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R.L. Welcomme and D.M. Bartley
FAO, Fisheries Department, Rome, Italy


With increasing pressure on the world's inland and coastal marine fisheries increases in production and quality of yield are being sought through the application of a range of enhancement techniques. Which of these is applied depends on the attitude to the natural resource by societies at different levels of economic development. The range of enhancement techniques involves increasing levels of human input and control which raise productivity significantly but also raise costs. Introductions have raised production in many areas of the world at the price of risks of environmental disruption. Stocking is extremely widespread but has generally been applied uncritically. A variety of models are proposed to serve as a basis for more rigorous evaluation of biological and economic effectiveness of this practice. Fertilisation of water bodies is used to further raise levels of production. Elimination of unwanted species then becomes necessary to maximise benefits from the target species. Adjustments to the habitats within the water body assist in raising general levels of productivity which culminate in the conversion of areas of the water into fish ponds or for cage culture. This process has important implications for the social, economic and policy context which necessitates shifts in ownership, finance and education among populations where these types of development occur.


Present trends in the use of inland and coastal waters for fisheries indicate that the production to be anticipated from such systems is limited for two reasons. Firstly, declining quality of the aquatic environment resulting from eutrophication, pollution and habitat modification has led to increasing incapacity on the part of native fish assemblages to adapt and maintain their form, diversity and biomass. Secondly, poor fisheries management has resulted in an inability on the part of many fish species to compensate through natural reproduction for excessive or inappropriate fishing pressure.

As a result of these two stresses catch from inland water fisheries based on naturally reproducing fish populations is declining world wide and many of the coastal marine resources are similarly threatened. The response to this crisis in management has been to increase the level of human interventions through a series of activities which may be individually termed enhancements or, collectively, intensification of production.

Two main strategies for the management of inland waters for fisheries are being adopted based on differing societal views of natural resources and their use (Table 1) and these condition the approach to enhancement.

Table 1. Differing strategies for management of inland waters for fisheries in developed and developing countries.

ConservationProvision of food
MechanismsSport fisheriesFood fisheries
Habitat restorationHabitat modification
Environmentally sound stockingEnhancement through intensive stocking and management of ecosystem
 Extensive, integrated, rural aquaculture
Intensive, discrete, industrialised aquaculture 
EconomicNet consumerNet producer
Capital intensiveLabour intensive

In developed countries attempts to manipulate species composition in rivers and lakes to correspond to the preferences of sport fishermen led to an early adoption of enhancement techniques. In fact stocking coupled with habitat maintenance became the major form of inland fisheries management. More recently, aquatic resource use is becoming increasingly subordinated to conservation. Production facilities are generally isolated in carefully controlled fish farms and the inland waters are destined mainly for aesthetic and recreational uses. This means that enhancement of the system is now tending to be accomplished more by rehabilitation and restricted access. Ideally, in this view, stocking programmes are limited to low numbers of large fish in support of native species or restoration of endangered ones. Stocking practice also now emphasises protection from undesirable genetic effects. Exceptions to this general trend exist in some lakes where capture fisheries are supported by large-scale stockings with target species and in some recreational fisheries which are still highly intensive and are maintained by high stocking rates with smaller fish.

Stock enhancement of marine and coastal areas initially started because of the belief that harvest could be increased simply by releasing large amounts of early life history stages, e.g. eggs or larvae, into marine coastal waters. This overly simplistic approach was followed in both the USA (MacCall, 1989; Larkin, 1991) and Norway (Griffin, 1993) for over half a century with no demonstrable effects on the targeted fishery. More recently, many marine stock enhancement programmes in developed countries have been established to enhance fisheries that have declined as a result of habitat degradation and overfishing (Danielssen and Moksness, 1994). Nearly all of the marine enhancement programmes, with the exception of commercial aquaculture, are government supported, thus they also serve as a visible indication that the government is trying to manage and/or promote marine resources. This promotional or publicity aspect of marine stocking programmes may be an extremely important factor in the establishment of marine hatcheries.

The objective of many developed countries' stock enhancement programmes is to maintain a quality of life and a profession (fishing) that is of historical significance, but that is being threatened by the very processes that make the country “developed”. This is in contrast to the developing country goal of improving conditions through increased production.

In developing countries there is a large food deficit, especially of animal protein, and inland fisheries are being called on to maximise supplies for human consumption. This means that most inland water systems reached their maximum potential under natural production regimes some time ago, and rising demand is now pushing many tropical waters to maximise yields through enhancement. In many countries this process is now advanced with the development of infrastructure to cope with the required production of fingerlings for stocking. In other areas lack of funding and infrastructure is delaying the process despite there being adequate physical potential. In many cases there is a close integration between capture and culture through extensive and semi-intensive management of artificial water bodies and rice paddies.

