THE lipids in fishes can be separated into liquid fish oils and solid fats. Although most of the oil usually gets extracted during processing of the fish meal, the remaining lipid typically represents between 6 and 10% by weight but can range from 4 to 20%.
Fish lipids are highly digestible by all species of animals and are excellent sources of the essential polyunsaturated fatty acids (PUFAs) in both the omega-3 and omega-6 families of fatty acids. The predominant omega-3 fatty acids in fish meal and fish oil are linolenic acid, docosahexaenoic acid (DHA) and eicosapentaenoic acid (EPA).
Both DHA and EPA fatty acids are produced and passed along the food chain by small-size algae and zooplankton, which are consumed by fish. Fish meal and oil contain more omega-3 than omega-6 fatty acids. In contrast, most plant lipids contain higher concentrations of omega-6 fatty acids. For example, oil extracted from soybeans, corn or cottonseed is rich in linoleic acid, an omega-6 fatty acid. Some oils, like those from canola and flaxseeds, contain linolenic acid (of the omega-3 family); however, its conversion into essential DHA and EPA by most animals may be limited.
The beneficial effects of lipids in fish diets are particularly evident in structure and function of cell membranes. The cell membrane is a semi-permeable and flexible layer that encloses each cell in animals and controls the passage of nutrients and other substances into and out of the cell interior. It protects the cell and is composed primarily of lipids, proteins and some carbohydrates. Because of their fatty acid composition, lipids permit cell membranes to maintain their fluidity in decreasing or increasing water temperatures and cushion cells from the dramatic pressure changes that fish encounter at varying depths in the water column.
Essential fatty acids are necessary for normal larval development, fish growth and reproduction. They are important in normal development of the skin, nervous system, brain and visual acuity. PUFAs appear to assist the immune system in defense of disease agents and reduce the stress response. Fish meal also contains valuable phospholipids, fat-soluble vitamins and steroid hormones.
Energy in fish meal
The lipids in fish meal not only impart an excellent profile of essential fatty acids but also provide a high content of energy to the diet. Since there is very little carbohydrate in fish meal, the energy content of fish meal relates directly to the percentage of protein and oil it contains. The quantity and quality of oil in fish meal will in turn depend on the species, physiology, sex, reproductive status, age and feeding habits of the captured fish and the method of processing.
The lipids in fish meal and fish oil are easily digested by all animals, especially fish, shrimp, poultry, pigs and ruminants--such as cows, sheep and goats. In these animals, the lipid digestibility is 90% or greater. The high digestibility of fish lipids means they can provide lots of usable energy. If a diet does not provide enough energy, the fish or shrimp will have to break down valuable protein for energy, which is expensive and can increase production of toxic ammonia.
Good-quality fish meal contains antioxidants or compounds that reduce the possibility of damage from highly reactive toxic substances that are continually produced at the molecular level in animal cells. For example, lipids, especially PUFAs, are easily damaged and become rancid when exposed to oxygen, a process known as oxidation and one that releases heat.
The use of antioxidants in the preservation of fish meal is essential in order to stabilize its energy value because of high quantities of PUFAs present in the oil. Without stabilizing fish meal with antioxidants, the available energy content of the meal may be reduced by as much as 20%; the oxygen will modify (damage) the chemical structure of PUFAs, and therefore, less energy is available to the animal.
Prior to the development and use of antioxidants by the fish meal industry, it was common practice to turn piles of processed meal in order to dissipate the heat arising from oxidation. Occasionally, fish meal would ignite spontaneously and cause fires while being shipped or stored. Historically, it was known that ships sank at sea due to fires caused by spontaneous combustion of the fish meal they were transporting. Today, antioxidants added to fish meal prevent such catastrophes.
Mineral, vitamin value
When a sample of feed is taken to the laboratory and analyzed for nutrient content, the procedure involves burning a portion of the sample. Ash is the material remaining after the feed sample is completely burned. Normally, the ash content of a good-quality fish meal averages between 17 and 25%. More ash indicates a higher mineral content, especially calcium, phosphorus and magnesium. Calcium and phosphorus constitute the majority of the ash found in fish meal.
Unlike the phosphorus in plants, phosphorus in fish meal is in a form highly available to most animals. The phosphorus in plants is not as readily available to monogastric animals (having a one-compartment stomach like pigs, dogs and humans) because it is primarily in the organic form known as phytate. Ruminants such as cows, sheep and goats are able to utilize phosphorus in phytate due to the microbial population in their rumen, which is one of the four compartments in the stomach of ruminants.
The vitamin content of fish meal is highly variable and influenced by several factors, such as origin and composition of the fish, meal processing method and product freshness. The content of fat-soluble vitamins in fish meal is relatively low because of their removal during extraction of the oil. Fish meal is considered to be a moderately rich source of vitamins of the B-complex, especially cobalamine ([B.sub.12]), niacin, choline, pantothenic acid and riboflavin.
