Can mussels be a secret answer to sustainable aquaculture?

When we talk about food security we tend to think about livestock or land production. However, many people around the world, especially those in coastal areas, also heavily rely on fish and seafood. In some countries, fish accounts for more than 25% of animal protein intake and some low-income countries depend on fish as a staple food even more.

Fish farming – or aquaculture is probably the fastest growing food-producing sector and is practised both by multinational companies and smallholders. You might consume more fish from aquaculture than you think – whether you are a fan of sushi or prefer fish and chips, most likely, every other fish on your plate was produced on a fish farm. Today, almost half of the world’s fish comes from aquaculture. This fact looks even more impressive when we put it into perspective – back in the 1970s, aquaculture produced only 6% of all food fish. Moreover, the demand for farmed fish is predicted to rise further because of an emerging global middle class alongside increasing population. 

I am a big fan of sushi myself and that in combination with the rising global demand for fish got me interested in looking at fish farming more closely. I had an opportunity during my bachelor’s studies in environmental science when I participated in an experimental research study for my thesis. One of my teachers, Prof. Håkan Olsén at Södertörn University was looking for two students who would continue his research work on sustainable fish feed. So I, together with another equally enthusiastic student, whom I was lucky to get to know thanks to the project, carried out this exciting experiment.

Coming into the project we believed, as many people do, that increased fish farming although not free of ecological impacts, relieved pressure from ocean fisheries. However, we soon came to learn that some aquaculture systems put even more pressure on wild fish stocks. This is greatly due to the high demand on carnivorous species such as tuna and salmon, that require large inputs of wild fish for feed. At the same time, 53% of the world’s fisheries are already fully exploited due to overfishing. So, to contribute to a more sustainable future, our experiment set out to examine if fish can be brought up on a diet consisting of mussel meal instead of fishmeal and fish oil.

Mussels are natural filter feeders that remove particles of organic matter from waterways. Their natural filtering power can be used to strain out pollutants and thus improve water quality in lakes over-enriched with nitrogen and phosphorous. Mussels can also help reduce the environmental impacts of fish farms by filtering their wastewater. Furthermore, in 2013 Swedish scientists discovered an effective way of getting mussels out of their shells by converting them into a liquid state, enabling mussels to be used more profitably as feed in future aquaculture.

Professor Olsén is somewhat of a fish fanatic (however his professional knowledge on the matter should not be underestimated) and has a garden pond where he keeps crucian carp. Much of his previous research was done on this fish species and he lent us a group of 21 carp to conduct our research on. Crucian carp are freshwater fish that have been cultivated in China for nearly 2000 years in ponds and rice paddies. This method, with little need to nurture the animals, is still widely practised. Also, the production of crucian carp has increased considerably in China, especially for local consumption by relatively low-income households.

Crucian carp is also commonly eaten in some parts of eastern Europe. The fish is delicious in soups or simply fried, has good meat quality and high nutritional value. However, it contains a large number of fine inter-muscular bones, which makes it difficult to eat for people who are not used to picking bones from their meal. As a result, crucian carp is not particularly popular in western countries, where production of salmon, shrimp and other high-value species are increasing instead.

Crucian carp is an omnivore and doesn’t require the same type of protein as carnivorous fish, so we were eager to find out if the species would acknowledge mussels as food. More precisely, the aim of our study was to evaluate if the extract of mussel meal can induce foraging in the crucian carp. Induced foraging means that the fish starts looking for food and this could indicate that the fish could become accustomed to eating mussels, even though mussels are not part of its natural diet.

The experiment was conducted in a dark and quiet laboratory (to ensure the fish wouldn’t become stressed) equipped with aquariums. Very simply put, we were pumping mussel meal extract through a small plastic tube into each aquarium while recording the behaviour of the fish during 5 minute periods. Foraging in the crucian carp consists of very distinctive mouth movements, that can easily be observed and counted. Then, we tested various other extracts in different concentrations, consisting of amino acids and regular carp feed. The results between the different extracts were later compared and statistically tested.

As it turns out, the crucian carp is not a fan of mussels. In fact, our results revealed that the fish were more attracted to almost every other extract tested. However, the only compound that was statistically proven to induce foraging was regular carp feed. This may be due to the fact that that’s what our fish has gotten accustomed to eating since it has been their regular meal in the pond.

However, even though crucian carp might not be a fan of eating mussels, similar studies have been conducted on other fish species with promising results. Turbot, common sole, arctic char and olive flounder, all of them carnivores, have accepted mussel meal as feed or an attractant in previous studies. Some species even preferred mussel meal over fishmeal and gained weight better eating mussels.

So far, a perfect replacement for fishmeal in aquaculture has not been found. However, future fish feed might actually be a blend of alternatives. Considering, for example, that different fish species seem to react differently to mussels as feed, the ideal solution might be to customize fish feed for each species. Research on this matter is not an easy task, but an important one if we want to save the remaining fish stocks from extinction and ensure that future generations can enjoy sushi, like us!

You can access the full study here!

This blog post is written by Ninél Sukovich, communications intern at SIANI and recent graduate with a BSc degree in Environmental science from Södertörn University, Sweden.