About 70% of fish and crustaceans produced in aquaculture are fed with protein-rich feed. If the aquaculture sector is to reach its growth targets by 2025, annual feed production must increase by approximately 38 million tonnes. This is not possible with today’s protein sources. The search for alternatives has become the central question in feed research on which the further development of aquaculture depends.
Global production of fishmeal is currently fluctuating around 4.5 million tonnes per year, of which 69% is used for fish feed. For fish oil, aquaculture already uses 75% of the annual production of around 0.9 million tonnes. Both resources are therefore scarce and correspondingly expensive. In order to meet the rising demand for protein sources for fish and shrimp feed, economical alternatives must be found that fulfil several criteria.
Modern aquaculture feed is already an artificially balanced blend of the most varied ingredients, which only together optimally meet the nutritional requirements of the respective aquaculture species. Both plant-based protein components such as soy meal, corn gluten meal or rapeseed meal, as well as animal byproducts (e.g. meat, bone or poultry meal), are used as alternatives to fish meal in aquaculture feed. Thanks to new technologies, fungal and algal proteins, insect protein, olive pomace and even a variety of grasses are also being considered. The overarching goal of these efforts remains to reduce the demand for fishmeal and therefore the proportional demand for the wild catching of industrial fish for aquaculture feed. But using plant-based ¬components in feed does not necessarily mean greater sustainability. The cultivation of soya beans, for example, has led to deforestation in some regions, which significantly harms biodiversity and the climate.
It has already been demonstrated in feed experiments that the cultivation of carnivorous fish species such as salmon, trout or saltwater fish with feed that does not contain fishmeal is possible in principle. However, across-the-board use of this scientifically blended feed in practice is not possible at the moment, because the cost is still too high and would make cultivation of the fish unprofitable.
Microbial biomass and insect meal
An interesting approach to solving the protein problem is the use of microbial biomasses, such as those concentrated in microbial flakes (biofloc). These protein-rich resources can be exploited as wet or dry meal and are consumed by many herbivorous and omnivorous fish species. Tests with tilapia hybrids show that with bioflocs, up to 50% of the protein in the feed can be provided without seriously restricting the growth of the fish. However, the quantities available are still far too small to be of practical relevance for the time being. Irrespective of this, significant potential is attributed to microbial proteins or single-cell proteins made up of bacteria, yeasts and microalgae for aquaculture feeds. These organisms have a high protein content of 45 to 65% and an amino acid profile that is comparable to fishmeal. The nutritional profile of yeasts and bacteria can also be controlled through the composition of the culture medium. The suitability and usability of these protein sources, must, however, be separately assessed at species level, because the absorption rates, digestibility and bioavailability of nutrients in the microbial biomasses can vary.
This also applies to insect meals, which would also be a promising protein alternative due to their nutritional profile. The main focus, apart from the yellow mealworm (Tenebrio molitor), the common housefly (Musca domestica), tropical house cricket (Gryllodes sigillatus) and Turkestan cockroach (Blatta lateralis) as well as the domestic silk moth (Bombyx mori), is mainly on the black soldier fly (Hermetia illucens), the larvae of which come relatively close to the raw protein content of fishmeal. However, the biochemical composition, amino acid and fatty acid profile of the larvae vary significantly depending on the stage of development and feed substrate, making their exploitation in feed difficult and costly. If the larvae grow on food waste, which would be very advantageous from a hygiene perspective, they put on a lot of fat, which however contains hardly any omega-3 and more omega-6 fatty acids instead. Since lipids are prone to oxidative decomposition, larvae meal such as this usually needs to have the fat removed, which means additional costs. Generally, the lipid quality of the insect larvae can be manipulated through the substrate used. In order to increase the EPA and DHA content, the larvae can be fed on fish waste, but this can also be used directly as raw material for fishmeal, which is more cost-effective. The nutritional value and composition of the larvae also depend very significantly on their stage of development. The closer they are to pupation, the greater their chitin content, which influences the proportion of fibre and thus digestibility.
Do insect meals improve the immune system?
Properly prepared, dried and de-fatted, and appropriately dosed, larvae meal of the black soldier fly can replace a portion of the protein in fish feed. For cultivation of young sea trout, a test was done with feed that contained 20% insect larvae meal, and no difference in growth was detected compared to the standard feed containing fishmeal. This is not surprising, since flying insects are also part of the food spectrum of numerous wild salmonids in natural waters. It is more unusual that larvae meal is also suitable for the cultivation of shrimp. For experiments with Pacific white shrimp (Litopenaeus vannamei), 20% of the fishmeal was replaced with de-fatted insect meal without the growth of the crustaceans being negatively affected. The researchers suspect that the insect larvae meal improves the immune system and intestinal microbiome of the shrimp. They detected an increased resistance to AHPND (acute hepatopancreatic necrosis disease). This only works if the proportion of larvae meal in the shrimp feed is not too high, however, as replacing 30% or more of the fishmeal results in degenerative effects.
