Aquaculture production in Asia surpassed capture fisheries in 2012. From 1990 to 2008, demand for fish oil for use in commercial aquaculture feed increased considerably with an estimated aquaculture share of global use of fishmeal at 10% to 58% and fish oil at 17% to 81%. As such, the demand for fishmeal and fish oil particularly for aquaculture use, is estimated to increase, consequently reducing availability and increasing the price.

A survey published in 2011, from 1995 to 2010 showed that replacement of fishmeal in aquaculture feed for marine fish and shrimp improved feed conversion ratio (FCR) when fishmeal inclusion levels were decreased from 50% to 26% and 28 to 16%, respectively. This indicated a successful substitution of fishmeal in aquafeed, but utilisation of fishmeal still increased due to rising production of aquaculture feed. A progressive reduction of wild fish into aquafeed is vital for sustainability of the industry and one of the top international research priorities.

For decades, studies on fishmeal substitution were conducted on many cultured aquatic species and the growing number of scientific publications indicated that some could be grown on fishmeal-free feeds with amino acid supplements. Current studies on potential replacements for fishmeal and fish oil not only assess their effects on growth but also include fish health and meat quality. The long list of feed ingredients evaluated for fishmeal and fish oil replacements include plant-based sources, rendered by-products, terrestrial/marine animal sources, and some unconventional sources. Plant-based ingredients are considered infinite sources and thus many kinds of legumes, cereal grains, oil seeds and leaf meals have been assessed in a large number of aquaculture nutrition studies. Digestibility studies are usually conducted for these sources. Use of plant-derived materials is limited due to a wide variety of anti-nutritional factors (ANFs) such that there is only a certain range of fishmeal protein replacement from these sources. A review on ANFs in 2001 tentatively concluded that levels usually present in fish diets using commercially available plant derived sources are unlikely to affect fish growth performance. Recent findings, however, showed that interactions between effects of ANFs appear very important and microbiota in intestine may modify the effects of ANFs. Removal of ANFs by improved processing methods and supplementation with limiting amino acids and incorporation of enzymes can increase inclusion levels of plant-based ingredients. However, plant protein sources can replace fishmeal at lower levels compared to animal protein sources.

Soybean meal

Generally, the basis for using feed ingredients as replacement for fishmeal or fish oil are nutrient values especially crude protein and lipid quality, acceptability by consumers, market price, commercial availability, yield of good growth and healthy fish, and preference for sustainable and renewable materials. Among the plant protein sources, soybean is quite adequate in these respects as it is also a source of lecithin and oil aside from a good level of crude protein. Soybean meal is widely used as a major plant protein source in aquafeed and the most-studied replacer for fishmeal. However, the soybean mel market price is unstable as there are other industrial uses that can increase the price. This development has driven research efforts to assess other plant sources suitable to substitute soybean meal in formulations in addition to fishmeal replacement. Other soybean-based products such as defatted soybean meal (50-54% crude protein), soybean protein concentrate (SPC, 67-72% crude protein), soy protein isolate (SPI, 90-92% crude protein), and fermented soybean are also utilised to reduce fishmeal inclusion either singly or in combinations with other plant-based ingredients as vegetable blends, with good results. The level of fishmeal substitution by soybean meal can be significantly increased by gamma irradiation. A fishmeal-free diet (30% crude protein) of soy composite diet containing poultry waste was most economical compared with other plant-based ingredients in tilapia. However, a well-documented limitation of soybean meal is the resulting inflammation of the distal intestine in some farmed fish at certain inclusion level.

Other plant sources

Plant-based sources include peas, lupins, corn, rice, canola, rapeseed, barley and wheat. Utilising these sources to replace fishmeal is not easy particularly in formulations for carnivorous species that require high dietary protein. Improved processing techniques enhance the nutritional contents of plant-based sources and increases utilisation as fishmeal replacement. Processing can remove ANFs and increase crude protein levels. Published information shows that it is possible to substitute most if not all the fishmeal in practical diets with plant proteins in combination with other plant protein sources or with proteins from other sources. In some species, plant-based ingredients promote growth with high quality fillets while in others total fishmeal replacement is possible but with lower biological performance. One particular plant-based ingredient is rice protein concentrate, which is a good raw material due to its high crude protein (75%) and lipid (11%) content with no adverse effect on fish growth up to 20% inclusion level. However, the base material is rice and its use may not be encouraged in areas where it is a basic foodstuff of the people.

