Abstract.-- Over $9.6 billion worth of fish and fishery products were imported into the United States in 1989. The global catch of 98.4 million metric ions of fish in 1988, the most recent year for which data are available, was not adequate to meet current and projected demands. The Food and Agricultural Organization estimates that industrialized nations will need an additional 22.5 million metric tons and non-industrialized nations 5.9 million metric tons of fish and fisheries products by the year 2000. Aquaculture has been expanding globally, but present aquacultural production will not satisfy the projected demands for fish and fishery products.

Increasing salinization of freshwater supplies limits traditional agricultural production in many arid and semi-arid regions. Saline water unsuitable for traditional crops can be used to culture marine or brackish water algae and fish. These aquaculture products rosy be marketed as food, bait, or recreational species or serve as the source for high value pharmaceutical and other biochemical products. Specifically, production of algae and fish using animal wastes from cattle and poultry production systems and saline water can produce a new agribusiness in arid and semi-arid regions where intensive solar radiation is conducive to high primary production.

The global catch of fish from the oceans increased rapidly from 21 million metric tons in 1950 to 76 million tons in 1970 but only slowly to 93 million metric tons in 1986 and a record 98.6 million metric tons in 1988 (National Marine Fisheries Service 1988, 1990). Experts consider it unlikely that the future sustained harvest of marine fish will exceed 100 million metric tons (National Marine Fisheries Service, 1988). The United Nations has predicted the world shortfall of land-based protein to be 39 million metric tons by year 2000 (Richmond 1986a). To offset the shortage of available protein, recommendations are to supplement the conventional crops with high-protein foods, such as those provided from marine life. This protein shortage cannot be provided solely by harvesting the renewable fishery resources of the oceans.

Although 60% by weight of the fish harvested from the oceans is consumed directly by people (Food and Agriculture Organization 1980), fish and fish byproducts have other important uses. One such use is production of fish meal. Fish meal provides the bulk of animal protein formulated into feeds for livestock and poultry. The demand for fish meal increased in the Far East, China, the Scandinavian countries, and the United Kingdom in 1988 as a result of expansion of the aquaculture and poultry industries. As demands increased in other parts of the world, the cost of fish meal increased. United States' imports of fish meal decreased by 30% between 1987 and 1988 as the cost increased and the value of the U.S. dollar in the world market declined (Ramada and Purinton 1989). As a result, animal producers in the limited States were forced to feed less desirable feeds formulated with higher levels of plant protein.

For current land-based animal production methods and technologies. the net effect of a limited global fish supply is a limited global protein supply. Not only will the oceans' fishery products be insufficient to provide needed protein for human consumption, they will be insufficient to provide the feeds necessary for intensive production of animals in confinement.

The farming of fish and other aquatic plants and animals is often touted as the solution to the global need for protein. If we assume aquaculture may contribute substantially to the amount of needed protein (we will ignore the often inverse relationships between protein needs and protein production in different geographic areas), we have to ask "why hasn't it already been accomplished" and "when will it make a significant contribution to the needed protein supply?"

Aquaculture is an emerging industry in the United States and has tremendous potential in Texas and other southern states (Joint subcommittee on Aquaculture 1983a, 1983b). Only 14% of the world supply of fishery products was produced on farms in 1985 (Ratafia and Purinton 1989), but aquacultural production is projected to increase to 25% by the year 2000 (Ratafia 1990). Present aquacultural production has created an unprecedented demand for fish meal. Since 1980, feed required by aquaculturists rearing aquatic animals (102 types of fish, 32 crustaceans, 44 molluscs and 3 miscellaneous) on farms has increased from 1980 over two-fold to 3.6 million metric tons in 1988 (Ratafia and Purinton 1989). Farm-raised aquatic animals are expected to require 4.3 million metric tons of feed by 1990 and 14 million metric tons by year 2000 (Ratafia and Purinton 1989). Many fish presently reared in aquaculture facilities typically require about 2 kg of dry feed to gain 1 kg of weight. Fish feed is normally formulated with 25-50% protein of which a large portion may be fish meal. The juvenile stages of fish and carnivorous species typically require higher levels of fish meal protein. It should be clear that aquacultural practices will not greatly increase the net availability of fish and fishery products unless an alternate protein source is found to supplement or replace fish meal.

Economic incentives exist for further development of aquaculture. One percent of the United States' fish supply was farrn-reared in 1970; by 1987, 7% of the fish consumed in the United Stales were produced by aquaculture. In 1989, fish reared on American farms were valued at about $750 million. However. imports of fish and fishery products into the United States were valued at $9.6 billion in the same year (National Marine Fisheries Service 1990). Clearly, economics alone are not sufficient to increase aquacultural production in the United States.

