Aquaculture: Challenges and Promise

Aquaculture is the culture of aquatic organisms, which includes fish, mollusks, crustaceans, algae and plants. People have been involved in different forms of aquaculture for thousands of years, with early documented evidence dating back as far as 500 BC in China (Ling 1977). Today, the practice of aquaculture spans the globe, with the exception of the extreme polar regions. Many of the basic goals have not changed significantly, however their methods of achievement have. There are two overarching goals of aquaculture: maximizing growth rate and minimizing production costs. A rapid growth rate minimizes the time to achieve a marketable size and decreases risk. The reduction of production costs makes an operation profitable. To accomplish this, there are a number of strategies, all of which are typically utilized to some extent. These strategies include: maximizing food conversion, and reducing water, power, processing and storage costs.

Food is critical to aquaculture because it usually constitutes over 50% of production costs, and also because it provides the energy inputs necessary to achieve maximal growth. In aquaculture, feeds range from live to formulated diets, and are often changed as species develop and mature. For example, as a larval fish hatches from an egg, the immediate source of energy is derived from maternal stores in the attached yolk sac. This energy source is rapidly exhausted, and the developing fish must transition to exogenous foods. Further, the capacity to store food in the gastrointestinal tract at this stage is limited. As a result, the greatest mortality generally occurs during this critical stage. For the aquaculturalist, this means that there needs to be continuously available sources of food to prevent starvation and promote rapid growth. Food must be readily accepted and easily digestible. As a result, live or high protein diets are often initially utilized, although fish are transitioned to a cost-effective formulated diet as soon as possible due to ease of feeding, and better nutrient consistency, availability and storage. Formulated diets are made of natural products, such as fish meal, soybean meal, and corn meal, and will also include a complement of essential amino acids, vitamins and minerals. Due to cost and the need to reduce the environmental footprint of aquaculture, the amount of fish meal is reduced as much as possible as the fish grow, and replaced with alternative protein sources, such as soybean meal. Every species has specific nutritional requirements, and the diet that is administered needs to meet minimum levels of these requirements (i.e., protein, lipids, carbohydrates, vitamins and minerals) and have high digestibility to ensure maximum growth rates.

After food, water is obviously at the heart of aquaculture, and questions such as the quality and quantity of the source that will be used are integral to the success of an operation. Key parameters necessary to the survival of most species are dissolved oxygen, temperature, salinity, hardness, ammonia, nitrite and pH. The goals of most operations are to maintain these water quality variables within ranges that will ensure maximal growth, while reducing water use, and minimizing effluent. The diet type often plays directly into water quality, because uneaten food, and food that is not highly digestible, result in nutrient inputs into the water. These nutrient inputs can fertilize the water surrounding the fish and the resulting effluent from the operation. Microbes, phytoplankton and plants can be stimulated by the nutrient inputs, and may result in poor water quality, with the amount of nutrients in effluent directly related to water retention time and hydraulic turnover rate (Tucker et al. 2005). Therefore, water quality is directly related to the intensity and type of aquaculture system being utilized.

There are a number of different types of aquaculture systems. In general, aquaculture can be divided into extensive or intensive production. Extensive aquaculture provides little control over the environment of the cultured organism, with cultured organisms subjected to limitations of natural food sources and environmental conditions. Examples of extensive aquaculture are oyster farming by spreading oyster shells along a region of shoreline. In contrast, intensive aquaculture is highly controlled, with many conditions such as temperature, dissolved oxygen, and diet maintained within specific desired levels. An example of intensive aquaculture would be a recirculating system or raceway system for tilapia fed a complete diet. Within the parameters of extensive and intensive, there are a number of different systems utilized for aquaculture. Some of the main categories of systems are earthen ponds, raceways, cages, net pens, and recirculating systems. There are a variety of different types of designs within each of these categories, which are limited by the species under culture and the creativity of the system designer.

One example of a common type of aquaculture system utilized around the world, are earthen ponds (Figure 1). Ponds are relatively simple to construct, and have the benefit of low nutrient input into effluent due to long water retention times and nutrient absorption by sediment. However, these systems are subject to erosion and upkeep. In contrast, cages and net pens, which can be placed into existing water bodies, may have much higher nutrient inputs to their immediate surroundings, which can be offset by choosing a location with sufficient current to minimize compounding effects in one area. Maintaining cage or pen integrity can be difficult and expensive. An old system which is receiving new attention is aquaponics. Aquaponics combines a recirculating system with a hydroponic system, in which effluent is utilized to grow plants. The plants utilize the nutrient rich water, and by removing nutrients, improve the water quality for the fish. Both the fish and the plants can then be sold profitably. This technology is still developing, although progress has been rapid. A similar type of system for earthen ponds is known as a partitioned aquaculture system. These systems divide a pond into two portions, with a partition in between. Paddlewheels keep water circulating from the fish side to the non-fish side. Phytoplankton in the non-fish side utilize the nitrogenous wastes and phosphorus while producing lots of oxygen that can be utilized by the fish. The result is that much higher densities and total numbers of fish can be produced than if fish were stocked into the total area of both pond portions. Current research is focused on understanding the processes involved and the appropriate stocking densities.

