The purpose of this report is to review what is known about the environmental impact of inland water cage and pen fish culture, and to examine possible methods for estimating carrying capacity. Efforts have been made to deal not only with intensive culture in temperate countries, but also with the more extensive methods practiced in the tropics, and to choose predictive models which are comparatively simple and inexpensive to use.
The report proper is preceeded by a brief account of cage and pen culture, and its relative importance in contemporary aquaculture.
There is some confusion concerning the terms ‘cage culture’ and ‘pen culture’ in fish farming. Both terms are often used interchangeably, particularly in North America, where ‘sea pens’ and ‘sea cages’ describe the same method of culture (e.g. Novotny, 1975, Saxton et al, 1983), or the general term ‘enclosure culture’ is used to describe what more precisely could be defined as cage or pen culture (e.g. Milne, 1979). Both cage and pen culture are types of enclosure culture, and involve holding organisms captive within an enclosed space whilst maintaining a free exchange of water. The two methods, however, are distinct from one another. A cage is totally enclosed on all, or all but the top, sides by mesh or netting, whereas in pen culture the bottom of the enclosure is formed by the lake or sea bottom (Fig. 1).
Like most other types of aquaculture, cage culture began in Southeast Asia, although it is thought to be of comparatively recent origin (Ling, 1977). It seems to have developed independently in at least two countries. According to Pantalu (1979), the oldest records of cage culture come from Kampuchea where fishermen in and around the Great Lake region would keep Clarias spp. catfishes and other commercial fishes in bamboo or rattan cages and baskets until ready to transport to market. In captivity, the fishes were fed kitchen scraps and were found to grow readily. This traditional method of culture has been practiced since the end of the last century, and is now widespread throughout the lower Mekong area of the country (Ling, 1977). From here it has spread in recent year to Viet Nam, Thailand and other Indo-Chinese countries.
A similar type of cage culture, using floating bamboo cages to grow Leptobarbus heoveni fry captured from the wild, has been practiced in Mundung Lake, Jambi, Indonesia since 1922 (Reksalegora, 1979), and has since been extended to other parts of southern Sumatra. Yet another form of cage culture seems to have begun independently in Java, where Vass and Sachlan (1957) reported that the capture and enclosure of carps in submerged bamboo or ‘bulian’ cages has been practiced since the early 1940s. Cages were usually anchored to the bottoms of small, organically enriched streams, where the captive carp fed and grew on organic material and benthic organisms carried in the drift. However, this method of culture is still almost solely restricted to west Java and Sumatra (Sodikin, 1977), and has had little influence on cage culture practices in other countries.
In the last 15 years or so, the practice of cage culture in inland waters has spread throughout the world to more than 35 countries in Europe, Asia, Africa and America, and by 1978 more than 70 species of freshwater fish had been experimentally grown in cages (Coche, 1978a). In all but a few areas, new materials such as nylon, plastic, polyethylene and steel mesh which although much more expensive have a much longer life-span and permit better water exchange, have superceded wood and bamboo. Most designs currently in use are of the floating type, and rely on a buoyant collar constructed either from locally available materials (e.g. wood, bamboo), or from steel or plastic pipe, and from which is suspended a synthetic fibre net. Styrofoam or oil drums are frequently used for supplementary flotation.
Cages are usually floated in rafts, and either anchored to the lake/reservoir/river bottom, or alternatively connected to shore by a wooden walkway (Fig. 2).
In some parts of the world such as China and the Philippines, fixed cages are used in shallow waters (<8m) with appropriate muddy bottoms (FAO, 1983). Synthetic fibre net bags are attached to posts driven into the substrate. They are simpler and cheaper to construct as they don't involve the construction of a buoyant collar, which can account for more than 50% of the capital outlay (see IDRC/SEAFDEC, 1979). However, fixed cages are often poorly constructed, and thus may be less able to withstand adverse weather conditions. For example, in July 1983 almost all of the fixed cages in Lake Buhi, Bicol Region in the Philippines were destroyed by Typhoon Bebeng, whereas most of the floating cages survived.
There are approximately ten species of fish which are commercially cultured in cages in both temperate and tropical waters, and these are listed in Table 1.
The origins of pen culture are more obscure, but it also seems to have begun in Asia. According to Alfarez (1977) and others, pen culture originated in the Inland Sea area of Japan in the early 1920s. It was adopted by the People's Republic of China in the early 1950s for rearing carps in freshwater lakes, and was introduced to Laguna de Bay and the San Pablo Lakes in the Philippines by the Bureau of Fisheries and Aquatic Resources (BFAR) and the Laguna Lake Development Authority (LLDA) between 1968 and 1970 in order to rear milkfish (Chanos chanos) (PCARRD, 1981).
Pens are still constructed in much the same way as they always were, except that nylon or polyethylene mesh nets have replaced the traditional split bamboo fences. The nets are attached to posts set every few metres, and the bottom of the net is pinned to the substrate with long wooden pegs. Buttressing may be used to strengthen the structures in exposed areas. Pens are usually built in shallow (<10m) waters, are 3–5m deep, and 1–50 ha in size (IDRC/SEAFDEC, 1979). Soft substrates are preferable.
The development and adoption of inland water pen culture has been much less dramatic than that of cage culture, and at present it is only practiced on a commercial basis in the Philippines, Indonesia and China (Dela Cruz, 1980, 1982; Lam, 1982). The principal species being cultured in these countries are milkfish and carps (e.g. grass carp, Ctenopharyngdon idella; bighead carp, Aristichthys nobilis; silver carp Hypophthalmichthys molytrix (Table 1). Some experimental pen culture of carps has been carried out in pens in oxbow lakes in Hungary (Muller, 1979; Muller & Varadi, 1980), and other countries such as Bangladesh and Egypt have expressed interest in their use (Ishak, 1979; Karim and Haroud-al-Rashid Khan, 1982). The production of tilapias in net pens is also currently being evaluated in the Philippines (Guerrero, 1983).
Because of their smaller size (generally 1000m2 surface area) and because they are easier to manage, cages are more adaptable than pens and can be used not only for grow-out of fish to market size, but also for breeding and fry production of fishes such as the tilapias (Pagan-Font, 1975; Rifai, 1980; Guerrero, 1983; Beveridge, 1984) and for nursing of the planktivorous juvenile stages of carps, white fish and pike (Bronisz, 1979; Jäger and Kiwus, 1980). Pens are largely restricted to lentic water bodies, whereas fixed and floating cages are also used in rivers and streams. However, in most cases both systems are used for monoculture.
Cages and pens have several advantages over other methods of culture (Table 2). Because they use existing water bodies, require comparatively low capital outlay and use simple technology, they are popular with farmers, extension workers and development programmes. They can be used not only primarily as a method for producing high quality protein cheaply but also, as is happening in Malaysia and Singapore, to clean up eutrophicated waters through the culture and harvesting of caged planktivorous species (Yang, in press; Awang Kechik et al, in press) and to improve conditions in acid lakes in Scandinavia (Swedish Research Council, 1983). Thus, despite accounting for only 5–10% of current inland water aquaculture production, growth in this sector is rapid.
However, concern is growing about the environmental impact of these methods. Intensive culture is believed to accelerate eutrophication, and extensive cage and pen farming has had a record of high initial promise, followed by decreasing production figures. Subsequent sections of this report classify and review different methods of enclosure culture, discuss environmental impact and attempt to model the effects of culturing fishes in cages and pens in inland waters.