Considerations
The high quality and concentration of essential nutrients, especially of well-balanced amino acids, essential fatty acids and energy content, make fish meal an indispensable ingredient in diets of most aquaculture species and many land-farm animals. Because of its nutrient content, high digestibility and palatability, fish meal serves as the benchmark ingredient in aquaculture diets.
The majority of fish meal produced is incorporated into commercial diets fed to fish, shrimp, swine, poultry, dairy cattle and other animals such as mink (Table). It is unlikely that supplies of commercially available fish meal and oil will be able to keep pace with the projected increase in worldwide production of aquaculture and terrestrial animal feeds.
In most recent years, aquaculture has used approximately 46% of the total annual fish meal production, a figure that is expected to rise as demand for aquaculture products increases in the next decade.
Optimal use of fish meal in practical aquaculture diets is necessary to minimize feeding costs that can account for 40% or more of operating expenses. The concentration of high-quality nutrients, especially protein, makes fish meal one of the most sought and expensive feedstuffs.
The cost of high-quality fish meal (65% protein) has ranged from approximately $385 to $554 per ton since 2000, or 2.0-3.5 times the price of soybean meal.
Unfortunately, the specific dietary requirements for energy, essential amino acids, fatty acids and other nutrients are not yet known for many aquaculture species. Also, the digestibility of different feedstuffs has not yet been established for many of the commercially important fish species. Therefore, there has been a reluctance to diminish the total amount of fish meal used in many of the different aquaculture diets.
Simply over-supplementing the diet with high-quality fish meal is an easy and very successful way of overcoming lack of knowledge about aquaculture diets in relation to specific nutrient requirements and digestibility of feedstuffs.
The best approach in feed formulation is to use high-quality feedstuffs to manufacture a diet that meets the nutritional and energy requirements of the aquaculture species in question. Fish meal contains the profile of amino acids that most closely meets the amino acid requirements of fish. If a portion or all of the fish meal in a diet can be replaced successfully with other high-quality protein sources, doing so will contribute greatly toward protecting the surrounding environment and promoting a sustainable aquaculture industry.
New information on nutrient requirements of aquatic organisms, coupled with advances in feed technology, indicates that species-specific fish diets can be made by partial or total replacement of fish meal with other plant and animal proteins.
All-plant protein-based diets containing soybean meal, cottonseed meal and middlings from corn and wheat, supplemented with lysine and methionine, have been used successfully to grow juvenile catfish, carp and tilapia to market size. However, larvae and young fish still require fish meal to grow at an optimal rate.
Animal proteins and fats--byproducts of the animal rendering industry--can be used in aquaculture diets because they also provide essential amino acids and fatty acids. These 'fish meal substitutes' will be used more extensively by the aquaculture industry in the future.
Fish meal belongs to a short list of excellent feedstuffs that provide essential nutrients in a highly digestible, concentrated form. The use of fish meal in domestic and farm animal diets will remain a core and efficient practice, particularly for young, rapidly growing and high-producing animals like maturing fish, berried (egg-laden) shrimp, poultry and lactating dairy cattle.
The beneficial effects of eating wholesome foods will increase the worldwide demand for seafood products, resulting in increased use of fish meal. Fish fed diets formulated with a high percentage of fish meal will contain high concentrations of PUFAs in their tissues. These fish and their fillets are very beneficial to people due to the well-known attributes of PUFAs, especially of the omega-3 family.
PUFAs are essential for human biological functions, particularly the production of prostaglandins. PUFAs and prostaglandins may ameliorate many human health disorders, such as high blood pressure, heart disease, arthritis, migraines, diabetes and cancer. Incorporation of DHA and EPA found in fish meal into the diets of fish and other farm animals is an efficient method to ensure a proper concentration of these important omega-3 fatty acids in the human diet.
Balancing nutrients in diets by using the minimum amount of fish meal to meet specific amino acid requirements for fast growth and reproduction and reducing feed costs constitutes one of the principal objectives in formulation of fish feeds. Another important aim in feed formulation is to increase dietary nutrient density and digestibility of the feed to increase biological performance and to reduce nutrient leaching and water-quality degradation.
The aquaculture industry must continue to seek out alternative sources of high-quality plant and animal-based protein ingredients for their feedstuffs. Presently, this is an active area of research in aquaculture nutrition.
R.D. MILES and F.A. CHAPMAN*
*R.D. Miles is a professor in the department of animal sciences and F.A. Chapman is an associate professor in the department of fisheries and aquatic sciences at the University of Florida. (The first part of this article ran in the Jan. 1 issue of Feedstuffs and can be found at www.feedstuffs.com.)
Use of fish by agriculture, % Sector 2002 2010 Aquaculture 46 56 Pigs 24 20 Poultry 22 12 Ruminants 1 <1 Other 7 12
