Using Microalgae In Sustainable Aquaculture
Wild fish populations are in decline across the world, as they are being exploited beyond their ability to replenish themselves. Aquaculture, or the farming of aquatic species including finfish, crustaceans, mollusks and aquatic plants, such as algae or seaweed, is seen as a promising solution to meet the world’s growing population’s growing demand for seafood.
The aquaculture industry has seen increased interest over the last few decades and is expanding three times faster than the terrestrial food production industry. An estimated 114.5 million tons in live weight of aquaculture produce was cultivated in 2018, according to the Food and Agricultural Organization of the United Nations (FAO). In response, the aquafeed market has also grown exponentially as around 70% of farmed fish is made up of direct-fed species, approximately 68% of which are dependent on commercially produced fish feed.
Fishmeal And Fish Oil
As you can imagine, the diet of terrestrial farm animals and aquatic farmed animals is significantly different. While most people have an awareness of what it takes to feed cows and sheep, fewer people are familiar with the dietary requirements of fish and other commercially farmed aquatic species.
Fishmeal and fish oils are commonly used as aquafeed as they are high in essential proteins, lipids, vitamins, minerals and pigments that are required for the optimum health and growth of aquacultured species. In particular, fish oil is included in aquaculture feed because it contains high levels of omega-3 long-chain polyunsaturated fatty acids that are not available from other sources.
Global reliance on fish meal and fish oils for aquaculture feed has been classified as a threat to both marine biodiversity and human food security. Around 70% of the world’s fish oil production is consumed by aquaculture farms.
Fishmeal and fish oil are produced from the trimmings of edible fish and from wild harvested populations of small, boney and oily fish that are often unappetizing for direct human consumption. Commonly used species include capelin, sand eel, anchovy, horse mackerel, pilchard and menhaden. Many fishmeal fisheries fish stocks have been reported to be fully exploited, which means that they are operating at the maximum sustainable yield. This is concerning as the demand for seafood and therefore aquafeed is only likely to increase in the coming years, especially with growing economies in the East. The gap in supply and demand has already resulted in a 300% increase in the price of fish feed.
Given the limited resources available for the production of fish meal and fish oil, researchers began looking for alternative solutions, such as terrestrial plant-based alternatives.
However, plant-derived protein alternatives were often not sufficient alternatives because of the low digestibility of tough plant cell walls and because most crops were found to be deficient in a number of essential amino-acids required by marine species. The deficit of digestible protein and suitable amino acid concentration was shown to affect the quality of the fish produced for human consumption.
While some fish feeds are supplemented with plant-based oils, such as rapeseed oil, these oils are often not optimal for marine species because they have high levels of omega-6 fatty acids but are low on omega-3 fatty acids.
Microalgae is believed to be a viable and sustainable alternative, or at least a substantial supplement, to fish meal and fish oil feed. Microalgae are either prokaryotic or eukaryotic microorganisms that are able to photosynthesize and therefore function as primary producers in a variety of aquatic ecosystems. They have a simple cell structure and are found either individually or in chains. They require only light, water, CO2 and nutrients, such as phosphorus and nitrogen, to survive.
Nutritional Value Of Microalgae
Microalgae is considered to be a promising alternative solution to fishmeal and fish oil because of the high nutritional quality and positive effect on growth rate found in feeding trials in a number of marine species. Trials found increased triglyceride and protein deposition in fish muscles, along with increased omega-3 fatty acid content and improved resistance to disease, decreased nitrogen output to the environment and increased physiological activity and carcass quality.
Commonly used microalgae in aquaculture currently include:
• Chaetoceros gracilis
• Tetraselmis suecica
• Pavlova lutheri
• Skeletonema costatum
• Dunaliella tertiolecta
• Nannochloropsis sp.
• Phaeodactylum tricornutum
• Chlorella sp.
Advantages Of Microalgae For Sustainability
Since fishmeal and fish oil production rely on wild harvests, the global supply is limited and, almost fully exploited as is. Cultivating microalgae on a commercial scale offers a viable solution to provide a sustainable source of feed to meet the growing demand from aquaculture. Microalgae are also able to grow in a broad range of habitats, they have very simple nutritional requirements, they can accumulate useful metabolites and some species also have a several-fold higher biomass production than terrestrial plants. In addition, algae can be grown in waste streams and absorb large volumes of CO2. In Norway, a ferrosilicon-producing company called Finnfjord is experimenting with trying to reduce their carbon footprint by feeding some of their carbon emissions straight into an algae pool. Another Norwegian company, AlgaePro, is experimenting with technology for cultivating microalgae using biowaste from municipal outputs and CO2.
Drawbacks Of Microalgae Production – And Possible Solutions
Currently the high cost of production of microalgae presents a challenge to the industry, as growing, harvesting, drying and creating pellets out of the microalgae requires significant quantities of time and effort and there is very little technology that has been developed specifically for this industry. However, as the industry grows and technology such as closed photobioreactors become more affordable and specialized mass culture facilities are built, it is likely that the costs will decrease. In addition, there is also a growing interest in biofuel and researchers are considering the possibilities of using the co-products produced by industrial algae for biofuel production in aquafeeds, which would also likely decrease the cost of production.
From a biological perspective there are other barriers to microalgae replacing fishmeal and fish oil production such as the hard to digest cell walls of some microalgae species. Indigestible cell walls can impact the availability of nutrients as fish are not able to extract the nutrients from within the cell if they cannot break down the walls. A possible solution to this is to add an additional step in the pellet production which breaks down the cell walls – though this may increase production costs.
There is also a risk of contamination if microalgae are grown at a commercial scale as diseases are able to spread very easily in monocultures (a culture with only one species). However, sterilization steps can be taken to reduce the risk of contamination.
Various microalgae also produce toxins – while commonly used microalgae have been tested for toxin production, there is often variation between strains even within the same species. The use of microalgae strains in aquaculture, however, needs to be approved by a number of regulatory authorities such as the Food and Drug Administration (FDA) and the European Food Safety Authority (EFSA).
While microalgae as replacement aquafeed is a promising solution to the global shortage of fishmeal and fish oil, there are a few challenges that still need to be addressed. It will be particularly interesting to see the development of technologies that allow for commercial scale production of microalgae that is economically profitable.