In the United States and throughout the world, most farm-raised foodfishes are cultured in ponds or raceways in warm climates with long growing seasons that are conducive to high annual production. Although these methods are economical and productive, they require large amounts of land and large volumes of high quality water. For pond production, the land should be fertile, have high clay content, and be in an area with a supply of high quality water. Production in raceways is limited by the amount of high quality water available to continuously pass through fish rearing units. Much of the area, at least in the United States, that has desirable conditions for pond or raceway aquaculture already has been developed. It is important to note that the land and water requirements for present aquacultural production also are needed for traditional agricultural crops. Therefore, further development of conventional aquacultural systems will reduce production of crops, such as corn, cotton, and rice.

As the world increasingly recognizes the importance of the amount and quality of water, waste discharges from aquaculture facilities must also be considered. In the United States, water discharges are affected by increasingly stringent regulations. These regulations will increase the cost of production and curtail some aquacultural operations.

Commonly, fish reared on fish farms, particularly in the United States, are economically desirable for fish farmers. Rearing fish for stocking recreational fisheries (sportfish) or for aquarists (hobby fish) can be lucrative; however, rearing sportfish and hobby fish does little to satisfy the need for protein. For the many farmers producing foodfish that are "economically desirable" means rearing high-value fish that typically require large quantities of expensive protein in their diets. Recycling plant and animal protein into high-value fish does little to reduce world-wide protein needs.

Food quality is an important consideration for aquacultural production. Aquaculturists have often enjoyed receiving a higher price for farm-raised fish than wild-caught fish because of the consistent high quality of farm-raised fish -- free from environmental contaminants with good flavor and texture. Although farm-raised fish are generally high quality, their fatty acid profiles are similar to that of soya protein, one of the main

ingredients of fish feeds as currently formulated, and does not contain omega-3 fatty acids at levels found in wild-caught marine fish (Stansby et al. 1990). Marine algae. the predominant synthesizers of omega-3 fatty acids, are at the base of the marine food chain through which these nutritionally important lipids enter fish and, ultimately, the human diet. Recent studies have suggested various benefits of omega-3 fatty acids to human health (Stansby 1990, Land 1986). Studies by The National Institute of Health and other health organizations have shown that omega-3 fatty acids are essential in the human diet (Hunter 1987,1988, Lees and Karel 1990). The effects of dietary omega-3 fatty acids on farm-raised fish is now being investigated at several locations including Texas A&M University, Mississippi State University and on fish farms in Japan.

A large portion of the earth has an arid or semiarid climate. In the United States, irrigation technology and development of drought-tolerant crops has resulted in agricultural development of arid and semi-arid lands. Irrigation has been a tremendous boon to our agricultural production and economy, but it is not without drawbacks. Water used for irrigation on U.S. farms was only 270 million m³ annually in 1940 but peaked at 570 million m³ in 1980 (U.S. Bureau of Census 1990). This increase was not due to population growth alone; daily per capita consumption of water increased from 3.90 m³ in 1940 to 7.42 m³ in 1989 -- an increase of 90% in 40 years. Withdrawal of groundwater has increased to meet the greater demands; in the High Plains withdrawal from the Ogallala Aquifer has supported population growth and extensive agricultural production. In many areas, withdrawal of water for irrigation has been blamed for alarming declines in the level of water in the vast Ogallala and, due to concern or the increased cost of pumping water, some farmers have ceased irrigation. Irrigated agriculture is responsible for salinization of soils and water (Camp 1963). Some areas of the arid southwest the formally produced agricultural crops are now fallow because of the presence of subsurface saline water and an accumulation of salts at the surface. The relationship of these changes with water withdrawal from the Ogallala is presently unclear.

Natural sources of salt water are a liability to agricultural development. Brine springs are found throughout much of the arid western United States. Several retention dams have been built or are planned to contain the salt cater and protect downstream fresh water supplies needed for agricultural and domestic purposes. The oil industry in Texas, valued at $10.7 billion in 1988 (Kingston 1989), produces considerable quantities of Oil-contaminated brine that must be contained to prevent environmental degradation.

In addition to water consumption by crops, large cattle feedlots that also require relatively large amounts of fresh water have been developed in this mosaic of rangelands and row crop agriculture. Cattle are usually maintained in feedlot pens for about 5 months while they gain weight from an initial 200-400 kg to about 500 kg at time of slaughter In 1989, over five million head of cattle were shipped from feedlots in the Southern High Plains (Texas Agricultural Statistics Service 1989). Recent trends have been to develop large feedlots; in 1989, 198 feedlots accounted for 51% of the nation's output of fed cattle. A typical feedlot has 50,000 to 100,000 head at one time. Each feedlot steer annually excretes 10 metric tons of waste per 1,000 kg of live weight (Midwest Plan Service 1975). The concentration of waste resulting from these and other intensive agricultural operations such as swine, poultry, and dairy creates severe waste management problems. In addition to potential environmental damage, economic aspects are associated with disposal of this waste. The Environmental Protection Agency has recently fined dairy farmers for ground water pollution. Several fines were in the $10,000 range and at least one fine was $85,000.

