Man has raised fish for thousands of years, but the development of modern, intensive aquaculture is relatively young at only some 30 years old. Science and technology is racing to find solutions to the unique and complex problems of breeding, feeding and raising the many hundreds of different fish, shrimp, lobster and shellfish that we like to eat.
The growth of aquaculture has been estimated at a rate of between 10% and 15% a year for the past several years. As wild stocks of fish decline, new species are cultivated, and population and wealth increases, the developing world — particularly China, where the vast majority of the world’s aquatic species are cultured — is making the transition from extensive to intensive farming practices, the rate of which is expected to be maintained for at least the next decade. The Foreign Agricultural Organization predicts that by the year 2030, more than half of the fish consumed is likely to be farmed.
This projected growth will only come about, however, if feed development can keep pace. The wide diversity of species presents particular problems: aquatic animals come in all shapes and sizes, and each has its own feeding pattern and nutritional requirements at each stage of production. Living naturally in warm or cold water oceans, estuaries, lakes or fast flowing rivers and even migrating from ocean to river during their life cycles, their environmental requirements are specific, and the nutrient interactions between the fish and its ecosystem must also be understood and controlled.
The fishmeal trap
Since the natural food of many of our favorite fish, such as salmon, trout, eels and flatfish is other fish, fishmeal is the perfect protein source. In 1989, the aquaculture feed industry used just 10% of the global fishmeal production. Today it accounts for 34% of the 6.5 million tonnes of fishmeal produced annually. This level is expected to rise to 48% by 2010, based on the predicted growth of aquaculture — especially carnivorous species that require high-protein feeds — and the transition of aquaculture in the developing world, particularly in China, to formulated feed.
China accounts for 70% of the world’s aquaculture and is rapidly modernizing production. Because of pollution and disease problems, Chinese aquaculture, with the encouragement of the government, is progressively replacing the estimated 3 to 4 million tonnes of "trash-fish" it currently feeds, with pelleted feeds.
The impact of China cannot be overestimated. Even at conservative projections, Chinese aquaculture will consume at least 30% of the total fishmeal supply by 2015, a proportion not much lower than the consumption by the whole of the global aquaculture industry today.
Fishmeal availability, however, is expected to stay the same, although this prediction is based on complex factors. Thirty million tonnes of mostly inedible, bony fish and trimmings from human food grade fish are used in the production of fishmeal. While tightening quotas and increasing demand for fish for the table will reduce the amount of catch available for fishmeal, by-catch, which is currently discarded, will become more available as pressure is brought to bear on fishermen to land everything in the net. Additionally, trimmings from aquaculture are a growing source of raw material.
The International Fishmeal and Fishoil Association, IFFO, says indications are that fishmeal supplies for aquaculture will be generally plentiful as a result of diverting use from land animal feeds, such as poultry and pigs, where it has less value. Others are not so sure.
Alternative protein sources
Feed accounts for up to 50% of an aqua farmer’s costs. The uncertainties surrounding the price and sustainability of fishmeal in the coming years has made the development of suitable replacements a high research priority — especially for marine species, salmonids (salmon and trout), shrimp and eels, whose diets can contain from 20% fishmeal in the case of shrimp to 45% in the case of marine carnivores, such as tuna and flatfish.
Aquaculture needs proteins that are affordable, are acceptable to the animals, provide rapid growth and survival rates and good Feed Conversion Ratio (FCR). They must also give good end product: they must not impair the nutritional value of the fish (by reducing the long-chain fatty acids Omega-3 and -6, for example), change the flavor, smell or appearance of the meat or in any other way cause the rejection of the product in the market place.
Cereals and oilseeds already make an important contribution to aquaculture feed formulations, as sources of carbohydrate, protein and as natural pellet binders, especially for shrimp feed. Canada’s National Grains Bureau has estimated the potential market for high-gluten wheat in the shrimp feed industry at 500,000 tonnes a year. However there is scope for a much higher use of grains and grain derivatives as fishmeal replacers.