Marine stocking in developing countries is currently uncommon. The oldest examples involve high value species, such as shrimp, e.g. Penaeus chinesis, in China, salmon, e.g. Oncorhynchus kisutch, in Chile, and sturgeon, e.g. Huso huso and Acipenser spp., in Iran, that can generate foreign exchange (Bartley, 1995). More recently, stocking of marine invertebrates in small island states of the Pacific have developed around high value species that can be easily processed and stored to accommodate the remote location of many island nations (Bell, 1996). Here also, the stocking programmes are in direct response to degradation of the marine habitat and overfishing.

Enhancement programmes have generally been pursued uncritically with little scientific evaluation of their success or failure. Whilst the primary motive for stocking was political, either to convince recreational fishermen that government authorities were looking after their interests or to subsidise fishermen's communities which would otherwise have insufficient catch, the need for evaluation was not pressing. However, where stocking is used as a tool to maximise yields from a fishery, the outlay in stocking material represents a sufficiently large percentage of total cost as to warrant a more coherent approach to the efficient use of resources. Because of the general low level of evaluation and because the use of stocking as a tool to systematically raise production from large food fisheries is comparatively recent, the literature surrounding enhancements is surprisingly small. This paper seeks to synthesise what data are available on enhancements, particularly those aimed at increasing food supply, as a step in developing guidelines for sound fisheries management in rivers, reservoirs, lakes and coastal marine environments.


There are several enhancement practices which together contribute to a process that can be termed the intensification of production. These practices cover a range between culture-enhanced capture fisheries to intensive aquaculture. They are often adopted in a stepwise manner leading to a progressive increase in fishery production per unit area of water through increasing human controls on essential parameters of the fish assemblages (Fig. 1).

The range of techniques in rough order of magnitude of resources required is as follows:

The increases in productivity associated with the various steps of this process are described in Figure 1.

Figure 1. Production from different capture and culture systems.

Figure 1.

2.1 Introductions

Welcomme (1992) listed 1673 introductions of 291 freshwater species (primarily fishes) into 148 countries. Since that date over 1000 more introductions into inland and marine environments have been reported (Bartley and Subasinghe, 1996; FAO unpub. data). Most introductions have been made initially into closed systems for aquaculture although many later escaped to the wild. Introductions made initially into the wild were for recreational fisheries in 266 of the 2875 introductions and for improvement of wild stocks for commercial fisheries in 282 of the cases. Introductions to improve habitat by means of filling a vacant niche, providing forage for important species or for biological control of unwanted organisms were cited as reasons in 113 of the cases. Many introductions were for more than one purpose, for example the introduction of grass carp may be for weed control, fishing and aquaculture. Such introductions are primarily:

To establish new fisheries through the introduction of new species that are more resistant to fishing pressure or have greater market value than native fishes for commercial fisheries, or in recreational fisheries to improve the variety available to anglers by introducing species of particular trophy or sporting value.

To fill a vacant niche where impoverished fish faunas do not fully utilise the trophic and spatial resources available. These can arise in natural waters where geographic accident has resulted in there being few native species, as in some islands, or areas where faunas have been wiped out through glaciation. More commonly the need for such introductions arises as a consequence of human activities. Thus many new reservoirs, especially in Latin America, lack native species capable of fully colonising lentic waters (Fernando and Holcik, 1982). In many river basins regulation of flow by dams has eliminated or drastically reduced the native rheophilic faunas leaving the waters open to colonisation by introduced species.

Despite the large numbers of species that have been moved around the world, successful introductions for inland fisheries have been based on surprisingly few species. Introduced tilapias, Oreochromis and Tilapia, have played a major role in increasing production from lakes and reservoirs. For example, in Sri Lankan reservoirs production rose from practically nil to 30,000 t/yr, 90% of which were tilapia (de Silva, 1988). Twenty-four percent of the 240,000 t of freshwater fish caught every year in the Philippines and up to 95% of catches in some lakes in Indonesia are tilapias (Petr, 1987). In Madagascar, tilapias and common carp, Cyprinus carpio, make a major contribution to the expansion of inland fish production to 40,000 t/yr. Mexico, with an insignificant inland lake fishery, expanded its production from 2,000 to 123,000 t/yr (in 1988) of which 65% were tilapia and 24% were common carp. Cuba developed fisheries in its reservoirs to produce 15,000 t in 1988, 93% of which were tilapias. The fisheries of the Departamento Nacional das Obras Contra as Secas' area of Brazil produce 17,000 t/yr of which 30% are tilapia.

The common carp and a number of Chinese and Indian carp species have been introduced into the countries of the Indo-Pacific region (Petr, 1989), where they form important elements of the capture fisheries of reservoirs and natural lakes. Equally the migratory Barbus javanicus was introduced to colonise rivers. These fishes have made an especially important contribution to the island countries to the east of the Wallace line which previously lacked freshwater species suitable for fisheries and aquaculture. Three species of Chinese carp, Ctenopharyngodon idella, Hypophthalmichthys molitrix and Aristichthys nobilis have also been widely distributed for aquaculture. These have escaped into local river systems and have even become established in major rivers such and the Danube and Mississippi where they have increased the potential for fisheries.