Terrestrial animal by-products

The good nutritional quality of terrestrial animal by-product meals make these suitable substitutes for fishmeal. Blood meal, meat and bone meal and poultry by-product meal have been used in aquafeed production and it is estimated that quantities of these by-products are higher than fishmeal. These ingredients however, should be processed satisfactorily to prevent disease transmission. Although there has been an importation ban of these products by some countries years ago, evidence suggests that the harmful mammalian causative agent in these by-products is different from that of fish and apparently is not able to cross the intestinal barriers of fish. A good source of crude protein is meat and bone meal (50%) but the high ash content of 29% limits its use as replacement for fishmeal. An improved processing method to decrease ash level could make this material a valuable alternative protein source.

Non-traditional feed Ingredients

Some other sources which can be considered renewable have been evaluated as fishmeal replacement, including insects, bacterial proteins, earthworms, silk worm pupae, mopani worm, and maggot meal, among others. These have good potential as replacers for fishmeal, but there are several considerations for their usage such as production cost, acceptability by consumers, and commercial availability. Increased research efforts on these ingredients coupled with technological innovations could make these potential fishmeal substitutes important and widely used in the future.

Fish oil

Fish oil is good source of omega-3 long chain polyunsaturated fatty acids (n-3 LC-PUFA) and is a high-cost ingredient in aquaculture feed with current prices more expensive than most protein sources. For every 100 kg of raw fish about 25kg of fishmeal is produced and only 5 kg fish oil. An important consideration in fish oil replacement studies is the “fish in to fish out” ratio (FI/FO) as there are farmed species that require n-3 LC-PUFA that exceeds the supply of residual oil in fishmeal. Studies on fish oil replacement are numerous and most substitutes are vegetable oils or blends of vegetable oils or with terrestrial animal fat. Most cultured fish require n-3 LC-PUFA and research is focused on how to supply sufficient amount to maintain level in farmed fish similar to wild fish. The fatty acid profile of a lipid source is reflected in the lipid profile of fed fish which could affect taste. Farmed fish feeding with vegetable oil in formulations are provided with finishing diet containing fish oil for a certain period to restore level of n-3 LC-PUFA at harvest. Lipid metabolism in fish is also affected by high level of carbohydrate content in plant protein sources. In a crustacean species, digestibility of lipid in protein-rich animal feedstuffs is low compared to carbohydrate-rich plant feedstuffs. Some lipid sources used as fish oil substitutes include oils from seeds, beef tallow, poultry fat, and DHA-rich microalgae. Crude lipid in commercial feed for low value species or herbivores is about 2-3%, and as such, fish oil substitution may not be necessary.

Issues and gaps

The nature of alternative sources for fishmeal and fish oil may affect quality of fish fillet and consumer acceptance; hence, sensory qualities of aquaculture products derived from using alternative sources need to be evaluated. An economic evaluation and assessment of available quantities for commercial utilisation should be addressed, and attention to possible industrial users of these alternative sources is important as these may compete on volume for aquaculture use.

Way forward

Quantities of fishmeal and fish oil in aquafeed formulations in recent years have been substantially reduced and publications suggest that it is still possible to reduce inclusion levels without sacrificing growth performance and utilisation of nutrients. To attain this objective there should be:

• A sustaining effort in improvement, refinement and modifications of aquaculture feed formulations through focused research.
• Creation of a well-coordinated group among concerned stakeholders (government fisheries research agencies, public and private sector) in the conduct of important research to improve amino acid profile of legumes and increase n-3 LC PUFA of plant-derived oils, and genetic selection for a fish strain/stock that can efficiently utilise plant-based or non-traditional ingredients.
• Technological innovations on less costly processing methods.
• Information or research results on alternative sources for fishmeal and fish oil for different growth stages of farmed fish such as digestibility, processing of ingredients, anti-nutritional factors and optimum inclusion levels without affecting fish health should be made available to users.