Wheat gluten contains 70% to 80% protein that is highly digestible to rainbow trout, coho salmon and other fish species. According to Dr. Ronald Hardy, head of the Hagerman Fish Experiment Station, University of Idaho, U.S., up to 25% of fishmeal can be replaced with wheat gluten without negatively impacting growth or FCR. Even higher levels can be achieved for trout when combined with lysine supplementation.
Wheat gluten however is expensive since it is produced as a high-end, human food-grade product. If less expensive, feed-grade wheat gluten were developed, this ingredient could become an important aquaculture feed ingredient, Hardy said.
Maize gluten is much better priced. It does not have the same functional properties as a pellet binder for shrimp feeds as wheat gluten, but it is another excellent protein source, containing a minimum of 60% protein, which is 97% digestible to trout. It can replace 25% to 40% of fish meal without negative effect of growth on FCR.
The drawback with maize gluten is that high inclusion levels make the flesh of trout slightly yellow. Hardy reports that 5% to 10% can be used in trout diets, for production of white-fleshed trout and when pink-fleshed trout or salmon are raised, corn gluten can be included at up to at least 20% of the diet, because canthaxanthin and astaxanthin, used to produce pink-fleshed trout, masks the yellow color in fillets. White maize gluten meal might be the answer to the problem. It is being evaluated for salmonids, and Hardy said initial results look promising.
Antinutritional factors (ANFs) are a serious limiting factor for the use of grains in aquafeed. They can reduce palatability, protein utilization and growth in aquatic species, just as they can for terrestrial animals.
There are two categories of ANFs: those that can be neutralized by heat-treatments, such as phytates, lectins, goitrogens, trypsin inhibitors and antivitamins, and heat-stable ANFs, such as fiber, saponins, estrogens, allergins, and lysinoalanine.
Canola. Canola meal has been used as an aquafeed ingredient for many years. As might be expected, herbivorous and omnivorous fish can tolerate a relatively high level of canola. A U.S. Department of Agriculture’s Agricultural Research Service’s scientist, Dr. Chhorn Lim, has reported that 31% canola meal can be included in channel catfish diets without negative effect on performance. Canola and rapeseed meal are also commonly used in carp diets.
Salmonid diets can include up to 20% canola meal in the formulation, but higher rates of inclusion have been limited by the negative effects of glucosinolates that suppress growth rates, and high concentrations of fiber and phytic acid, a bound form of phosphorus, that inhibit digestive absorption of some minerals and amino acids. Salmon lack the enzyme phytase to digest phytic acid, which means that most of the phytic acid is excreted into the water, causing algal blooms that deplete the oxygen in the hatchery or grow-out system. However, the use of low-phytate canola and the use of phytase supplements for salmon diets are under investigation. A high-protein concentrate from canola meal, where the glucosinolates, fiber and phytate are reduced and the protein level increased, is being developed as a fishmeal substitute.
Barley and Maize. At the USDA-ARS Small Grains and Potato Germplasm Research Unit, Aberdeen, Idaho, U.S., plant geneticist Victor Raboy has developed and patented a procedure for breeding grains like barley and maize that have less phytate than conventional commercial varieties.
Preliminary results from feeding low-phytate barley feeds to rainbow trout indicate that as the amount of phytate in grain is reduced, there is a corresponding increase in the availability of usable phosphorus and calcium — another mineral that’s essential for healthy fish. In addition, trout fed a low-phytate barley absorbed more iron and more zinc — also essential minerals — from their feed.
Soy. There has been more research on soybeans in aquatic diets than most other ‘vegetable’ proteins. With protein ranging from about 45% for soybean meal to more than 70% protein for soy protein concentrate, and a favorable amino acid profile, it offers good potential as a fishmeal substitute. More than 10 years ago, Dr. Dean Akiyama, American Soybean Association, Southeast Asia, Singapore, showed that Tiger shrimp (Penaeus monodon) fed a diet with 72.2% of the total protein in the feed coming from soybean meal and wheat products, performed as well as those fed fishmeal-based diets. Soybean meal, especially when heat-treated, can be used at relatively high levels for trout feed but is less well tolerated by salmon. In the U.S., catfish diets typically contain 45% to 50% soybean meal, with less than 10% fishmeal.