The pelagic zone of reservoirs is the most difficult for riverine species to colonise and several species of pelagic fish have been successfully introduced. A small clupeid, Limnothrissa miodon, was introduced into Lake Kariba in 1973 and subsequently increased yields by a factor of 10 to some 35,000 t/yr (Welcomme, 1995). The same species has colonised Lake Cahora Bassa downstream of Kariba where a fishery for it is developing rapidly. It was also introduced into Lake Kivu in Rwanda where it now forms the basis for a valuable fishery yielding 3,500 t/yr.

In temperate zones, the major introductions have involved salmonids including coregonines. The productivity of northern and Alpine European lakes has been improved through the introduction of coregonines such as Coregonus lavaretus (introduced into Belgium, Germany and Italy). Micropterus salmoides, the largemouth bass, has been introduced into many reservoirs and lakes in warm temperate South America, including Mexico. Members of the genus Stizostedion, especially the zander in Eurasia (S. lucioperca) and the walleye in North America (S. vitreum), have been widely introduced on their respective continents.

Introductions into the marine environment are primarily for aquaculture of high value species in both developed and developing countries. Marine shrimp (Penaeus spp.), molluscs and salmonid aquaculture is well established in many parts of the world (FAO, 1996). Introduction of marine species for fishery enhancement has had some success from initial transplantings, e.g. striped bass (Morone saxatilis), and American shad (Alosa alosa), transfers in North America, rainbow trout, Oncorhynchus mykiss, introduction to New Zealand and Chile, Kamchatka king crab to the Barents Sea, but in general dedicated marine hatcheries providing exotic species to the marine environment have not been established.

Introduction of new species carries with it serious risks of corrupting native fish communities through predation, competition and disease (Courtenay and Stauffer, 1984). The very irreversibility of successfully introducing a new species calls for special caution (Turner, 1988; Coates, 1995). Unfortunately, it has been pursued somewhat uncritically in the past and numerous examples where species have either declined in abundance or disappeared altogether, can be found in the literature (Welcomme, 1988). Most notable in recent years has been the exchange of views as to the advantages and disadvantages of the introduction of Nile perch into Lake Victoria which has raised production from Lake Victoria from 100,000 t in 1980 to 450,000 t in 1990 at the probable cost of the loss of several hundred native species (Witte et al., 1992; Reynolds and Greboval, 1988; Pitcher and Hart, 1995).

The number of introductions indicates that this practice is viewed as a legitimate management tool and, as such, will probably persist. There is, however, strong international pressure to regulate the movement of species to reduce the risks of damage to the environment, to the native fish stocks and to the genetic composition of resident and introduced fish. There is also concern about the co-introduction of disease and accompanying fish species. A slowing in the number of new introductions at the species level has been detected, possibly because most practical introductions have already been made. However, given the new awareness of local genetic diversity and the current efforts at domestication, selective breeding and genetic engineering in favour of better varieties for aquaculture, it is probable that a future wave of introductions at the level of strains, varieties and hybrids of existing species will take place.

Several international organisations and fora are calling for a “precautionary approach” to resource management. The precautionary approach to fishery management requires that adverse effects to the aquatic environment be reversible within a time frame of 2–3 decades (FAO, 1995). Therefore, species introductions into the wild would generally not be precautionary, especially in the marine environment where containment and reversibility would be nearly impossible. However, by applying other elements of the precautionary approach to species introductions, such as risk analysis, implementing a monitoring system with defined acceptable impact levels, and establishing corrective measures in advance of adverse impacts (i.e. contingency plan), the chances of their adverse effects can be reduced (FAO, 1995).

Although information is lacking on the long-term effects of most introductions, present performance leaves little doubt that on the basis of aggregate yield, the inland fisheries of the world have greatly benefited from well-planned introductions. However, in the marine environment, natural fisheries have seldom been enhanced by the transplantation of exotic fishes. Exceptions are certain species fish, such as the striped bass introduced to the west coast of North America and some molluscs. For example, culture of the Pacific oyster, Crassostrea gigas, in Australia yielded nearly 3000 t in 1994 (FAO, 1996), however, it is threatening local species of molluscs through competitive exclusion. There is a modest fishery in central Chile based on exotic Pacific salmon (Oncorhynchus spp.), however, it is generally believed that these fish are escapees from the extensive cage-systems in the area and do not represent commercially viable salmon populations.

2.2 Stocking

2.2.1 Types of stocking

Stocking is the most widespread measure for management of inland fisheries in use today. Although it is not as widely used in marine and coastal areas, stocking of anadromous species such as salmon and sturgeon is common and the practice is being extended to other high value species. Most countries report stocking to some degree. As more conventional approaches to management by control of the fishery have proved incapable of limiting effort, compensation for shortfalls in recruitment caused by overfishing and environmental damage has been sought through the addition of young fish to the system. In fact several major types of stocking can be identified:

compensation to mitigate a disturbance to the environment caused by human activities;
maintenance to compensate for recruitment overfishing;
enhancement to maintain the fisheries productivity of a water body at the highest possible level;
conservation to retain stocks of a species threatened with extinction.

