduce low-trophic species, products, and processes in marine aquaculture value chains across the Atlantic.

So, while trophic levels alone can’t tell us everything we need to know about how sustainable a particular species is to farm, the environmental and social benefits of increasing the diversity of our farmed seafood to include more low-trophic species are hard to deny. For large producers of high-trophic species, it yields an opportunity to buffer uncertain climates, offset their impacts and create a more circular economy. For small producers and lower-income regions, it provides a real low-input, low-cost and sustainable route towards improved food security and livelihoods.