Imagine a system that not only used limited quantities of high quality water and produced energy but also used animal wastes and salt water to produce an agricultural crop high in protein and desirable for human nutrition and formulation into animal feeds. This system would be built in an inland environment safe from devastating tropical stomps, on lands too arid for conventional agriculture, and would not discharge environmentally damaging wastes. Aquaculture can be such a system -- specifically aquaculture of marine microalgae and fish reared in recycled water systems.

Marine microalgae

Microalgae are now commercially produced in several locations throughout the world (Richmond 1986). The primary market is for specialty items such as vitamins, pigments, pharmaceuticals, the human health food market, and feeds for larval aquatic organisms or ornamental fish of high value (Shelef and Soeder 1980, Cohen 1986). Spirulina is the principal marine microalga produced commercially and has received the greatest research and development interest. Richmond (1986) reported the worldwide production in 1984 of Spirulina for food was 850 metric tons produced on 10 farms with a total of 35.6 hectares in cultivation. Spirulina is particularly attractive for aquaculture for several reasons. Several papers included in Richmond (1986) indicate that Spirulina has high levels of protein (50-70%), lipids (7-16%), and vitamins, especially vitamin A and the B complex; is tolerant of wide ranges of salinity (salt at 20-70 g/l is optimum but 0 to 270 g/l is tolerated; and can be cultured in the salt water is presently an environmental threat. Digested feedlot wastes can be used as a nutrient source to culture Spirulina. The helical shape of Spirulina makes it easily harvested by flotation and filtration. Spirulina.. has been intensively cultured in raceways and shallow ponds both indoors and outdoors.

The climate and resources of arid and semi-arid regions, especially in the Southern High Plains, are conducive to intensive culture of Spirulina (Figure 1). Waste from cattle feedlots would be digested anaerobically and mixed with saltwater to provide the nutrient-rich media for algal culture. The persistent sunshine characteristic of arid and semi-arid climates provides the essential energy to convert nutrients to biomass. Operations placed in solar-heated greenhouses would support production throughout the year and allow high-production aquaculture in climates previously considered too cool for outdoor aquaculture. Additional heating can be achieved by combustion of the methane produced by digestion of the feedlot waste. Algae and associated bacteria, fungi, yeast, zooplankton, and detritus could be used directly by fish or harvested for processing into formulated feeds (Garrett et al. 1976, Matty and Smith 1978).

Figure 1. Flow diagram for a vertically integrated system using cattle manure to produce algae for incorporation into fish feed and extraction of high value products.

The maximum production level of farm-raised algae is 200 metric tons/hectare (dry weight) annually. Typically, only 10-50 metric tons/hectare are produced on the few farms producing algae today (Richmond 1986), clearly indicating the need for improved efficiency. Over 2 million metric tons of algae could be produced annually from the wastes of the 5.3 million head of cattle on the Southern High Plains.

The high protein and lipid contents of Spirulina make it desirable for formulation into animal feeds. Spirulina has proven to be a valuable component in fish and shrimp feeds. The high omega-3 fatty acid content makes it a desirable substitute for fish meal in the diets of fish, swine, and poultry.

There is a market for Spirulina. Marine microalgae has been anaerobically digested to produce methane, used as a fertilizer, and harvested and sold as a health food. Several products of current or potential commercial value have been extracted from algae (Table 1; Cohen 1986). A few of these products include pigments for food coloring and animal product pigmentation, food flavoring, agar and a food product base, cosmetics, pharmaceuticals and therapeutic chemicals, and chemical reagents for clinical research and industrial processes. Research continues to show that new products can be obtained from marine microalgae.

Fish Culture in Water Reuse Systems

Fish production in ponds is based on surface area because natural ecological processes in ponds convert waste products to nontoxic forms. High densities (weight/volume of water) of fish can be produced if water quality can be maintained. In current practice, waste metabolites produced by fish in raceways are diluted and discharged by flowing large volumes of high-quality water through the fish rearing units. By recycling the water with a variety of biofiltration technologies, the demand for large volumes of fresh water could be greatly reduced and discharges of nutrient laden water could be virtually eliminated. By providing rapid turnover of high quality, recycled water, standing crop and production can be intensified and spatial requirements per unit of production are reduced. Attaining high production in limited water volumes allows fish culture to move indoors where temperature can be controlled. Control of temperature allows continuous production, thereby increasing total annual production, and allows the culture of warmwater fishes in locations that would otherwise be climatically unsuitable.