2.2.2 Major stocking programmes

Although 94 countries have reported stocking to FAO as part of their fish ery statistics, only a few countries have documented the success or failure of their programmes. Examination of the scientific literature produces some 4847 references extracted from ASFA (1978–96) dealing with stocking of finfish, molluscs or crustaceans into inland or marine habitats. However, most of these do not present any data which help understand the rationale of the stocking exercises. Most documented evaluation of stocking programmes in inland waters appears in national or international reports and is drawn from very few countries, including China, Cuba, India, Mexico, Poland, Russia and Sri Lanka, whereas marine stocking programmes mainly are documented in North America, Japan, and Iran (Caspian Sea).

Inland stocking programmes

India adopted stocking as its main policy for reservoir fisheries management as early as the 1960s. Despite attempts to develop such fisheries for tilapias, native carps proved much more popular. Early stocking regimes were arbitrary and achieved poor results, but the rationalisation of stocking programmes in some reservoirs in the 1980s has allowed tenfold increases in yield (Sugunan, 1995) over the earlier regimes. Stocking has been less successful in Sri Lanka as the numerous tanks and reservoirs are managed mainly for self reproducing tilapias (Amarasinghe, 1997). Seeding is then only necessary in seasonal waters. Stocking has produced variable results over the rest of Southeast Asia where it has been possible to raise catch levels from 20 to 100 kg/ha in some waters in Thailand, Indonesia, Philippines and Malaysia (Fernando, 1977). Constraints on further spread of stocking are mostly socio-economic, although it has proved difficult in some cases to trace the success of stocking due to the high level of self-reproduction in the stocks. Stocking and other enhancements are central to Chinese inland fisheries management policy and have achieved notable increases in productivity (see Fig. 2). As one of the best documents of enhancement systems Chinese fisheries are referred to throughout this paper.

Figure 2. Yield per unit area from Chinese reservoirs and lakes (data from Li and Xu, 1995).

Figure 2.

Stocks of coregonids, perch and zander are maintained in many alpine and northern European lakes in support of commercial as well as recreational fisheries. Similarly, fisheries in lowland reservoirs and lakes in Poland and the countries of the former Soviet commercial Union are maintained by stocking, fertilisation and other enhancements. In the Russian reservoirs yields were considered to be far higher in managed lakes at about 250–500 kg/ha than in unmanaged ones (Berka, 1990). The collapse of the centrally planned economies which supported such fisheries with subsidies led to the collapse of the sector, although this is now being reconstructed on private lines in some countries. Enhancements are also used widely throughout the United states and Canada where the emphasis is on management for balanced populations for recreational fisheries (Moehl and Davies, 1993).

Stocking is widespread in Latin America where the greatest development has been achieved in Cuba, Mexico and parts of Brazil. However, more recently other countries, including Colombia and Venezuela, have adopted the practice (Quiros, in press). In general, enhancement programmes have been successfully developed in support of commercial fisheries, although in Brazil, where stocking was a statutory requirement for mitigation of the effects of dams, less success has been registered.

Stocking and other approaches to enhancement have not been confined to lakes. Rivers have more generally been associated with attempts to sustain endangered species, mitigate for adverse environmental impacts or improve angling conditions. More recently efforts to increase productivity have led to stocking of floodplain lakes. Such programmes have been developed particularly in Bangladesh where substantial increases in yield of valuable carps of up to 594 kg/ha from deep oxbow lakes and 2,800 kg/ha from shallow scour lakes have been achieved (PIU/BRAC/DTA, 1995; Rahman, 1995).

Marine and coastal stocking programmes

Marine stocking is primarily practised in developed countries. However, the technology is also used in some developing countries and there is an increased effort to transfer the technology to certain developing areas and small island states (Bell, 1996).

The most widely stocked group of fish into marine and coastal areas are the various anadromous salmonids. Canada, Iceland, Japan, the UK, and the USA release billions of juvenile salmon into the coastal environment or inland rivers. In the USA, Federal and State salmon hatcheries release over 2 billion Pacific salmon (Oncorhynchus spp.) smolts per year with the State of Alaska contributing more than half of this total (Isaksson, 1988a; Seeb et al., 1993; Bartley, 1995). In the Pacific Northwest, Pacific salmon populations have more than doubled over the last twenty years due to a combination of favourable ocean conditions and artificial enhancement (Bigler et al., 1996). Japan also has an extensive stocking programme primarily for chum and pink salmon that accounts for approximately 90% of Japan's Pacific salmon catch (Isaksson, 1988a; Kitada, 1996). Atlantic salmon (Salmo salar L.) are also stocked on the east coast of North America, in Europe and in Scandinavia.