Feed proteins made from leaf and grass biomass
Plant-based biomasses harvested from leaves and grasses are raw materials that are available on a huge scale and attractively priced, but are technically demanding to process. The few studies carried out to date on Leucaena and cassava leaf meal as well as with leaf protein concentrate from ryegrass and alfalfa have shown that plant leaves can be used as dietetic protein sources in fish feed.
Recent experiments with protein from the drumstick tree (Moringa oleifera), a fast-growing, drought-resistant plant from the Indian subcontinent, have achieved promising results with tilapia, grass carp, rainbow trout, shrimp and other species. Moringa meal has a high nutritional value, is rich in fibres, proteins, vitamins, minerals and lipids and also has remarkable pharmacological qualities. It is claimed that these plants, among other characteristics, have anti-inflammatory and anticarcinogenic, antidiabetic, antioxidative, antimycotic and antibacterial properties. However, further studies are necessary before they can be used in aquaculture in order to determine the appropriate dosing.
Dry meals made from Kikuyu grass (Cenchrus clandestinus) have already been successfully tested as a protein replacement in diets for tilapia rendalli. Kikuyu grass has a relatively high protein content and a good amino acid profile. It comes from the east African highlands but has spread worldwide, is easy to cultivate, still largely unexploited and is correspondingly cheap. An initial feed test with 20% Kikuyu grass meal shows that it is highly usable. However, for high admixtures around 30%, the growth of the fish decreased, which the researchers attributed to the increased fibre content as well as the lack of important amino acids, above all methionine and lysine.
A Danish aquafeed manufacturer, which is testing the usability of 7,000 tonnes of an organic protein concentrate made from grass from BioRefine in trout feed, demonstrates that feeding fish with grass protein is not just a fad among eccentric researchers. The protein concentrate has a similar nutritional profile to soya bean meal and it can also be produced regionally and in a climate and environment-efficient way.
Olive pomace and micro and macroalgae
Due to their high concentration of valuable lipids, water-soluble carbohydrates and polyphenols, the byproducts of olive oil manufacturing are also being considered as potential raw materials for fish feed. Byproducts can be used as an alternative to wheat bran. This raw material has already been successfully tested in feed tests on the African Clarias catfish and tilapia.
However, the greatest hopes of the feed industry are currently resting on the production of micro and macroalgae, especially as they contribute not just to the solution of the nutrition problems in aquaculture, but also to the improvement of marine health and climate protection (algae bind carbon dioxide and reduce the quantities of phosphorus and nitrogen in the seas.) Macroalgae forests also act as a nursery and hiding place for many marine animals, thereby promoting biodiversity underwater.
Microalgae have the potential to replace fishmeal and fish oil in aquaculture feeds. With raw protein contents of up to 65% and lipid contents of up to 40%, their nutritional profile is very comparable to terrestrial plant and animal sources. However, the current global production of microalgae (autotrophs and heterotrophs), at an estimated 50,000 tonnes per year, only covers 0.7% of the actual protein demand of the feed industry, which is much too little to substitute for fishmeal protein in fish feed. In addition, there is a significant difference between with prices of microalgae and soya meal. It can therefore hardly be expected that microalgae will become a viable alternative protein source for fish feed. However, interest from the industry is significant and even growing, since it was recently -discovered that the photoautotrophic marine diatom Phaeocystis pouchetii not only contains high concentrations of the omega-3 essential fatty acids EPA and DHA and vitamins, but can also deter sea lice when added to salmon feed. The results of feed experiments show that the number of lice parasites on salmon that were fed with feed containing diatoms was significantly lower. The scientists suspect that this effect was caused by oxylipin, a fatty acid-based compound containing oxygen. However, -microalgae are also increasing in importance for aquaculture as a source of minerals. For example, the microalga Nannochloropsis oceanica is rich in selenium, which is indispensable for the healthy development of the fish and the ¬functioning of their immune system. Selenium deficiencies can inhibit growth, increase mortality and hinder immune response. Algae selenium also has high bioavailability, i.e. it is present in a form that can easily be absorbed by fish intestine.
Although the potential of macroalgae is often overestimated (the proportion of raw protein is very variable and ranges from less than 1% to 48% of the dry weight of the biomass), it could also play a certain role in fish nutrition in aquaculture, because its proteins have just as many or more essential amino acids as agricultural crops and fishmeal. If the high content of complex polysaccharides (fibre), which negatively affects the digestibility of the algal protein and the nutritional value, can eventually successfully be reduced, macroalgae could also be considered for aquaculture feed. All the more so since they contain bioactive compounds that are linked to higher stress resistance and improved immune function for fish. It appears that macroalgae in feed stimulate the appetite and thereby indirectly contribute to better growth of fish and crustaceans.
Finally, however, none of the alternative protein sources can currently completely replace fishmeal. A high degree of flexibility is being demanded of the feed industry regarding the composition of its aquaculture feed, especially since raw material prices are volatile and are likely to rise further.