Small-scale indoor fish production units have proven technically successful (Broussard et al. 1973, Parker and Simco 1973, Northeast Regional Agricultural Engineering Service 1991). However. few, if any, commercial size indoor water reuse systems have remained in operation for as long as 5 years indicating that they have not been able to effectively compete with aquaculturists using more traditional production systems (Losordo et al. 1989). The increasing concern for water quality and greater demand and higher price of fish products has developed renewed interest in water reuse systems. Several large-scale units ($5-15 million each) have recently been built or are under construction in California, Mississippi. Tennessee, Virginia, and West Virginia (Van Olst and Carlberg 1990).

Environmental control in water reuse systems allows great flexibility in the types of fish that can be produced. However, two constraints are: (1) the products must be sufficiently high in value to compensate for the additional costs of construction and operation of the facility (fish produced in an indoor, water reuse system can not compete economically with pond-reared fish), and (a) the animals reared must tolerate high-density crowding.

Fish can be produced for various markets - recreational, commercial, ornamental, etc. - and may be sold as live fish, whole fish on ice, processed (gutted and skinned or scaled), fillets, or as value-added products such as ready to cook meals. Fish that could be produced in arid climates include blue tilapia Tilapia aurea, red drum Sciaenops ocellata, and paddlefish Polyodon spathula. The tilapia (Cichlidae), a group of fishes native to Africa that are very similar in appearance to North American sunfishes (Centrachidae), grow rapidly and can be cultured at high densities. Tilapia are important foodfishes worldwide and have retailed in the round for about $8.00/kg. The red drum has been classified as a sport fish in most areas of its range along the Gulf Coast and southeast Atlantic Coast and commercial harvest of those wild stocks is restricted. The red drum became commercially important almost overnight as the popularity of blackened redfish swept the South in the early 1980's. These fish can be easily maintained indoors and induced to spawn repeatedly by manipulating temperature and photoperiod. Paddlefish, closely related to sturgeon, have provided a source of caviar for about 10 years in the United States. Populations of paddlefish have been depleted in many areas due to loss of habitat and overfishing. A 20-kg paddlefish will produce about 5 kg of eggs (or caviar) valued at $10/kg to fishermen. The eggs can be taken without loss of the fish and it is expected that, under culture conditions with manipulation of temperature and photoperiod, more than one annual harvest of caviar could be taken from a mature female. Culture techniques for paddlefish have been developed (Dillard et al. 1986) but are not yet the basis of foodfish production.

In arid regions of the world it may be prudent to rear and sell fry and fingerlings to be stocked and grown by other fish farmers with more abundant water supplies. Other species could be produced specialty markets and could include hobby fish. molluscs, shrimp, and bait fish.

Locations in the arid southwest and especially in the Southern High Plains have the natural resources to develop efficient. commercial-scale systems for culture of algae and fish using the plentiful livestock waste from feedlots as a nutrient base and saltwater as a culture medium. Because aquaculture is a form of agriculture, the Southern High Plains area has the necessary infrastructure to support aquaculture. Potential products in such a system include algal protein for direct consumption by fish or zooplankton that in turn are important food organisms for larval fish and several adult fish. Other products include foodfish (tilapia and redfish), caviar from paddlefish, and harvested algae. The harvested algae provide animal feeds, health food, and a variety of extractable compounds.

The increased demand and declining supply of seafood products identify aquaculture as a growing industry. The commercial value of some products (fish and caviar) are known but the value and identity of other potential products remains unknown. As a food source for livestock, algal protein will be valued in comparison to the cost of soya protein. The cost of algal protein would be lowered by the high value of other products extracted from algae (e.g., pigments, pharmaceuticals, reagents, etc.); processing the algae to obtain the high-value products leaves algal protein as a value-added byproduct. Algal lipids containing omega-3 fatty acids will be valued in comparison to fish oils; however, algal-derived fatty acids could command a higher price if consumer concern about contaminants in marine fish increases. Engineering improvements in the development of water reuse systems to conserve fresh water and development of aquaculture production systems to use saline waters would be marketable technology of increasing value worldwide.

The potential for aquaculture to expand as a new agribusiness seems to be unmatched by any other sector in the agricultural community. Arid regions can support significant aquaculture operations if approaches are taken to use natural resources locally available and to produce products of special interest and high value.

We thank S. Dernarais, Texas Tech University, and N. Mathews, U.S. Fish and Wildlife Service, for their review of the manuscript. This is Technical Article T-9-611 of the College of Agricultural Sciences, Texas Tech University. The Texas Cooperative Fish and Wildlife Research Unit is jointly sponsored by Texas Parks and Wildlife Department, Texas Tech University, The Wildlife Management Institute and the U.S. Fish and Wildlife Service.

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