CHAPTER 12
AQUACULTURE
What is aquaculture?
Aquaculture may be simply defined as the growing of aquatic organisms under controlled conditions(Bardach, et al., 1972). As such, aquaculture can take many forms. Strictly speaking, any intervention in the life cycle of an aquatic organism intended to increase the production of that organism for the benefit of mankind might be classified as aquaculture. Hawaiians, for example, transplanted juvenile fish from the ocean into confined coastal ponds where the supply of natural foods stimulated the growth of the fish. The practice of merely transplanting an aquatic organism to a habitat more favorable for growth is a simple form of aquaculture. A more sophisticated form of aquaculture would involve manipulation of the pond environment and the supply of food for the fish. In some cases the natural productivity of the system is stimulated by the addition of fertilizers, in which case the supply of food for the cash crop is controlled only indirectly. Such schemes, for example, have been used to culture milkfish and shrimp in Southeast Asia. Similar strategies may be used to grow shellfish, which feed by filtering particles out of the water. In other cases the cash crop is nourished directly through the use of pelletized feed, the culture of catfish and trout being cases in point.
Regardless of the mechanism of feeding the cash crop, control of water quality invariably becomes an issue in aquaculture. An adequate supply of oxygen and a mechanism for removing metabolic waste products are essential. Temperature and, in some cases, salinity, must be kept within bounds favorable for growth. When organisms become stressed they become susceptible to infection and disease. Stresses created by poor water quality can therefore impact yields both directly and indirectly.
Strategies for dealing with water quality vary greatly, depending on the availability of resources and the characteristics of the cash crop. Where there is an abundant supply of clean water, the simplest way to control water quality is to operate in a flow-through mode. When this is not an option, water quality is typically controlled by operating what amounts to a polyculture system in which the role of some organisms is to maintain oxygen levels and/or remove metabolic wastes produced by others. Aquatic plants, for example, often perform this service in aquaculture systems in which fish or crustaceans are the cash crop.
Table 12.1. Fish production in properly managed ponds under different management regimes. Source: Hepher and Pruginin (1981)Management regime / Yield (tonnes ha-1 y-1)
Fertilized ponds, no supplementary feeding / 2.4
Heavily manured ponds, some feeding with rice bran / 6.2-7.0
Heavily manured ponds, no supplementary feeding (results extrapolated to 240 days) / 4.2-7.1
Fish fed on protein-rich pellets / 7.3-10.9
High stocking rates (more than 14,000 per hectare), fish fed on protein-rich pellets (results extrapolated to 240 days) / 12.2-35.7
Monoculture of carp in running water / 1,360 – 1,800
Table 12.1 rather dramatically illustrates the impact of different feeding and water quality management regimes on fish production in aquaculture systems. Yields of a few tonnes per hectare (ha) per year are achievable in closed ponds in which the food for the cash crop is provided by the natural food chain in the pond. Production in such cases is stimulated by the addition of fertilizer or manure. When the fish are fed more directly with protein-rich pellets, yields are on the order of tens of tonnes ha-1 y-1. Yields increase dramatically when water quality is controlled through rapid water exchange. It is important to bear in mind, however, that the effective size of a running water system is actually much larger than the nominal area of the culture facility.
Aquaculture and Agriculture
It is fair to say that the husbandry of aquatic organisms is de facto a much younger science (or art) than the husbandry of terrestrial organisms. The origins of agriculture go back roughly 7,000 to 10,000 years to the Fertile Crescent (Natufian or Sumerian culture), East Asia (rice), and Central America (maize, squash). Oyster culture can be dated to ancient Rome and Gaul, and there are less certain reports of fish culture in China roughly 2,500 years ago (Bardach, et al., 1972). Given its longer history, it should not be surprising to find that agriculture has achieved a degree of sophistication not observed in most aquaculture enterprises. This is indeed the case, and it is instructive to compare the two husbandry systems to perhaps gain some insight about the future of the aquaculture industry.
Agriculture is very much focused on the production of plants and herbivores. This should not be surprising when one considers the inherent inefficiency of food chains in transferring organic matter from one trophic level to the next (Chapter 1). As in agriculture, the husbandry of aquatic animals has tended to focus on organisms that would logically be assigned to a low trophic level. Unlike agriculture, there has been some success in raising aquatic animals that are not strictly herbivores or detritivores, but the commercial value of carnivorous fish must be high indeed to warrant the expense associated with feeding them. Most agricultural production is in fact accounted for by the raising of plants, not animals, and the agricultural production of crops such as rice, corn, and wheat figures prominently in human nutrition. As cash crops and sources of human nutrition, plants are much less important in aquaculture than in agriculture, although they are obviously essential to the production of herbivores and perhaps less obviously to the maintenance of water quality.