Japan has an extremely ambitious marine stocking programme; approximately 80 species of finfish, shellfish and crustaceans are being stocked or are being researched for stocking. The Japanese programme is at least partially in response to the ratification of the UN Convention on the Law of the Sea that acknowledges countries' 200 mile Exclusive Economic Zone and the resulting decrease in Japan's foreign fishing rights. Scallops, Kuruma prawn Penaeus japonicus, red sea bream Pagrus major, and flounders Paralichthys olivaceus, as well as previously mentioned salmon, are the principal species stocked. Japan has an extensive continental shelf area that enables stocked animals to be harvested by the country's coastal fisheries. It has been estimated that stocking accounts for approximately 90% of the chum salmon fishery, 50% of the Kuruma prawn catch, 2–75% of red sea bream catch, nearly all of the scallop harvest, and from 2–40% of the flounder fishery (Kitada, 1996). Habitat restoration, predator removal, and behavioural conditioning of fish are also included in the Japanese stocking programme.

Norwegian stock enhancement programmes started at the end of the last century with Atlantic cod, but then were abandoned in the 1970s because of a lack of a demonstrable increase in the cod fishery (Danielssen and Gjosaeter, 1994). There is now renewed interest in stocking cod and other marine organisms such as Arctic char, Atlantic salmon, brown trout, and lobster. Commencing in 1985, 186,000 to 300,000 fry and juveniles were released per year from a variety of sites (see Bartley, 1995, for summary).

The former Soviet Union and the Islamic Republic of Iran actively stock the Caspian Sea with sturgeon Huso huso and Acipenser spp.. In Iran stocking started in the early 1970s with sturgeon in order to increase diminishing stocks due to loss of riverine spawning habitat. In 1995 over 9 million sturgeon fingerlings were released into the Caspian sea and an estimated 12 million will be released by the end of 1996 (Abdolhay, 1996). Because of the degraded spawning habitat in rivers entering the southern Caspian Sea, nearly all of the fishery is based on stocked fish. Other salt-tolerant fish such as Rutilus or mahi sephid, (Rutilus frisii kutum), pike perch (Stizostedion lucioperca), bream (Abramis brama) and Caspian trout (Salmo trutta caspius) are also stocked into the Caspian Sea. Stocking levels are substantial, e.g. 2.5 million perch, 100 million bream, 140 million mahi sephid, and correlate well with harvest levels. Many of these fisheries are also almost completely supported by stocking after their near collapse earlier (Abdolhay, 1996).

Marine stocking programmes target sport fisheries in many developed co untries. Striped bass (Morone saxatilis), red drum (Sciaenops ocellatus), white seabass (Atractoscion nobilis) and striped mullet (Mugil cephalus) as well as the salmonids mentioned earlier are stocked for recreational purposes in the USA. Many states prohibit commercial catches or sales of recreational species such as striped bass and red drum. Barramundi (Lates calcarifer) are stocked in coastal waterways and inland reservoirs in Australia and the stocking programme specifically targets recreational angling. The contribution of the enhancement programmes to the sport fishery is significant or has significant potential based on experimental pilot-scale releases and high value of these fishes in a recreational fishery. The red drum fishery in the Gulf of Mexico is composed of 20% hatchery fish. Stocked striped mullet increased both the sport and commercial fisheries by 33% and 13%, respectively in Hawaii (Leber et al., 1996; Leber and Arce, 1996).

Several other countries, such as China, Denmark, France, Iceland, Spain, Thailand and the UK also have activities in stocking marine species (see Bartley, 1995, for summary). Countries along the Persian Gulf are developing pilot-scale releases of grouper (Epinephelus spp.), sea breams (Sparidentex hasta and Acanthopagrus latus), and rabbitfish (Siganus canaliculatus), (FAO, unpublished report). Pacific island states such as the Solomon Islands, Vanuatu, and also Okinawa are developing or considering stocking programmes for shellfish such as giant clams (Tridacnidae), pearl oysters (Pinctada spp.), trochus (Trochus niloticus), green snail (Turbo spp.) and sea cucumbers (Holothuroidea) (Dalzell and Adams, 1995). These invertebrates can often be easily processed into a form that can be readily stored (e.g. pearls, bêche de mer, mother-of-pearl shell) and then transported to markets very sporadically. This is an advantage to many of the remote Pacific Island States that sell their products to distant foreign markets.

Stocking of marine areas is an extremely controversial subject at present. Many past programmes have been discontinued because of a failure to noticeably increase the targeted fishery. Over 100 years ago Norway established a private marine hatchery for cod; the USA began releasing cod in the Western Atlantic in 1890. Both programmes closed when cod populations continued to decline, the USA programme closed in 1952 and the Norwegian in 1971. In the 1940s 97% of the eggs the USA stocked were from cod, flounders, and pollack; these programmes were also stopped when the targeted populations did not increase (Larkin, 1991). The Japanese International Co-operation Agency (JICA) supported marine stocking of Pacific salmon in Chile in the 1970s–1980s, but hardly any fish returned and the programme was abandoned in favour of a very profitable salmon aquaculture development. In the face of technological improvements in marine fish husbandry and an appreciation of the role of ecology and genetics in stock enhancement there is new interest in marine stocking, but the controversy continues as to its effectiveness (see review in Bartley, 1995a). Similar to stocking inland waters, recent experience in stocking marine species has shown that, where healthy populations exist, stocking provides very little added benefit (Kitada, 1996).

