Are Farmed Fish and Shellfish a Problem for the Marine Environment?
This article was published in Forskerne Formidler on January 17, 2025.
The official dietary guidelines recommend eating 350 grams of fish per week.
That shouldn’t be too hard to follow — but in reality, Danes only consume about 200 grams per week on average. From a health perspective, there is good reason to eat more fish.
How does it affect the marine environment if you choose farmed fish?
As a biologist and researcher in aquaculture production — such as fish farming — I often encounter the belief that farmed fish and shellfish harm the environment.
But that is only partially true. The environmental impact depends on what is being farmed and the method used. That is what I hope to clarify in this article.
Production Method Matters for the Environment
When we talk about aquaculture potentially harming the environment, it typically relates to the emission of nitrogen, phosphorus, and CO?.
When it comes to fish farming, we must distinguish between different production methods. There are significant differences in emissions depending on the type of facility used.
There are three main types of systems. In Denmark, most fish are produced either in land-based ponds — known as model fish farms — or in large open sea cages, called marine net pen farms.
A third method is called recirculating aquaculture systems (RAS). These consist of fish tanks and a “purification system” that cleans nearly all the water before reusing it, minimizing water usage.
In land-based fish farms and RAS systems, nutrient emissions are significantly reduced through various filtration methods.
Of course, RAS systems also have high electricity consumption for pumping, water purification, cooling, and more. Many facilities compensate for this by using green electricity from wind turbines or solar panels.
What you farm also matters for the environment
To further nuance the picture, nitrogen, and phosphorus emissions from aquaculture depend heavily on what is being farmed.
For example, farming seaweed, mussels, or oysters reduces nitrogen, phosphorus, and CO? in the environment.
In other words, this type of farming may even benefit the environment — or at least not harm it.
Seaweed absorbs nitrogen and phosphorus and also converts CO? into oxygen through photosynthesis. When the seaweed is harvested, the nutrients it has absorbed are removed from the marine ecosystem.
Mussels and oysters filter large amounts of microalgae, which also absorb nitrogen, phosphorus, and CO?. Thus, when mussels or oysters are harvested, those nutrients are also removed.
However, it is important to note that oyster and mussel farming is often criticized for polluting the seabed directly beneath the farms.
That is because mussels produce so-called “pseudofeces,” similar to bird droppings. If you look beneath a mussel or oyster farm, you will find many of these droppings, which can cause local pollution.
However, a recent study from the University of Copenhagen shows the opposite: Denmark’s largest mussel farm had higher biodiversity beneath it than comparable reference areas.
In Denmark, a further challenge with seaweed, mussel, and oyster farming is the need for many lines, buoys, and pipes. One solution is to submerge the farms to make them less visible from the shore.
It is All About Location, Location, Location
Returning to fish, rainbow trout is the most commonly farmed fish in Denmark.
Rainbow trout begin their lives in land-based fish farms and are later, in the spring when they weigh about 600 grams, transferred to big open sea cages. They are harvested in the fall when they weigh between three and five kilos.
Currently, marine farms do not have the same ability to capture nutrient waste, organic material, medicine residues, or chemical additives as land-based systems do.
There is no doubt that marine farms are the most polluting method of fish farming in Denmark, as both nitrogen and phosphorus are released directly into the sea environment.
But if we compare this with other sources of nitrogen and phosphorus in our oceans, the picture becomes more nuanced.
According to the Danish Environmental Protection Agency (Milj?styrelsen), in 2021, Danish fish farming emitted 894 tons of nitrogen and 74 tons of phosphorus annually.
In comparison, wastewater treatment plants emitted three times as much nitrogen and phosphorus. It is important to note that emissions from agriculture are not included in the EPA's report.
In 2021, the average nitrogen emissions from agriculture into our waters were 55,000 tons.
In that perspective, aquaculture is not the major nitrogen polluter —we are, through wastewater systems and agriculture.
That said, we still need to be careful about the aquaculture. There’s no doubt that our coastal waters — such as fjords and Lilleb?lt (the Little Belt) — are under pressure from years of excessive nitrogen pollution.
We know we need to reduce nitrogen emissions to our marine environments by one-third to achieve what is known as “good ecological status.”
That’s why most marine farms today are in areas with strong water currents. And it makes little sense to place marine farms in the most coastal parts of Denmark, as they are already heavily polluted with nitrogen.
However, more offshore facilities in areas with little or no nitrogen stress could be a future option for expanding marine farming.
But why even talk about marine farms — or expanding them — in an already nitrogen-stressed marine environment?
Maximum Food, Minimum Emissions
It is about producing the most food per kilogram of nitrogen emitted. But how?
A simplified way to view this is through the feed conversion ratio or FCR.
FCR tells us how much feed is required to produce one kilo of meat. A lower FCR means the animal is more efficient at converting feed into body mass.
Salmons, for example, have an FCR of 1.1, meaning 110 kilograms of feed produces 100 kilograms of fish. In comparison, poultry has an FCR of 2.2, pigs 3, and cattle between 4 and 10.
When an animal is inefficient at converting feed into meat, the unused nutrients are released as nitrogen and phosphorus.
This means that per kilo of fish, emissions are lower than for other livestock. So in aquaculture we can produce the most food with the least emissions.
Other Challenges?
In Norway, one of the main problems with marine farming is sea lice — parasitic copepods that attach to the fish scales and cause open sores.
These lice can reproduce in large numbers within a marine farm and then spread to wild salmon.
However, this is not a problem in Denmark, as the salinity is too low for sea lice to survive in the waters where rainbow trout are farmed. It is something worth considering if marine farms are in saltier areas.
Another challenge is that marine cages can be damaged in autumn storms, potentially releasing farmed fish and causing genetic pollution in local wild salmon populations.
This is a limited problem in Denmark because we farm only female rainbow trout, which are not native here. Even if mating occurred, it would not be successful.
Furthermore, farmed fish are accustomed to feed pellets and are not competitive with wild salmon, meaning they would likely not survive in the wild.
However, escaped mature rainbow trout can attempt to spawn in rivers, disrupting natural spawning grounds for wild trout and salmon. Fortunately, marine cages rarely break or are damaged.
What Can You Do?
There are many nuances when it comes to fish farming. But as this article shows, eating farmed fish isn’t necessarily bad for the environment — especially if you make informed choices.
Look for the ?-label or ASC label (both labels for farmed fish), or the MSC label (for wild-caught fish).
If you want to dive deeper into the topic, I recommend my earlier article: Mythbusting: Farmed Fish Aren’t Full of Antibiotics and Won’t Pollute Your Body (Original: Mytedrab: Opdr?ttede fisk er hverken fyldt med antibiotika og sviner heller ikke din krop til)