Inland fisheries management strategies may be classified in many ways. For the purposes of this paper they are considered under three headings:
Management aimed at improving fish habitats and/or facilitating fishing operations.
Management measures aimed at conserving/protecting fish populations through control of fishing activities.
Management aimed at enhancing fish populations and yields.
Changes to the physical environment in and around the water body can be made to improve the living or feeding conditions for fish and/or to facilitate access for fishermen or use of certain gears.
Fertility of fresh waters can occasionally be too high, due to runoff of nutrients, holding of large numbers of animals around the shoreline etc. This can lead to excessive plankton blooms which produce anoxic conditions in the water when they break down. This problem is difficult to solve except by reducing the levels of nutrient input.
More commonly, however, the fertility of natural water bodies is below the optimal for fish productivity. Small static waters up to a few hundred hectares in area can be fertilized by application of organic or inorganic fertilizers in the same way as fish ponds. In seasonal water bodies or perennial ones with large annual fluctuations in water level, worthwhile fertilization can sometimes be accomplished by grazing animals on the exposed bed during the dry season.
The aim of increasing water fertility is largely to increase the phytoplankton population. This feeds through the food chain to increase biological production at all levels, including fish. In some waters phytoplankton development is impeded by excessive growth of macrophytes, especially floating weeds such as water hyacinth. Thus mechanical, chemical or biological removal of dense macrophyte cover is usually beneficial for fish yields. In very small waters, shading by trees overhanging the margin can be a significant impediment to phytoplankton development, and removal of these trees would improve productivity.
Rotting of inundated vegetation can cause water to become acidic, thus reducing productivity. Burning off grass and removing trees before inundation helps prevent this, and in small areas of water liming can be used to correct acidity as in artificial fish ponds.
Water level is very important in the breeding cycle of many fishes. Some species require the stimulus of rising water level to trigger spawning. These fishes generally move onto newly inundated areas at the water margins to spawn, and the resulting fry require the shallow flooded areas to be maintained through their nursing period. Thus the maintenance of constant water level by man-made control structures is not conducive to successful reproduction of such species, and instead allowance should be made for some water level fluctuation synchronized with the natural seasonal cycle.
In addition, fishes which nest in shallow waters around the shoreline may have their reproduction upset by falls in water level which leave their nests high and dry. Water levels should therefore be maintained during the breeding season if reproduction of these species is considered desirable. On the other hand, disruption of nesting might be a useful tool in controlling overpopulation of certain species, e.g. some tilapias.
Maintenance of minimum water level or the prevention of complete dewatering is clearly advantageous in maintaining broodstock through the dry season. Where water bodies normally become empty, refuges can be constructed by the building of water retaining embankments (forming artificial ponds), or excavation of fish pits.
Access to underutilized water bodies can be improved by construction of paths or tracks to the shoreline and landing facilities for boats, and by cutting dense rooted vegetation around the margins.
The use of certain gears, especially nets, may be facilitated by clearing obstructions (e.g. submerged trees) from the bottom and removing excessive vegetation. Operation of boats is also dependent on maintenance of open water by control of floating and rooted vegetation where necessary.
The construction of dams on rivers, flood control embankments, and water control structures in canals etc. can act as impassable barriers to the movements of migratory fishes, cutting them off from their breeding and/or feeding grounds. The obstructions can affect actively migrating adults moving either up or downstream, and the normal distribution of juveniles drifting passively downstream with the current. In some instances, passage of fish through artificial barriers can be restored simply by opening the water control structures, e.g. sluice gates, at times when migrating fish are moving. Where upstream migration across a large head difference is required, e.g. in the case of broodstock attempting to reach spawning grounds upstream of a dam, construction of specially designed fish passes or ladders can be the solution for species capable of using them.
Where access to breeding areas is impeded by natural obstacles, e.g. sandbanks across the mouth of a spawning stream, these can be removed or breached by dredging to permit fish passage.
It is sometimes practicable to improve the spawning grounds themselves, e.g. by planting suitable vegetation in shallow water to act as a substrate for spawning, or cleaning gravel beds to improve oxygenation and survival of fish eggs laid on them. In extreme cases, artificial spawning beds can be constructed.
Legal restrictions on fishing activity have the general aim of preventing over-exploitation of fish stocks. Regulations have two specific primary objectives:
Control of the age at which fish first become liable to capture, with the aim of postponing harvest until after they have had the opportunity to mature and breed.
Limitation of overall fishing mortality to ensure sufficient fish escape capture to maintain the population at a biologically and economically viable level.
Commonly applied regulations to prevent killing of immature fish are the setting of a minimum takeable size (length) limit, minimum mesh size for nets, and closure of the fishery during that period of the year when the majority of fish are still undersized.
Such measures must be species specific, and can therefore become very complicated in the type of multi-species, multi-gear fishery being considered here.
Regulations to achieve this goal include the designation of fish reserves, i.e. areas permanently closed to fishing, and closed seasons during which fishing is temporarily forbidden. Most commonly, the fishery is closed during the fish breeding season.
