Finding a replacement for fishmeal has been a particular focus of global aquaculture research for over two decades and has led to a notable expansion of the catalogue of potential feed ingredients for aquaculture. Due to this increase in the number of potential ingredients the composition of fish diets has become much more complex. This allows feed manufacturers more flexibility but also requires more detailed knowledge and know-how. Our knowledge of fish nutrition is still incomplete in some areas, and a lot of valuable findings are not directly transferable from one species of fish to another. Fortunately, fish have no particular need for fishmeal, but they do need those nutrients that are naturally present in fishmeal in an optimal, well-balanced mixture. This means that fishmeal can actually be replaced in fish feed as long as the required nutrients are mixed together “artificially” in the right proportions and in a digestible form.
Untreated vegetable feed is unsuitable for many fish species
What at first sight might appear simple, however, is in fact quite complicated in practice if the fish are to thrive on a diet that is largely free from fishmeal and still maintain their health and nutritional value for the consumer. In addition, fish feed must be affordable. And the sharp rise in the price of fishmeal since the beginning of the millennium is one of the main driving forces behind the search for cheaper alternatives. Initially, it was assumed that vegetable raw materials were fundamentally unsuitable as feed for carnivorous fish species. This is because carnivores can only digest and utilise carbohydrates to a very limited extent. Carbohydrate-rich food damages their intestinal immune system, and causes inflammatory changes in the intestine and liver. The amino acid composition of plant proteins is insufficient and they often lack lysine and arginine whilst, on the other hand, the leucine content is often too high.
In addition, antinutritional factors (ANF) can impede the availability of nutrients in the fish feed, impairing nutrient utilization and metabolic performance. Some polysaccharides, phytic acids, protease and lipase inhibitors and oxalates, for example, can have antinutritional effects. The need is therefore not only to find suitable alternatives to fishmeal and fish oil but also to understand their functions and effects in relation to fish digestion. Alternative raw materials are often only suitable for inclusion in fish feed after appropriate processing. It is not enough for the required components to be present in a raw material: In order to actually “feed” the fish, they must also be available in a form that is digestible and utilizable for the fish. And for it to be eaten at all – and thus reach the fish’s gastrointestinal tract – the feed must meet the fish’s requirements with regard to consistency and taste.
New animal raw materials from worms to insects
As is to be expected, animal proteins are most likely to meet the requirements of potential alternative protein sources for fish feed. In carnivorous species, in particular, their composition usually meets the nutritional requirements quite well. Not every animal protein source has the desired amino acid spectrum but, by cleverly combining different components, such deficits can be easily compensated for and acceptable growth rates achieved in the fish. A wide range of animal raw materials has been tested over the years and useful diets developed for numerous fish species. They are based on single celled organisms, yeasts, microorganisms or zooplankton from copepods to krill, which are particularly rich in protein and omega-3 oils. Feed protein can be obtained just as effectively from bacteria produced on the basis of methane or carbonaceous waste. However, the quantities involved here are currently far from sufficient to make an effective contribution to solving the feed shortage. At present, worldwide availability of bacterial protein is probably hardly more than 20 to 50,000 t. By 2025, the quantity could grow to 200 to 400,000 t, but even that is still too little.
In the search for protein sources research is also focusing on invertebrates such as polychaetes, earthworms and insects. Bristle worms, for example the marine polychaet Nereis virens, are already used in feed for fry and for encouraging the maturation processes in some marine fish. Among terrestrial earthworms, species such as Eisenia foetida and Endrilus eugineae have proved particularly suitable. They are rich in high quality protein (60 to 70 per cent in dry matter) with a high proportion of essential amino acids, especially lysine and methionine. Earthworms are relatively hardy and can be easily produced in simple composting processes at low production costs.
There are said to be more than 3,000 composting plants in Japan producing earthworms for feeding to farmed eels (Anguilla japonica). Studies show that earthworm meal can replace up to half the fishmeal in the feed of many fish species without impairing their development. The only digestive problem is the high chitin content (a polysaccharide component) in the outer shell of the worms, but this can be reduced somewhat by using soft, nutrient-rich culture substrates.
A good source of protein from insects are the pupae of silkworms which contain high-quality pro-tein as well as valuable fatty acids. However, the available quantities are too low to noticeably alleviate the feed industry’s problem in its search for alternative protein sources. Nevertheless, hopes are high that commercial insect breeding will make progress for it offers two important advantages. Firstly, insects belong to the natural food spectrum of many fish species, for example salmonids, and secondly, they are relatively easy and cheap to produce. Feed trials confirm that insect meal can partially, and in some fish species even fully, replace fishmeal in the feed. More than 30 companies worldwide are already involved in the commercial production of insects, mainly the black soldier fly (Hermetia illucens). Their larvae are particularly large and can be dried and crushed after only 14 days of development. Global production volume, which is currently hardly more than 30,000 t, could rise to 100 to 200,000 t by 2025. But even that is far too little to make an effective contribution to protein supply for aquaculture.
In its search for a solution, the aquafeed industry now sometimes resorts to unusual raw materials. The Norwegian technology company Hyperthermics, for example, extracts the proteins contained in fish faeces. (The protein content in fish sludge is more than 40 per cent!).
Meal from bones, poultry, blood, feathers and meat are highly digestible protein sources but are not permitted for use in animal feed in all countries. They can replace fishmeal in fish diets to a high degree without significantly affecting weight gain, feed con- version or body composition of many fish species. In individual cases, however, feed additives may be required to complete the amino acid spectrum. Lysine and methionine are often added to rainbow trout feed, for example, to compensate for deficits that occur when replacing fishmeal.
