This article is a slightly edited version of the original in:FAO. 1999. Review of the state of world fishery resources: inland fisheries.FAO Fish. Circ. No. 942. Rome, FAO. 54pp.

Introduction

Most inland capture fisheries that depend only
on natural production are exploited above or close to their sustainable maxima and as a result global capture fisheries production is increasing at a slow rate. Increasing pressure on the fishery resources, environmental degradation of aquatic habitats and poor fisheries management have contributed to this situation. Conventional fisheries management measures such as regulation of minimum mesh sizes, closed areas and closed seasons are used to counteract this situation, but these measures can be difficult to enforce. In such cases, other techniques can be used. These techniques can be collectively termed as enhancements and include various possibilities to intensify fishery production. These include:

• introduction of new species to exploit under-utilized parts of the food chain or habitats not colonised by the resident fauna;

• stocking natural waters to improve recruit-ment, bias fish assemblage structure to favoured species or maintain productive species that would not breed naturally in the system;

• fertilization to raise the general level of productivity and hence growth of the fish;

• engineering of the environment to improve levels of reproduction, shelter, food resources and vital habitat;

• elimination of unwanted species that either compete with or predate upon target species.

• constituting an artificial fauna of selected species to increase the degree of control and the yield from the system;

• modification of water bodies to cut off bays and arms to serve for extensive and intensive fish ponds to increase control and nutrient flows;

• introduction of cage culture and parallel intensification of effort of the capture fishery;

• aquaculture through management of the whole system as an intensive fish pond;

• genetic modification to increase growth, production, disease resistance and thermal tolerance of the stocked or cultured material.

Inland fishery enhancements are becoming a central theme in inland waters and are widely applied nowadays. The exact contribution of enhancements to the total inland capture fisheries production is difficult to estimate, but it is believed that many inland fisheries are supported by various forms of enhancement, often combined with conventional fisheries management practices.

FAO has been increasingly involved with the promotion of inland fishery enhancements; however, this is the first attempt to globally and comprehensively characterize inland fishery enhancements by taking into account en-hancement types, their geographic distribution and the species employed. The results are summarized in the following sections.

Of the first four enhancement types mentioned above (i.e. introductions, stocking, engineering of the environment and fertilization), intro-ductions and stocking are the most commonly applied (Table 1). Introductions and stocking programmes are mostly applied to lakes, reservoirs and rivers and less information is available on the enhancements of fisheries in other kinds of water bodies, although specific examples exist. There exist also a large number of small water bodies such as village ponds, and small irrigation tanks that have potential for enhancement. Stocking of fish in these smaller

water bodies has been generally more successful because these are easier to manage, do not require large amounts of stocking material and are often more productive. In contrast, introductions of new species with the aim to establish self-reproducing populations (auto-stocking) have been more effective to enhance fisheries in the larger water bodies.

Most enhancements are carried out to produce food and generate income (fisheries production), but also for the benefit of recreational fisheries, to restore collapsed fisheries and to control pests (aquatic weeds, mosquitoes).

Table 1. Available information on enhancements in Aquatic Sciences and Fisheries Abstracts ¹

¹ ASFA was searched as follows: 1/1978 _ 8/1996 for stocking; 1/1978 - 12/96 for introductions; 1/1978 _ 6/97 for engineering and fertilization

The most widespread introduced species that have established self-reproducing populations in open waters and globally contribute significantly to food production are Mozambique tilapia, common carp, rainbow trout, Nile tilapia and brook trout (Table 2). Mosquito fish and guppy have been widely introduced for mosquito control and grass carp for the control of aquatic weeds. The goldfish has been mainly introduced for ornamental reasons. The distribution of introduced species that established in the wild is given in Figure 1.

World Perspective on Stocking

With regard to stocking, common carp, rainbow trout, Atlantic salmon and brook/sea trout are relatively widespread as can be seen in Table 3. Most of the species in the table are stocked for food production, income generation and for recreational fisheries. Common carp and rainbow trout are also important introduced species that successfully established, but these species are also supported by hatchery production. The geographic distribution of stocked freshwater species is given in Figure 2. This figure is primary based on hatchery production data provided to FAO by its member countries, but for some countries with incomplete hatchery production data (e.g. China, Canada), species have been added on the basis of scientific literature. 