From a nutritional standpoint aquatic organisms differ from terrestrial plants and animals in some important ways. Aquatic organisms have little need for support tissue, since their density is similar to that of water.[1] Phytoplankton in particular may be as much as 50% protein. As noted in Chapter 1, fish are also an excellent source of protein, and their lipid tissue is one of the few good sources of-3 polyunsaturated fatty acids (PFA’s), whose presence in the human diet is needed to balance the intake of -6 PFA’s from other fats and oils (Chapter 1). Although it is fair to say that most aquaculture production is targeted for human consumption, this is by no means true in every case. Pearl oysters and ornamental fish are two cases in point. Some hatcheries rear and release fish to satisfy the needs of sports fishermen, and bait fish may be grown in culture for use by both recreational and commercial fishermen. Seaweeds are an excellent source of colloids such as agar, algin, and carrageenan. Although many uses of these colloids involve foods, some do not. Carrageenan, for example, is used as a stabilizer in toothpaste and as a gelling agent in air freshener, and agar is used in the production of solid growth media for the cultivation of bacteria and microalgae.
One of the important differences between aquaculture and agriculture is the extent to which breeding has impacted the latter. Virtually every important agricultural crop is the result of generations of selective breeding of varieties with desirable traits. The history of maize cultivation (Mangelsdorf, 1947)is certainly a case in point[2], but it is only one of numerous examples that could be cited. Although some selective breeding has been done with aquacultured species (e.g., tilapia), overall breeding has had much less of an impact on the aquaculture industry than the agriculture enterprise.
Overview of Aquaculture
Figure 12.1 shows how world aquaculture production has grown since the middle of the 20th century. Compared to capture fisheries, aquaculture production was virtually nonexistent in 1950, but at the present time aquaculture accounts for about 36% of global fisheries production (capture fisheries plus aquaculture) by weight and about 43% of the economic value of the catch. The wholesale value of the aquaculture catch is about $1.17 per kilogram, which is substantially higher than the corresponding figure of $0.84 kg-1 for capture fisheries.
Empirically freshwater fish have proven much easier to culture than marine species, a fact that is evidenced in the comparison of freshwater and marine fish aquaculture production in Table 12.2. The reason for the greater success with freshwater fish is the fact that many marine fish, and most aquatic invertebrates, produce small eggs that hatch into tiny, delicate larvae. In the case of aquatic invertebrates, the larvae may go through many developmental stages, each with distinct environmental and nutritional requirements (Bardach, et al., 1972). Rearing such sensitive and finicky organisms to adulthood is a difficult task. When it comes to reproduction, freshwater fish are more toward the K end of the r-K spectrum. They tend to produce fewer but larger eggs, and their larvae are generally hardier than marine fish larvae. A number of the intensively cultured freshwater fish protect their eggs in some way[3], whereas marine fish simply release their eggs into the water. The net result is that it is much easier to rear freshwater fish through the egg and larval stages than is the case with marine fish.
Figure 12.1. World aquaculture production since 1950 and the economic value of the produce since 1984.
Table 12.2. Breakdown of world aquaculture production by major category of cash crop in 2002.Category of cash crop / Percentage of aquaculture production
Freshwater fish / 42.7
Mollusks / 23.0
Aquatic plants / 22.6
Diadromous fish / 5.0
Crustaceans / 4.1
Marine fish / 2.3
The biological issues associated with a successful aquaculture operation can be summarized with four words: seed, feed, weed, and breed. Problems and opportunities that arise within these four areas are summarized in the following paragraphs.
FEED. Any cultured organism must be fed. The easier it is to identify and obtain the right kind of feed, the more successful the operation is likely to be. There is no question that some of the species that have been grown with great success in aquaculture systems have been very easy to feed. Problems associated with feed are minimized if the natural food web in the culture facility can provide the nutrition for the cash crop. Analogues in agriculture include animals like cattle and sheep that are content to eat grass. In aquaculture systems, carp, tilapia, mullet, and milkfish are all capable of using natural pond productivity. Milkfish, for example, can subsist on a diet of benthic algae and associated protozoa and detritus. Mullet feed on plankton, benthic algae, and, in ponds, decaying higher plants (Bardach, et al., 1972). Tilapia will consume phytoplankton, but supplemental feeding of tilapia ponds is desirable. However, the supplemental feed need not be expensive. Kitchen refuse, rotten fruit, coffee pulp, and mill sweepings have all been used to stimulate tilapia production.
Carp come in a variety of species that, when used in combination, can make remarkably efficient use of natural pond productivity. Table 12.3 lists the species of carp used in classical Chinese carp polyculture and their feeding niches. A well managed carp polyculture pond requires little or no supplemental feeding. The food web is stimulated with fertilizer and/or manure. A key to the success of the polyculture system is the right mix of species and stocking densities. A typical system favors planktivorous fish, since production in these highly eutrophic ponds is usually dominated by the plankton community. An interesting variation on the fish polyculture model is an integrated aquaculture/agriculture system in which animal (typically pig) waste from the agricultural component is used to fertilize the fish pond, and sludge from the bottom of the fish pond serves as a soil conditioner and fertilizer on the agricultural land.