Vegetable raw materials are available in large quantities
There is a particularly wide and promising selection of alternative raw materials in the case of plant-based products from agriculture, for example soya or various types of grain. Plant proteins are generally available in much larger quantities than fishmeal.
While hardly more than 5 million tonnes of fishmeal are produced worldwide each year, the quantity of soya alone is already more than 650 million tonnes, and additional sources are corn, rice, wheat and numerous other agricultural products which – after appropriate pretreatment – are suitable for fish feed. One advantage is that, due to the large quantities involved, prices are usually lower than for fishmeal, and they do not fluctuate as much. Initially, vegetable proteins were mainly used for omnivorous fish species but carnivorous species such as salmon or trout are now also reared on feed containing vegetable proteins. Of course, carnivores cannot be biologically reprogrammed and turned into herbivores. The feed manufacturers’ clever trick is to use special processing methods to give the plant concentrates and vegetable components properties similar to those of fishmeal.
The vision of a vegetarian trout that can be reared inexpensively on vegetable feed lives on! Scientists of the Agricultural Research Service in the US Department of Agriculture (USDA-ARS) breed vegetarian rainbow trout strains that perform well on soybean based diets. Since the genetic selection programme began about 20 years ago measurable progress has been made from generation to generation in the use of pure plant protein diets. Nutritional studies on the selected trout strain have shown that the fish absorb plant proteins better, process amino acids effectively, and show good protein retention.
The use of agricultural raw materials in fish feed is promising, but not without its problems, for the feed industry has to compete with a number of other user groups on the agricultural market. Wheat, for example, is needed for bakery products, rice is one of the staple foods in many countries of the world, soya and corn are fed to cattle, poultry and pigs. Due to the growing demand price fluctuations on the international markets are very frequent; since 2005, the raw material price index has risen by almost 50 per cent. Unfavourable weather conditions and severe storms, floods or droughts associated with climate change further exacerbate the problems. The use of genetically modified plants which are tailor-made for the requirements of fish nutrition could relieve the tense situation, but this technology is currently not acceptable for the majority of consumers in Europe. Genetic engineering methods could, for example, be used to control protein content and amino acid composition and align it more precisely with the fish’s requirements or to reduce the concentration of antinutritional factors. On the other hand, methods such as the biological improvement of raw materials using microorganisms, for example fermentation with yeasts, bacteria or fungi, are gaining more acceptance. Feed manufacturers are currently focusing mainly on the culture of microalgae from which high-quality omega-3-rich oils and proteins can be obtained. Microalgae cultures can be scaled to meet demand at reasonable cost and already account for a considerable proportion of the feed industry’s raw material supply. Their potential is far from exhausted and is expected to increase further in the near future.
The use of plant protein sources is currently concentrated in several areas:
• Development of technologies for the processing of plant material to reduce the effects of antinutritional factors and improve the nutritional value
• Cultivation of plants wiht less antinutritional inhibiting fac- tors and an amino acid profile optimized for fish nutrition
• Increased aquaculture of omnivorous fish species with lower protein requirements, such as tilapia or (better still) pure herbivores
• Development and use of new plant species as protein sources in fish feed
Protein concentrates increase the value of plant raw materials
A helpful way of developing vegetable raw materials for fish nutrition is to enrich the protein contained in them to produce protein concentrates. This allows the commercial use of raw materials that previously received little attention. Barley, for example, is a nutritious and inexpensive grain that is produced in large quantities. It meets almost all sustainability criteria, is comparatively undemanding, thrives in both cold and warmer climates, and requires hardly any irrigation, little fertilisation or treatment with pesticides. Worldwide, more than 150 million tons of barley are produced annually, and it is at the lower end of the price spectrum for feed ingredients. Normal feed-grade barley costs about 160 US dollars per tonne.
The US company Montana Microbial Products says it is about to start commercial production of Barley Protein Concentrate (BPC). Feed trials have shown that BPC is easily digestible for several fish species – even at high replacement rates – and can therefore be used as a protein base in fish feed. Trout reared with 30 per cent BPC in the feed showed no differences in feed conversion rate (FCR), growth, taste, meat quality or colouring compared to conventionally farmed fish. Another advantage is the relatively low fibre content of BPC, which consists of less than 5 per cent crude fibre.
Montana Microbial Products uses a process called “enzymatic fractionation” to produce one tonne of 60 per cent BPC from five tonnes of barley. Its market price is said to be 1,000 to 1,100 US dollars per tonne, which would be about a third cheaper than the same amount of fishmeal.
Corn gluten meal (CGM), which is produced during the wet milling of corn, has similarly good properties. With a protein content of between 60 and 70 per cent in dry matter CGM is an inexpensive alternative to other vegetable protein sources. CGM already replaces fishmeal in the feed of some fish species, for example cobia, sea bass and gilthead sea bream, without negatively affecting the growth performance of these carnivorous fish.
The potential applications of plant raw materials are quite impressive. Recent feeding trials have confirmed that alfalfa concentrates are suitable as natural colour enhancers for shrimps. They give shrimps from aquaculture, which are often somewhat underpigmented and pale in appearance, an attractive reddish colouring which is considered an important quality criterion that is also taken into account in pricing. Alfalfa concentrates, which are obtained by dehydrating the plant, are a cost-effective alternative to traditional carotenoid-rich products such as spirulina algae, paprika or synthetic astaxanthin, particularly since shrimp feed containing alfalfa apparently not only intensifies colouring (differences in colouring compared to the test group were already visible three weeks after starting the feed), but also improves the crustaceans’ performance, especially weight gain.
Manufacturers of feed for aquaculture are under pressure to find adequate, high-protein and preferably