It can also be used extensively in tilapia and carp feeds at similar levels.
Dr. Hardy believes that enzyme supplementation may be a key to fully exploiting grains in aquaculture diets. Specific enzymes that enhance the digestibility of carbohydrates are widely used in pig and poultry diets, but while phytase is beginning to be utilized for some diets, other enzymes have not been evaluated fully in aquafeed applications.
Rendered products, meat and bone meal and poultry by-product meal, would appear to have good potential use in aquaculture. Constraints are the inconsistency of quality, composition and nutritional value between producers and even between plants belonging to the same company, high ash content, insufficient research and almost total utilization of available stocks by animal feed and fertilizers.
Perhaps the answer would be to develop blends of animal, fish and plant proteins. This would provide an optimal amino-acid profile that most closely matches that of fishmeal. This thought could have dramatic implications for the cost efficient production of aquafeeds.
Dr. Dave Higgs, a researcher from Fisheries and Oceans, Canada, West Vancouver Laboratory, together with experts from CSH Innovations, Ltd., Vancouver, BC, a feed equipment and feedmill design and implementation firm, has developed a simple and economical process for preparing nutritionally upgraded oilseed protein and lipid sources for use in aquafeeds that concurrently make use of poorly or inefficiently utilized animal protein sources.
The process that has just been developed, involves heat treating and front-end dehulling of oilseeds, cold pressing the dehulled oilseeds, and then co-processing the resultant protein and generally lipid-rich products with ground animal offal or whole animals (fish or poultry without feathers), antioxidant, and supplemental water to yield an array of value-added products.
The main products that have resulted from laboratory tests of the process include protein- and lipid-rich oilseed meals or protein-rich, low-lipid content oilseed meals that are, respectively, suitable for inclusion in high energy and low energy diets. The meals derived from the processing of four oilseeds — canola, soybeans, sunflower, and hempseed — had reduced fiber content. The protein concentrations based on samples of the seeds varied from about 51% to 63% on a lipid-free dry weight basis (LFDWB) and they had attendant low concentrations of heat sensitive antinutritional factors (except phytic acid). Protein concentrates were produced that were extensively comprised of protein from one of the above dehulled oilseeds (oilseed protein generally furnish from 39% to 89% of the initial concentration) with the remainder being protein from whole herring or poultry offal. These concentrates generally contained about 66% to 77% protein (LFDWB). They had depressed concentrations of heat sensitive antinutritional factors, lower levels of crude fiber and other indigestible carbohydrates, reduced concentrations of water soluble antinutritional factors, improved essential amino acid profiles, and higher concentrations of bioavailable minerals than in the respective oilseed meals. The digestibility of protein in the concentrates ranged from 89% to 100% in Atlantic salmon held in seawater.
The group now plans pilot-scale implementation of its process to extend the range of products that can be produced, further improve cost efficiencies, and enable widespread testing of the new animal and protein lipid products.
Sniffing Out The Answer
Getting carnivorous fish to recognize grains as food and to eat them is another story and one to which U.K. scientists from the Centre for Environment, Fisheries and Aquaculture Science (CEFAS) may have found the answer: they have developed pheromones that trigger feeding behavior in fish.
Dr. Andy Moore and his colleagues from the CEFAS Laboratory in Lowestoft, Suffolk, U.K., have identified pheromones, which are released by fish and induce feeding behavior in other fish. They have now developed synthetic versions of the pheromones. By adding the compounds to the water, fish begin to search and will eat any food item present.
"A pheromone is a compound released by one fish detected by another fish normally by the sense of smell," said Dr. Moore. "It brings about specific responses such as migration, and is involved in reproduction and predator-prey interactions. So it’s a form of chemical communication between fish. We’ve been finding over the last two years that there are specific pheromones that do encourage fish to feed." Since the pheromones are naturally produced in the water by the fish it will not have an impact on the fish’s health or the wider environment.
CEFAS, an agency of the UK Government’s Department for Environment, Food and Rural Affairs, intends to work closely with the aquaculture and aquafeed manufacturing industries. Worldwide trials are due to start early next year.