The significant contribution of mollusks to aquaculture production is accounted for almost entirely by bivalves such as clams, cockles, oysters, scallops, and mussels. These animals are all filter feeders that consume suspended particles, including plankton. Feeding bivalve mollusks can be as simple as locating the aquaculture facility in a productive body of water. Some of the highest yields ever attained in aquaculture, 300,000 kg ha-1, have involved mussels grown on rafts in the Galician bays of Spain. This is another case where specification of yields on an areal basis is misleading, since currents and mixing effectively transport particles to the suspended mussels from a much wider area than is occupied by the raft facility. Nevertheless, from the standpoint of feeding the cash crop, the system is remarkably effective and inexpensive.
Table 12.3. Carp species used in classical Chinese carp polyculture and their associated feeding niches (Bardach, et al., 1972)Carp species / Feeding niche
Black carp / mollusks
Common carp / Benthic animals and detritus, including grass carp feces
Mud carp / Benthic animals and detritus, including grass carp feces
Silver carp / phytoplankton
Big head carp / zooplankton
Grass carp / Vegetable tops
Moving to the base of the food chain, we come to aquatic plants. Feeding aquatic plants is straightforward. They require sunlight and inorganic nutrients. From the standpoint of feed, manipulation of their growth environment involves nutrient additions, typically nitrogen and phosphorus. In very intense culture systems, inorganic carbon may also be added, typically by bubbling in carbon dioxide in a counterflow system. Large scale production of macroscopic aquatic plants such as kelp may simply involve setting out seed (gametophytes or sporophytes) on an appropriate substrate in a favorable growth habitat. Aside from habitat selection, there is no further effort to manipulate the supply of inorganic nutrients and sunlight. Intense culture of microalgae, sometimes the cash crop and in other cases a source of food for mollusks or larval stages of fish, often involves addition of inorganic nutrients and in some cases manipulation of the light regime by artificial illumination.
At the other end of the spectrum (or food chain) with respect to their food requirements are carnivorous aquatic animals such as catfish and trout. Culture of such organisms can be justified if the value of the cash crop is sufficiently high and a supply of the right kind of food available at a reasonable cost. Denmark, for example, achieved preeminence in commercial trout culture because of the availability of trash fish at Danish fishing ports (Bardach, et al., 1972). Carefully formulated pelletized feeds are an alternative to trash fish, and in theUnited States the rapid expansion of channel catfish aquaculture during the last two decades of the 20th century (Fig. 12.2) was due in no small part to the availability of pelletized catfish chow from Purina Mills. Channel catfish currently account for 50% of U.S. aquaculture production by value and 58% by weight.
Figure 12.2. Channel catfish production in the United States.
One interesting strategy for feeding carnivorous fish is to simply rear the fish to an age when they can presumably fend for themselves and then release them to the wild. This strategy is exemplified in salmon ranching. At salmon ranch hatcheries, adult salmon are spawned, the eggs are hatched, and the juvenile salmon are reared in tanks until they are ready for release. The salmon find their way to the sea, where they remain until they become sexually mature. They then return to the hatchery, where they are harvested. The profitability of the operation requires that at least 3-5% of the salmon in fact return to the hatchery, and therein lies the rub. Although the cost of feeding the salmon while they are at sea is zero, it is not uncommon for 98-99% of the salmon to be lost to natural and fishing mortality. The result is that privately funded ocean ranching of salmon has failed miserably. Most commercial aquaculture of salmon at the present time involves maintaining the salmon in pens or other enclosures throughout their life and feeding them with fishmeal typically made from clupeids or remnants from fish processing. As such, salmon farms are actually net consumers of fish. About five tonnes of landed fish are required to produce a tonne of farmed salmon. Atlantic salmon is the preferred species for farming because it grows rapidly and is more disease resistant than other candidate species such as Chinook and Coho. At the present time world production of farmed salmon is dominated by Norway and Chile (Fig. 12.3).
Figure 12.3. Aquaculture production of farmed salmon by the four major salmon farming nations.
The impact of feeding regime on fish production is dramatically illustrated by the results of experiments conducted with carp aquaculture ponds in Israel. In the studies summarized in Fig. 12.4, juvenile carp were stocked into ponds at the same initial density (1,000 fish per hectare), regardless of the feeding regime. The solid line shows the growth rate of the fish when the supply of food is not limiting.
Figure 12.4. Average growth rates of carp at 1,000 fish per hectare under different feeding and fertilization treatments at the Fish and Aquaculture Research Station, Dor, Israel (Hepher and Pruginin, 1981). The solid line is the potential growth rate of the fish when food is not limiting. The dashed lines show the growth rates of the fish when the critical standing crop is exceeded. Growth rates drop to zero at the carrying capacity. (A) no fertilization or feeding, (B) chemical fertilization, no feeding, (C) Chemical fertilization, feeding with sorghum.
The critical standing crop (CSC) is the biomass of fish (kg ha-1) at which the growth rate of the fish begins to drop below the solid line. In the case of the no fertilization and feeding management regime, for example, the CSC is 65 kg ha-1, which means that fish stocked at 1,000 per hectare would have an average weight of 65 grams. As the biomass of the fish continues to increase, their growth rate slows even further, and eventually growth ceases altogether. This point corresponds to the carrying capacity (CC) of the system under the given management regime. Table 12.4 summarizes the information on CSC and CC obtained from the experiments carried out by Hepher (1975).