As an example of aquaculture, the US catfish industry is the largest producer of any species group in the USA (Figure 2). Earthen ponds roughly a meter deep, which are fed by well-water or surface water provide the culture environment, and refinement and improvement of production processes are continuously being implemented. One example is through the culture of hybrid catfish. Hybrid catfish are produced by crossing a female channel catfish with a male blue catfish. The offspring benefit from the fast growth rate of channel catfish and the greater disease resistance of the blue catfish. Currently, the US catfish industry has been challenged by a number of issues, including plateaued product prices, increased production costs, international competition and economic recession. US market prices for catfish have remained relatively stable for the last twenty years (USDA 2010), while production costs, such as fuel, have steadily increased. Corn, soybean and grain costs, which are primary ingredients in catfish feeds, have also seen recent increases in costs, also resulting in reduced profitability. International competition has come in the form of tilapia (family Cichlidae) and catfishes mainly from the family Pangasiidae. Other examples of aquaculture, that demonstrate some of the variety of species under culture, include the culture of giant freshwater prawns (Macrobrachium rosenbergii; Figure 3), the culture of red swamp crayfish (Procambarus clarkii; Figure 4), and recently, the culture of algae for biofuels.

One of the appeals of aquaculture, both as a profitable business, and as a subsistence provider of protein for human populations, is in the efficient growth of fish. Unlike major terrestrial species under culture, with few exceptions, fishes do not thermoregulate, and after consuming food, they generally remain relatively inactive during digestion. Further, due to the specific gravity of water, they can maintain buoyancy relatively easily with an internal air bladder, meaning that they don’t expend energy on heavy support structures. All of this equates to energy savings, which provides more available energy for growth, resulting in the highest feed conversion efficiency of widely domesticated animals.

Globally and in the USA, the per capita demand for seafood is increasing. In the USA, demand for seafood has exceeded domestic supply, resulting in a 9.6 billion dollar trade deficit (FAO 2010). While the current state of the economy has restricted the growth of aquaculture, the long-term outlook is quite good. Similarly, the worldwide outlook for aquaculture is very favorable. Increasing human populations combined with limited natural resources in freshwater and the world’s oceans, which are currently near maximum harvest yields (Hilborn et al. 2003), mean that the demand for seafood must be met by aquaculture. In recent years, global aquaculture production has increased to the point where it exceeds 50% of commercial capture fishery production (FAO 2010).

Future directions for aquaculture involve the refinement of current techniques, such as through the use of hybrid catfish and partitioned systems in the catfish industry. Further, recirculating systems and aquaponic systems are likely to grow in production and technological advances. These systems facilitate aquaculture almost anywhere, including the culture of marine species at locations far from the coast. This means a fresher and more environmentally friendly product, with less fossil fuels and expense in transport, packaging and storage, as well as the appeal of a locally-grown product. Further, water quality can be maintained within strict standards, and specific size requirements desired by consumers can be reliably produced. Two other forms of aquaculture that are likely to grow in the future, are marine off-shore aquaculture, and culture of species for the aquarium and pet-trade industry. In terms of marine aquaculture, offshore areas offer great potential for culturing species due to a lack of space restrictions, good water quality, and the ability to culture species that are in great demand. In terms of the aquarium industry, culture techniques for new species are continuously being investigated and refined. Further, aquaculture production reduces pressures on natural stocks, and environmentally unsustainable methods of capture. Finally, aquaculture is a world issue. Developing and developed nations will both benefit from an increased realization of the potential for aquaculture, as well as the need for protein derived from aquaculture in the face of continuing population growth.

Images courtesy of J. A. Steeby.

Exogenous - Not arising within the organism; due to an external cause

Effluent - Water exiting an aquaculture system

Hydroponics - A method of growing plants in water that utilizes nutrient solution inputs

Phytoplankton - Autotrophs in the pelagic zone of a body of water; passively floating or weakly motile algae

Biofuels - Fuels derived from biomass

Thermoregulate - Balancing heat gain and heat loss to maintain a specific temperature or temperature range