The United States, Poland, Australia, Finland, France, Iran and Mexico report the highest number of species.

The available information on stocking, intro-ductions and translocations, derived from an extensive search in the Aquatic Abstracts and Fisheries Sciences database, shows that the highest number of references deal with North America, Europe and Russia (Figure 3). Various countries in Asia and Oceania (China, India, Australia, Thailand and Papua New Guinea) have a considerable information base. Published information in the ASFA database is relatively scarce for Africa and Latin America. However, as described in the previous paragraphs, these continents do have fisheries that are supported by stocked and introduced species. It is important that the results of such stocking programmes and introductions are published to allow further evaluation of these techniques.

In the following paragraphs, a brief overview is given for each region.

Asia and Oceania

A variety of species have been introduced to this region, mainly for the purpose of aquaculture. Many of these have established populations in open waters. Mozambique tilapia, common carp, rainbow trout and Chinese carps are important for commercial capture fisheries. For example, Mozambique tilapia established itself successfully in the larger reservoirs in Sri Lanka where it is now the main landed species (de Silva, 1988). This species also established itself successfully in many Indian reservoirs (Sugunan, 1995). Translocations of species within the country have created new fisheries in China. Most recently, icefish (Neosalax taihuensis and Protosanlax hyalocranius) was very successfully transplanted to various provinces of China. By 1997, the production reached 10 000 tonnes, of which transplanted icefish contributed as much as 8 000 tonnes (Miao, pers. comm.). Translocations of indiginous species have also been one of the major measures for enhancement of inland water bodies in Australia, mainly for the purpose of recreational fisheries (Petr, 1998). Introduction of Nile tilapia and common carp into larger reservoirs of the lower Mekong countries (Laos, Cambodia, Vietnam and Thailand) were generally successful whereas non-indigenous Indian and Chinese carps generally failed to establish themselves (Bernascek, 1997). Introduction of common carp in Indian reservoirs was not successful (Sugunan, 1997).

Stocking has yielded better results in the smaller water bodies,

though relatively large lakes and reservoirs have been stocked successfully in China. Bighead carp and silver carp are the main species stocked in China (60-80% of the stocked species). Chinese carps, Indian carps, tilapias, common carp and rainbow trout are the most common species used for stocking in this region. Stocking of the indigenous species (silver perch, golden perch and Australian bass) are important for recreational fisheries in Australia. Environmental engineering practices are not described frequently, but they do exist. Cage and pen culture are widespread, notably in China, Indonesia and the Philippines. Other measures include screening of inlets and outlets, protection and construction of spawning habitats, re-establishment of floodplain-river connections and the construction of brush parks.

Africa

The majority of countries in Africa report introductions. The introduction of the Nile perch (Lates niloticus) has created a major commercial fishery with substantial economic and nutritional benefits (Reynolds and Greboval, 1988) but at the same time it is held responsible for disruption of the Lake Victoria ecosystem. It has been a controversial introduction. The successfull introduction of the Lake Tanganyika Sardine (Limnothrissa miodon) into Lake Kariba, Cahora Bassa and Lake Kivu (Marshall, 1991; Lévêque, 1998; Spliethof, 1983) has contributed signi-ficantly to fishery output.

Apart from these often referred to introductions, tilapias, common carp and rainbow trout have been distributed over this continent. Largemouth bass and sunfish have been introduced for recreational fisheries in 11 and 6 African countries respectively. About 23 species were introduced into Madagascar, of which 14 established in open waters where they contribute to commercial and sport fisheries (Moreau, 1986).

Information on stocking practices in Africa is relatively scarce. Stocking of small man-made reservoirs in Zimbabwe, of floodplain pools in Nigeria and of seasonal small water bodies in Burkina Faso (<50 ha) is reported. Stocking of larger water bodies is reported for Lake Nasser and Lake Quarun in Egypt. Overall, it can be concluded that stocking of open waters is not very common in Africa.

Other enhancements reported include the brush park fisheries in lagoons in Côte d’Ivoire and elsewhere in West Africa and hydraulic mana-gement in lagoons, but the information is very scarce.