2.2.3 Origin of stocking material

There are two main sources of seed material for stocking - extraction of the products of natural spawning from rivers, lakes and coastal waters, and production from aquaculture installations.

Natural production

Initially nearly all material for stocking was drawn from natural sources, and seed material for several species, such as shrimp, milkfish (Chanos chanos), yellowtail (Seriola quinqueradiata) and eel (Anguilla anguilla), continues to be based on natural production today. Currently for many marine species, controlled reproduction is difficult and raising early life history stages of organisms such as crabs and palinurid lobsters is nearly impossible. Therefore, stocking is based on wild harvest of juveniles, but there are primarily pilot or research projects in developed countries such as Japan. The practice of extracting large numbers of young fish from rivers, lakes and marine coastal areas is a cause for concern to fisheries managers on the basis that this would damage the sustainability of stocks. In certain circumstances a considerable proportion of the young-of-the-year can be removed from floodplain rivers due to the overproduction of 0+ fish from such fluctuating systems (Welcomme and Hagborg, 1977). However, fish populations in lakes, regulated rivers and some marine environments are less tolerant of large-scale withdrawals of early life stages from the population.

Hatchery production

While most early stocking was carried out with material of natural origin the development and spread of techniques for controlled, induced spawning in the 1950s and 1960s generally freed the sector from natural sources of supply and developed an alternative source of seed for stocking a greater range of species.

With controlled reproduction comes the potential to rear and stock genetically improved species, as has already been done with common carp. For example, Iceland's stocking programme for Atlantic salmon includes a genetic improvement breeding plan to increase growth rate and rate of return in released fish (Jonasson et al., 1994). Research on controlled spawning and mass production of fingerlings have recently been initiated for grouper (Epinephelus spp.), sea breams (Acanthopagrus latus, Sparidentex hasta), and rabbitfish (Siganus canaliculatus) on a pilot scale in the Persian Gulf (FAO, unpublished report). Japan Sea Farming Association maintains statistics on seed production and release; in 1995 seed from 74 marine species were released into Japan's waters. Some of these releases are only experimental at present, but it is expected that some of these species will be stocked en masse in the near future. The number of farmed species has increased from about 174 in 1984 to over 260 in 1994 (Garibaldi, 1996), therefore it is expected that aquaculturists will soon be able to spawn and mass produce additional marine species.

Size of fish stocked

Two main factors influence the size chosen for stocking material, cost and survival. While biologically the penalty of high mortality acts against the stocking of fish at too small a size, the exponentially increased cost of the stocking material with increasing size, especially in slow growing species, tends to favour stocking of early life stages. The actual size chosen usually depends on an empirically determined balance between these two factors, as well as on the life history of the fishes. Migratory and anadromous fish such as salmonids are usually stocked at a small stage (fry) to acclimate to the natal river and to prepare for migration as their size increases. Cyprinids on the other hand are generally stocked at a larger stage (fingerlings). Recreational fisheries increasingly tend to rely on even larger fish of cacheable size and to rely less on grow-out in the natural environment, although behaviour considerations may limit the upper size at which fish can be stocked (Petr, this publication) due to conditioning in the hatchery environment.

Kent and Drawbridge (1996) compared probability of recruitment to the fishery and cost of production for various sizes of white seabass (Atractocion nobilis) stocked into southern California coastal waters, and determined that the most cost effective size of release is about 210 mm for white seabass; raising fish to a larger size greatly increases culture costs without greatly changing the chance that the fish will be harvested by the fishery.

Release strategies for marine organisms must take into account both size at release and season of release. Time of release is especially important in temperate areas where temperature, nutrients, waterflow, and current patterns vary seasonally. The Japanese chum salmon stocking programme was significantly improved by timing the release with the occurrence of an upwelling current and snow melt period (cited in Kitada, 1996).

Artificial reproduction allows the culturist to produce fingerlings all year round for many species. However, stocking of red drum (Sciaenops ocellatus) fingerlings that were produced and stocked “out of season”, i.e. abnormal spawning period in the spring, yielded only one return from 20,000 fish stocked, whereas the group produced and stocked “in season”, i.e. normal spawning period in the fall, yielded 821 recaptured fish per 20,000 (Willis et al., 1995). Studies on striped mullet (Mugil cephalus) in Hawaii further showed that appropriate size and time of release strategies can improve a hatchery's contribution by nearly four times (Leber et al., 1996).