During periods when fishing is allowed, restrictions may be placed on the number of fishermen or boats permitted to operate. Boat size, type (e.g. engined, wind or man-powered), power etc. can also be specified. This type of control is normally administered by some form of licensing system.
The type and number of gears which may be used by each boat or fisherman can also be legislated. Normally, “destructive” gears such as use of explosives, poisons and often electric fishing, are banned altogether.
Introductions of fish species which do not naturally occur in a water body, or which were formerly present but have disappeared due to some natural or man-made catastrophe, can be appropriate where a vacant ecological niche exists. For example, many eutrophic lakes support dense populations of phytoplankton or macrophytes, but few native fish species are capable of utilizing these. The term “introduction” or “transfer” is generally used to mean a “one-off” (or occasionally repeated until it is successful) release of a species which is capable of developing a self-sustaining, naturally breeding population in the recipient water. According to convention, an introduction generally refers to a species moved beyond its natural or current geographical distribution range, whilst a transfer is done within that range.
In Asia, the purpose of inland fishery activities is almost entirely to provide fish for human consumption (by contrast to certain western countries, where the primary aim is for sport or recreation). Since production of herbivores or omnivores yields a greater biomass of fish from a water body than would species higher up the food chain, it is not usual to introduce carnivores. On the contrary, frequently strenuous efforts are made to eliminate predatory fishes which reduce yields of other species.
Sometimes species other than fish, e.g. zooplankton and benthic invertebrates, are introduced with the aim of increasing food supplies for fishes. The same principle of bringing into productive use a previously underutilized food source (in this case, for example, phytoplankton or bottom detritus), applies.
There are relatively few fish species which are considered valuable as human food, but which are capable of efficiently utilizing some of the most abundant primary food sources in inland waters, especially plankton and macrophytes. Many of the most efficient amongst them, notably the Chinese carps and the Indian major carps, are riverine species usually incapable of forming self-sustaining, breeding populations in lakes, reservoirs etc. Consequently the use of these fishes requires repeated, frequently annual, releases of juveniles to maintain a population sufficient to support a fishery. Fry or fingerlings for release must normally be supplied by artificial propagation in hatcheries, and consequently this management system may be referred to as a “culture-based” or “culture-enhanced” fishery, or simply as “fish stocking”.
The above management approaches can be seen as progressive steps, each involving increased human intervention, towards maximizing fish production from natural waters through increasing intensity of fish culture. In small lakes or reservoirs up to a few hundred hectares in size, this can culminate in the water body being used in exactly the same way as a fish pond. Small perennial waters can receive fertilization and supplementary feeding, and in this case the majority of the harvested fish will be derived from stocked juveniles. Seasonal waters may be treated even more like fish ponds, undergoing annual preparation of the bottom, liming etc. during the dry season. Fry may be stocked artificially and/or allowed to enter naturally with the onset of the flood season.
For water bodies of many hundreds or thousands of hectares, and for rivers, however, intensive culture requires the use of some means to confine the fish to a manageable area and to prevent them escaping. Occasionally, shallow lakes up to a few metres deep are completely subdivided into workable sections by walls of netting suspended from timber or bamboo posts stuck into the bottom. More commonly, arms or bays of large lakes are separated from the main water body by a netting (or mesh made from other material such as bamboo or steel) barrier, or shallow areas adjacent to the shore are fenced off by nets or barriers inside which fish are grown. The latter system is referred to as pen culture. However, by far the most widely used method of confining cultured fish in a large water body is the deployment of cages.
In shallow water, net cages can be supported by bamboo or wooden poles stuck into the bottom mud, but floating cages are much more commonly used because they can be moored in almost any depth of water.
Floating cages come in a wide variety of sizes and construction materials. All have in common the basic elements of a floating collar supporting a submerged water-permeable enclosure within which the fish are confined. In Asia, most cage enclosures are made either of fish netting (nylon or polypropylene) or of slats made from split bamboo or timber. Occasionally steel, plastic or other mesh is used. Flotation is provided by bundles of bamboo, empty oil or chemical drums, or less commonly by specially made polystyrene or plastic floats. The floats are often surmounted by timber or bamboo walkways to make working conditions easier. Cages are moored either by ropes or cables to the shore or to anchors. The tops of cages are sometimes covered by nets to exclude predatory birds and animals. To prevent theft, it is often necessary to maintain surveillance, A hut or shelter is frequently built on top of each group of cages for the comfort of the watchman and as a storage facility for fish food and equipment. In some areas it is customary to erect permanent dwellings on top of the fish cages, where the whole family lives. Substantial floating villages, complete with shops and other facilities, are well established in some places.
Cage culture is a versatile technology suitable for farming a wide variety of fishes, ranging from carnivores (e.g. catfishes and snakeheads), through omnivores (e.g. cyprinids and tilapias) to herbivores (e.g. Chinese carps). The choice of species depends on local availability of juveniles (which are either captured from the wild or bought from hatcheries), availability of suitable foodstuffs, and market demand and price.