Tilapias have been widely introduced in this region and in many cases contributed significantly to fishery production (Juarez-Palacios and Olmos-Tomassini, 1991). Self-reproducing populations of tilapias were formed in lakes and reservoirs in Colombia and contribute significantly to the total fish production in the country (Castillo Campo, 1996). T. rendalli and O. niloticus form an important component of the fish catch in the reservoirs of northeast Brazil (Gurgel and Fernando, 1994) and in Cuba they are the main species in culture-based fisheries (see section on stocking). Common carp is also relatively wide spread in this region though they did not successfully establish in Northeast Brazil (Gurgel and Fernando, 1994). Salmonids (pacific salmon, brook trout) have been introduced to Chile and Argentina.

Stocking is practised in various countries, notably in Cuba (tilapia and Chinese carps in reservoirs), Argentina and Chile (salmonids), Brazil (tilapias in Northeast Brazilian reservoirs) and recently in Mexico (tilapias, common and Chinese carps in reservoirs). Cage culture is practised in reservoirs in Argentina and Colombia and experimentally in some other countries in the region. Information on environmental engineering as a tool for fishery enhancement in this region is not available from ASFA.

North America

Numerous species were introduced to this continent and a considerable amount of literature is available (697 references in ASFA). These mainly deal with invasions and the related environmental effects of various accidentally introduced species such as Zebra mussel (Dreissena polymorpha), ruffe (Gymnocephalus cernuus) and sea lamprey (Petromyzon marinus) into the Great Lakes area. Information on intentional introductions involve those of various species to enhance recreational fisheries in small impoundments. For example, in California, 30 exotic species were introduced to enhance recreational fisheries of which black bass, catfish (Ictalurus spp.), sunfishes (Lepomis spp.) and striped bass (Morone saxatilis) contributed 42 to 77 percent to the angling catch in this State (Lee, 1995). In Florida, butterfly peacock bass (Cichla occelaris) was introduced into 11 coastal canals, where it has established self-reproducing populations and contributes significantly to the sports fisheries in the area, without significant negative impacts (Schafland, 1995).

In Ontario, Michigan and New York States, introductions of Atlantic salmon (S. salar) into Lake Ontario were done to restore the population (Jones and Stanfield, 1993). Pacific salmon (Oncorhyncus spp.) was introduced in the tributary streams of Lake Ontario (Rand et al., 1992) and the introduction of white perch (Morone americana) was successful in that it has become a major commercial species (Haynes et al., 1982). Small numbers of pink salmon (O. gorbusha) were released into Lake Superior in 1956 and have established significant populations (Bagdovitz et al., 1986). In Florida, blue tilapia (Oreochromis aureus) was accidentally released into public waters where it established itself successfully, resulting in a commercial fishery for this species (Hale et al., 1995).

S. fontinalis was successfully transplanted in four lakes in Ontario (Fraser, 1989) but it has not been translocated extensively in North America. Lake trout (S. namaycush) has been widely introduced also in response to declining populations due to the invasion of the sea lamprey. Evaluation of 183 introductions of this species in Ontario, showed that lake trout failed to establish in shallower lakes with large littoral zones and richer fish communities with potential predators (Evans and Olver, 1995). Rainbow trout has been introduced into Appalachian streams where it dominates native brook trout (Clark, 1997).

Stocking of largemouth bass, its prey species, bluegill and other sport fishes is widely practised in the United States for recreational fisheries. Steelhead (O. mykiss) is extensively released to enhance recreational fisheries in British Columbia. Furthermore, grass carp is stocked in the Southern states to control aquatic weeds and various salmonid species are stocked in the Western States to restore and enhance river stocks. Important stocked species with high reported hatchery production in North America are salmonids, pikeperch, striped bass, largemouth bass and bluegill.

Europe and the former USSR

Relatively complete information is available about introductions in the former USSR countries, Norway, Sweden, Finland, Germany, Italy and France. Introductions have been widespread in Europe. Important species are common carp, rainbow trout, pikeperch (Stizostedion lucioperca), peled (Coregonus peled), grass carp and lake trout (Salvelinus namaycush). European eel (Anguilla anguilla) is also widely released.