The Fish Meal Dilemma - What are the Alternatives?

All involved in aquaculture are aware of the challenges, that the limited global supply of fish meal and fish oil represent for the industry. Numerous scientific papers and presentations, press news and discussions, as well as political documents address this situation. To secure high production efficiency, good health and welfare of the fish and crustaceans in this expanding industry, investments in basic as well as applied research and development are still needed. Actions are taken along the whole production chain to overcome the obstacles caused by this shortage of marine sources: new nutrient sources are being explored from the animal, plant, insect and microbial kingdoms; innovation is stimulated to find optimal processing methods and equipment for ingredients and feed; nutrient requirements of the various fish species are estimated and re-estimated to reach a higher level of accuracy in diet formulations; and breeding programs aiming to improve fish tolerance to new ingredients are underway. These developments take place in the light of rapid growth in the human population as well as in the standard of living with corresponding increase in demand for food in general and for high quality foods such as fish and meat. Other important factors include the increasing pollution around the world. Waste deposits grow and organic resources, including huge amounts of food, become unusable due to contamination with heavy metals and other pollutants. At the same time, sources of essential nutrients are in danger of being depleted. The world’s phosphate resources are of the most critical. Thus the search for alternatives for fish meal and fish oil must be carried out in the light of the global need for more food, waste reduction and resource preservation. The challenges that the aquaculture industry faces as an industry are considered to be of great importance for future food security around the world, and necessitate a holistic approach as outlined in the principles of circular economy (European Commission, 2015).

High quality fish meals and oils are important sources of long chain ω-3 fatty acids, i.e. EPA and DHA, essential for most animals, including humans, and in particular for a number of marine fish and shrimp species. Fish meals supply amino acids in a balance that outcompete most plant sources, and are also good sources of many essential vitamins and minerals. Moreover, the nutrient concentration per unit of available energy is high, while the level of indigestible material is low. Most fish and shrimp require diets with 20-30% essential amino acids, i.e. 40-60% total protein, and 1-2% essential fatty acids. Hence, relative to volume, the demand for amino acids is by far the highest. However, the demand for essential fatty acids may be the most critical at present, as the number of alternative sources is limited and most are marine based. Solutions for the shortage in EPA and DHA may lie in modern plant and microalgae breeding techniques, non-GM as well as GM, which already have come quite far in offering possible replacements for fish as sources of EPA and DHA. In the longer perspective, finding alternative protein sources may be the most critical.

The following presents an overview of resources suggested as alternatives to fish meal for aquaculture, many of which already are the focus of investigation around the world. The reviews of Sørensen et al. (2011), Tacon et al. (2011) and Olsen (2011) have supplied useful information for the present overview.

Animal sources

Although wild fish catches worldwide have reached a plateau and may show a decreasing trend, fish meal may continue to be an important nutrient source for aquaculture since it may also be secured by more efficient use of offal from the fish processing industry, in addition to by-catches from the commercial fisheries. Further improvements in preservation and drying technology will increase fish meal quality and may thereby aid in the optimal utilization of these highly valuable fish resources. Among marine nutrient sources other than fish that may supply nutrients for the aquaculture industries, are marine organisms belonging to lower trophic levels. Two crustaceans with body compositions closely matching nutrient requirements for fish and shrimp have been considered, i.e. the large planktonic copepod Calanus finmarchicus living in the Norwegian Sea and the Antarctic krill (Euphausia superba). Particularly, their content of long chain ω-3 fatty acids, mostly incorporated in phospholipids, as well as astaxanthin contribute to their high value. High palatability for fish is also an appreciated characteristic. Annual production of the former has been estimated to be 200-350 million tons, and harvest of as little as 1% would represent a valuable contribution to the nutrient supply for aquaculture. However, lack of suitable harvesting and preservation technologies, as well as discussions regarding possible impact on the natural fish stocks, challenge the use of these zooplankton as nutrient sources in the near future. Krill, for which annual biomass production is estimated to be more than 100 million tons and the catch quota is 5.61 million tons, is already harvested (0.2 million tons) and supplies the food and feed industry with high value products, but also at high costs. The cytoskeleton of krill may set a limit for full replacement for fish meal in fish diets due to high chitin, fluoride, copper and cadmium content. Cost efficient removal of the cytoskeleton from krill products can open up for full replacement of fish meal by krill meal. There is room for an increase in krill harvesting, but expensive improvements in the harvesting and processing technologies may be required.

Recently, mussel products have appeared as potential fish feed ingredients. Feed grade mussel products, i.e. by-products from commercial mussel production are available as feed ingredients. A new type of mussel production is emerging, which may supply larger amounts, i.e. mussels produced in co-cultures with fish and macroalgae. The aim of this multitrophic culture is to recycle nutrient losses from fish production to the environment by growing mussels and seaweed in the near vicinity. If harvested and processed properly, i.e. avoiding algal toxins, enzymatic and microbial deterioration, and removing excess shell material, mussel products have been found to be excellent nutrient sources for fish and shrimp. They also contain astaxanthin, an essential nutrient for fish and shrimp, for which there are also limited resources.