Stocking versus natural production

Where fish are stocked into populations where natural reproduction occurs the dynamics of the process become uncertain. Impacts can be anticipated particularly on density dependent factors such as feeding and population density where mortality may increase and growth rates decrease due to the addition of excess elements to the stock. Natural reproduction may also be inhibited where the fish used for stocking are drawn from a strain not adapted to the recipient water body. Clearly these effects will depend on the relative proportions of stocked fish to those originating from natural reproduction. It is clear that where there is adequate natural reproduction stocking may well be superfluous, as in the case of tilapias in Cuban reservoirs (Fonticiella et al., 1995). In some fisheries, such as the Finnish coregonid fisheries and the Polish lake fisheries stocking has made a long-term positive contribution despite there being naturally reproducing stocks of the target species (EIFAC, 1983). In other lakes, for example in Thailand (Bhukaswan, 1988), it has been impossible to evaluate the biological and economic success of stocking due to difficulties in separating stocked fish from natural production. Many salmon fisheries in North America and Europe are composed of both hatchery and naturally produced fish; how they interact and their relative contribution to the fishery is a topic of much discussion (see Campton, 1995, for discussion and references).

2.2.4 Dynamics of stocking

Analysis of the few data sets on stocked systems show a complex situation involving the interplay of two main variables - area of stocked system and stocking rates. In general these data are difficult to analyse for the following reasons:

  1. Yield is related to stocking rate. Relationships between stocking rate and catch have been obtained for one lake, for example the Nanshahe reservoir in China where yield in kg/ha (Y) is directly proportional to stocking density (D) [Y = 122.1 + 0.1140 D (r2 = 0.58)] (Li and Xu 1995). They have also been obtained for a series of lakes as in the case of Sri Lanka where Y = -1.829 + 0.3714D - 0.0011 D2 (r2 = 0.70) or Mexico where Y = 13.25 + 3.48 D (r2 = 0.66). Relationships are usually assumed to be linear but some interpretations, such as that of Amarasinghe (1997), predict a curvilinear relationship between stocking rate and yield with an initial rise in yield but a later fall as density dependent factors come into play. Alternatively, Quiros (in press) used a log-log relationship to describe the increases in yield with numbers stocked in 129 water bodies in tropical Latin America.

  2. Yield per unit area is inversely related to the area of the stocked system. Stocking has generally proved more effective in small reservoirs in many regions although larger water bodies are also stocked. Data sets from China (Li and Xu, 1995), Sri Lanka (Amarasinghe, 1997) and Mexico (FAO, 1992, 1993) (Fig. 3) all show strong inverse relationships between reservoir area and yield per unit area which are linear on log-log scales. Fonticiella, Arboleya and Diaz (1995) found a similar influence of area in Cuba where there was a very strong correlation between stocking density and catch in small reservoirs managed semi-intensively but a lesser correlation in extensively managed larger water bodies. The inverse relationship of yield against area would appear to be consistent with the biology of the systems concerned where the greater degree of control over small systems and the greater risk of predation and competition in larger ones would seem to favour the efficiency of stocking in water bodies with smaller areas. Furthermore, larger water bodies are more difficult to control socially and have a heightened probability of losses through poaching and unreported catch.

Figure 3. Plots of yield per unit area against area for various sets of reservoirs.

Figure 3.


  1. Stocking rate is inversely related to lake area. In nearly all countries practising stocking there is a tendency to stock fish at lower densities into larger water bodies (Fig. 4; Table 2). This is generally justified by: a) the supposition that competition and predation will be greater in larger water bodies and therefore survival rates will be reduced; and b) that other aspects of enhancement such as fertilisation, control of unwanted species or construction of artificial faunas are more manageable in smaller water bodies. If the first of these suppositions were true, then the efficiency of stocking in terms of yield per unit stocked would be expected to fall as the size of the stocked water bodies increased. In none of the cases examined was there a correlation of this type and efficiency of stocking remained roughly the same over a vast range of reservoir areas. It must be concluded therefore that, at present, the practice of using small water bodies for intensive and large ones for extensive stocking depends more on the high numbers and cost of fingerlings needed for stocking the larger water bodies, and the difficulties of controlling other parameters in the larger reservoirs such as poaching, than on any real evidence of lesser efficiency of stocking. Clearly the biological rationale for this needs further examination especially in species which have localised distributions within a larger system such as a marine area. For example, lobsters may be stocked into a 1000 km2 bay of which only 20 km2 may be lobster habitat.

Figure 4. Stocking rates into reservoirs of different areas in Mexico and Sri Lanka.

Figure 4.

Analysis of stocked systems in the marine environment is especially complicated because of the difficulty in defining an “area”; the marine environment is composed of inshore, offshore, coastal lagoons/bays, brackish waters and rivers, all of which are connected. Additionally, there exists a poor state of information on the survivorship and behaviour of hatchery-produced fish in nature.

Table 2. Stocking and production characteristics of reservoirs of different sizes in China.

