This report has set out to review present knowledge of the environmental impact of inland water intensive, semi-intensive and extensive methods of cage and pen culture with the aim of developing simple models which can be used to predict carrying capacity. Although a number of impact studies have been completed these have been largely concerned with the intensive culture of temperate water species, and have been focussed on qualitative rather than quantitative aspects. However, several studies are nearing completion, which should yield some of the data required to improve the models proposed above. Sadly, there are few such studies of extensive and semi-intensive enclosure culture in progress, despite rapid growth in these sectors of the industry.
Fish production from enclosures could be increased through the implementation of a number of strategies, all of which would result in a better utilisation of much pressurised resources. Wastes from intensive cage farms could be reduced by minimising P inputs to the water body and maximising P outputs. Inputs could be reduced by improving diet formulations, feed manufacturing technology, and methods of feeding fish. The P content of most commercial diets could be lowered, as P is usually present in excess of nutritional requirements, or in a form which is partially unavailable to the fish (see Section 4). The P-content of the diets is also highly variable, due to least cost formulation methods of manufacture (Tacon and De Silva, 1983). Thus, in theory the P content could be brought more into line with actual nutritional needs through improved formulations which would not only be lower in total P, but contain P in a more digestible form. Such feeds exist, but are more expensive to produce, and to date only one European manufacturer has found it profitable enough to market them. Advantages to cage fish farm operators not only include reduced risk/increased production, but also lower feed transport costs because of improved FCR (see below). However, an economic study of “low pollution” feeds and their use in cage fish farming is required in order to fully evaluate profitability.
Both extruded and expanded steam conditioned pellets have lower dust levels (Hilton et al, 1981), and the extruded type also float and have greater stability in water (Stickney, 1979), thus reducing the proportion of uneaten feeds. The FCR of extruded, steam-conditioned floating pellets seems to be better (Suwanasart, 1972; Hilton et al, 1981), although the carbohydrate fraction in the diet is increased to such an extent through the manufacturing process, that in rainbow trout at least, liver function could be impaired (Hilton et al, 1981). However, several novel feed manufacturing processes, which seem to improve pellet durability and which would thus reduce waste levels, are currently being evaluated (ADCP, 1983).
Little research has been carried out on feed presentation, and it is therefore difficult to conclude which method - manual/mechanical, automatic/demand - is best. Feed consumption pattern varies with species, size and temperature, but in the absence of hard data hand feeding is usually recommended for artisanal farming, whereas automatic feeders are recommended for more intensive operations. According to Goddard and Scott (1980), fish in cages should be fed over longer periods of time (i.e. the ration should be delivered to the cage at a slower rate), due to the relatively small surface area to volume ratio, compared with ponds.
However, the designs of present-day mechanical feeders used in cages are generally the same as those used in ponds and raceways, and should be examined more closely with the aim of reducing feed losses.
The net P loading to the environment could be reduced by application of a number of conventional lake and reservoir restoration techniques. Point-source control, or diversion of wastes from the water body, is a common method of reducing loadings (Welch, 1980), and has been demonstrated as technically feasible at enclosure fish farm sites by Tucholski et al (1980, 1980), who trapped the particulate waste fraction from cages and pumped them ashore. Sediment removal has also been used in restoration programmes (Jørgensen, 1980), but has not yet been attempted at inland water cage or pen sites. Submersible mixers, consisting of a large, electrically-driven propellor, have been used to disperse sedimented wastes from under marine cages, but would probably cause more problems than they would solve if used in inland sites. Here, the resuspension of sediments might halt localised H2S production, but would also be likely to stimulate algal production through increasing dissolved nutrient levels and destroying the thermocline. The actual removal of sediment from under cages is necessary, and this is prohibitively expensive (Welch, 1980). Tucholski et al's method of waste diversion would also be expensive, and impractical in commercial-sized operations.
Other, more practical methods of reducing impact from intensive farms include removal of mortalities and increased fisheries pressure. Penczak et al demonstrated that removal of dead rainbow trout from cages reduced the annual total-P loading to the lake by 10%. The capture and removal of escaped fish, through netting or angling can also help. In one cage rainbow trout farming operation in Scotland, for example, which produces in excess of 200 tonnes per annum, 10 tonnes were harvested through netting of escaped fish, whilst a further 2.5 tonnes were removed by anglers (A. Stewart, pers. comm.). This not only generated additional income to the farm, but also reduced the annual total-P loading to the lake by up to 1.3% (assuming 1.5:1 FCR, P content of feed = 1.5%, and P content of fish carcasses = 0.48% wet weight. See Section 4.4. Estimated reductions in waste outputs from intensive cage operations, based on methods suggested above, are summarised in Table 33.
Another, and as yet unresearched method of reducing the environmental impact of intensive cage fish farming, whilst improving the utilisation of water bodies for fish production would be to combine extensive with semi-intensive or extensive operations. In this way, expensive-to-culture fishes, such as gourami, which require high protein diets, could be reared alongside inexpensive species such as the tilapias or carps, the sale of which would help offset the costs of feed. The potential for such a scheme is considerable, and may make intensive enclosure culture, currently regarded as being marginally feasible in some tropical developing countries, a more realistic proposition. Such a scheme may also have potential in temperate countries, providing species suitable for extensive culture, from both technical and economic viewpoints, could be found. Greatest potential here probably lies in the use of carps, whitefish, and the planktivorous stages of carnivores, such as pike.
Despite careful planning, and minimising of any adverse impacts, it is highly probable that some types of inland water body will prove unsuitable for cage or pen culture. For example, in fast-flowing reaches of rivers and streams, high feed losses will affect the viability of intensive and semi-intensive operations (see Section 2.2). If extensive culture is practiced in such systems, then care must be taken to ensure that there is adequate natural food available for the particular species being farmed (see Othman et al, 1983). In some lentic systems there may also be insufficient food to support extensive culture. If, for example, primary production in a typically unproductive lake is around 50 g C m-2 y-1, then fish production of 50 kg ha-1 y-1, assuming a food conversion efficiency of 1%, might be expected. Thus a single cage measuring 5 × 5 × 5 m, and stocked with 5 fish m-3 would require all the algae produced in a 1 km-2 (10 ha) area, in order for the fish to reach a market size of 150 g. However it is uncertain (but doubtful) whether a single cage of fish would have access to all the algal production from such a large area, and at this level of primary production, the feasibility of extensive culture looks unpromising.