Byproducts from the land animal industry, such as meat, blood, skin, feathers and bone, have potential and many of these have already been used with success as replacement of fish meal in fish and shrimp feed. Challenges include their low level of long chain ω-3 fatty acids, the relatively high melting point of the lipid fraction that can limit suitability for cold-water fish species, low availability of cysteine in feather meal may impair amino acid balance, and a high iron level in blood meal may cause oxidation of essential compounds such as astaxanthin. Moreover, such products vary greatly in nutritional value due to variation in chemical composition, differences in solubility of the raw materials, as well as variation in processing conditions, for some products quite harsh. Improvements in processing technology are a R&D focus, however, with great potential for increased refinement, stability and nutritional quality of these land animal products, hence opening up for increased use in aquaculture. An additional challenge is the fact that, in the EU, such products have been abandoned for use in animal feed for many years following the BSE scandal. The ban for use in fish feed was recently lifted, but many feed companies are still reluctant to use them, mainly for fear of negative consumer reactions. Acceptance and use of such by-products is expected to improve with time. However, their potential as raw materials for products of even higher value, e.g. as foods and as nutra- and pharmaceuticals, may increase their price and thereby limit their use for fish feed.

Insect larvae products, e.g. from Tenebrio molitor (meal worm) and Hermetia illucens (black soldier fly), have gained much attention recently as they appear to have great potential as nutrient sources for animal production, including fish and shrimp. They are “nature’s own invention” for recycling and upgrading of organic material. Great research efforts are underway to develop suitable production technologies for various insect species, to clarify nutritional value and possible beneficial health effects, such as immune stimulation. Their chemical composition indicate high nutritional quality, although challenges regarding stability of the lipid component have been revealed. One reason for the optimism regarding insects as potential feed ingredients is their efficiency as converters of organic material, such as biproducts and waste from agriculture and the food chain, to high value products. Insects may upgrade waste, which presently are used for heat production, to high quality feed, and possibly food ingredients. Although insects are eaten by many populations around the world, and potentially may become food even for people in the Western world, their potential as nutrient sources for animals is most likely more immediately applicable. Also, earthworms with a high protein level in the dry matter, seem to have a similar potential as useful feed ingredients. They have, however, gained lesser attention, although their conversion of low-grade organic material may outcompete that of insects due to their symbiosis with microorganisms.

Plant sources

Protein-rich plant seeds and refined, high protein products of grains have been investigated as fish meal replacers in fish feed for more than three decades and are used more and more in fish and shrimp feed.

They are the nutrient sources that have allowed the increase in aquaculture production over the last 30 years in spite of limited fish meal and oil supplies. The most commonly used plant ingredients are whole or refined products of soybeans, rapeseeds/canola, sunflower seeds, lupine seeds, pulses, peanuts, corn and wheat. Legumes and grains are well-established ingredients in feed for land animals and are highly available globally at competitive prices. However, none have demonstrated potential as the sole source of protein, replacing all of the dietary fish meal. The reasons are several, e.g. the relatively low protein content in many of these products; suboptimal amino acid balance; high content of indigestible carbohydrates; and variable content of one or more of several so-called antinutrients or antinutritional factors (ANFs). The latter reduce feed efficiency, may cause health problems and can even render a resulting fish product of low value and low palatability. Mixing lower levels of various plant products is now common for optimization of nutrient balances and limiting levels and effects of ANFs. Regarding lipid quality, most plant oils have low levels of ω-3 fatty acids and show absence of the long chain fatty acids required by marine species, whereas levels of ω-6 fatty acids are high. The exception is rapeseed oil, which is useful due to its low level of ω-6 fatty acids and high content of monoene fatty acids, which allow for higher inclusion in fish diets without affecting the ω-3/ω-6 fatty acid balance. Most plant protein sources used in fish feed are defatted. By-products from the starch industry with their high protein content, e.g. maize and wheat gluten meals, have become useful in fish feed production. Also by-products from the beer and biofuel industries often contain high protein content, and therefore have the potential of becoming protein sources in fish feed. The condition is that the processing and handling leaves the amino acids available and that the hygienic quality is safe, e.g. regarding microbe and mycotoxin levels. Future developments and innovations in plant breeding and processing may improve the nutritional quality and reduce the content of ANFs, allowing higher inclusions in diets for aquaculture. However, as with bi-products of animal origin, improvements in nutritional value of plant-based product also make these products more suitable for human consumption, enforcing the discussion regarding the competition between animal and humans for food. An added disadvantage of using terrestrial plant and animal nutrient sources to supply aquaculture demands is the continuous and largely irretrievable loss of valuable nutrients from land to sea.

Microbial ingredients

Biomass with a high protein level, as well as appreciable content of ω-3 fatty acids, may be produced from a wide range of organic resources from microorganisms such as bacteria, yeast, fungi, and microalgae. The available nutrient substrates for their production range from the simplest organic molecule, i.e. methane, to complex byproducts from agriculture and food industries, from the forest and cellulose industry, from macroalgae etc. The nutrient composition and safety of such biomasses vary greatly, depending on the microbial species used as well as the production conditions. Researchers foresee that production of microbial biomass may be tailored to fit the requirement of specific fish species regarding amino acids, as well as lipid and other essential nutrients. The perspective indicates that, in the future, microbial biomass produced on waste or unused, organic resources, may become major nutrients sources for fish as well as other animals. However, the bioavailability of the nutrient sources, firstly of the substrate for the microbes, and, secondly, of the microbes for the animals, is a challenge. Innovations are needed for tailoring of enzymes, which can release nutrients from various substrates for growth of the microbes as well as for development of equipment for biomass production. High investment costs for the production facilities is foreseen. Another challenge is the cost of the research needed for characterization of nutritional value and safety of microbial biomass.

Conclusion

A great variety of feed ingredients partly replacing fish meal have been included in feed used in aquaculture over the last decades. More are in the pipeline that have the potential to further enhance the sustainability of the aquaculture industry. Realization of the forecasted growth in this industry demands continued efforts to find yet unexplored nutrient sources. They will most likely be found among the categories described above, but may also be developed from other resources, yet to be recognized.

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