Area of Reservoir (ha)Stocking densityFish yield
Small (<70)3000–7500750–3000
Medium (70 – 670)1500–3000450 – 750
Large (670 – 6670)750 – 1500225–450
Super (>6670)450–750150–225

Yields for marine stocking programmes are usually presented simply as per cent return rates or as proportion of a harvest that is due to stocked animals. Such yield estimates are extremely variable across species, within a single species, and even within a given hatchery stocking programme. In a brief review of marine stocking programmes, Bartley (1995) found return rates from 0.01 % for coho salmon in Chile to 32% for Arctic charr released at 2 years of age in Norwegian fjords. The fecundity of many marine species is very high, therefore hatchery production tries to compensate for low survival rates by stocking large numbers of fry. Preliminary data analysis by the Texas Parks and Wildlife Department for red drum stocking “reveal positive correlations between stocking and subsequent gill net relative abundance indices for at least four Texas bays” (McEachron et al., 1995).

Linear regression of stocking and number of adults returned have been determined for a few stocking programmes. For chum salmon stocked in Japan from the early 1970s to mid 1990s, Y = 36.35X - 5958.81 (r2 = 0.77) for releases around the island of Hokkaido and Y = 22.56X - 1960.94 (r2 = 0.95) for releases around the island of Honshu (X = numbers of recaptured adults and Y = millions of fingerlings released) (Kitada, 1996). Return rates have now stabilised at about 4% for Hokkaido and 2% for Honshu. For scallop ranching at three locations in Hokkaido, wild seed are collected and grown for one year (3.5 cm shell length) and then released before summer causes water temperatures to rise. The theoretical return rate for the scallop fishery is 0.3, but many catches in two regions of Hokkaido far exceed this indicating that a favourable environment and some other natural reproduction is contributing to the harvest (Kitada, 1996). In fact, the relationship in the two favourable environments appears exponential rather than linear (Fig. 7 of Kitada, 1996) again indicating additional natural reproduction.

2.2.5 Models of stocking

All models of stocking rate and cost developed here are applicable to populations where there is either no reproduction of the target fish or there is near-total harvesting. The unpredictable contribution of natural reproduction and any interactions between the stocked and native fish makes evaluation difficult. As a result, assessments of the stock would be needed which interpret the stock recruitment, mortality and growth relationships using models such as those of Lorenzen (1995). Where harvesting rates are low and the target species are long lived, stock densities may rise over a number of years. In such cases models can only be applied after a relatively stable situation has been reached where stocking produces no further rises in annual yield.

Stocking rates

Welcomme (1976) suggested that the number of fry needed to stock a body of water can be obtained by inverting the standard mortality formula:

No = Nc exp(z(tc - to))

Where No is the number to be stocked, Nc the number desired at age-of-capture c and z the total mortality (m for the age group), tc = age at capture, to = age at stocking, tc = age at capture: to = age at stocking.

Numbers to be stocked should also be related to the potential productivity of the water body. Several systems have been used for this ranging from generalised equations such as the morpho-edaphic index to specialised indexes based on benthos or zooplankton densities. These can be incorporated into the general formula as follows:

S = (qp / w)exp(-z(tc - to))

Where S is the number to be stocked, p the natural annual potential yield of the water body (MEI or alternative estimator), q the proportion of the yield derived from the species in question, W the mean weight at capture,

More empirical expressions are used by Chinese reservoir fishery managers for arriving at the number of fish for stocking (Li and Xu, 1995); for instance:

d = f / WR

where d is the annual stocking density (fish ha-1), F the annual fish productivity (kg ha-1) as estimated from food organism abundance, W = average weight of fish at harvest (kg) and R = return rate.

Food biomass indicators are used to establish the productivity and carrying capacity of the water to be stocked in both China and Russia (Li, 1988; Berka, 1990).

Experience has shown that the higher the rate of stocking the greater the yield up to a certain limit after which yields decline. At the same time, as the stocking density increases the mean weight of the fish caught falls until they are no longer acceptable on the market. Thus a balance has to be maintained between the stocking rate and the size of the fish produced.


Once regular stocking has been accepted as a management tool, the stocked population may rapidly exceed the natural carrying capacity of the recipient water. Stocking alone will result in little sustainable increase unless accompanied by other measures to increase the productivity of the water body concerned. Efforts to remedy this include the fertilisation of the waters, stocking with balanced groups of species and the modification of the physical form of the environment. Fertilisation is usually the first of these measures to be adopted and once systematic stocking programmes to raise yield are initiated it appears inevitable that some form of fertilisation will follow. Fertilisation may be through the discharge of nutrient rich agricultural waste waters into the water body, through the addition of inorganic fertilisers or through the addition of solid organic material of various types. As an example of this widespread practice, the Chinese increased yield from 58.5 to 540 kg/ha in reservoirs of the Wan county through application of inorganic fertilisers (Lu, 1992). Alternatively 3000–6000 kg/ha of green manure may be used. Inorganic fertilisers are widely used in Russian reservoirs where the application rates are determined according to the trophic status of the water body. These range from 30 kg/ha of superphosphate and 50 kg/ha of ammonium nitrate for oligotrophic lakes to 15 kg/ha and 20 kg/ha in eutrophic ones. Treated lakes show increases in the order of 5 times in the number of benthic and planktonic food organisms (Berka, 1990). Schedules for the fertilisation of North American lakes have been proposed which vary according to species, type of lake and climatic region (Moehl and Davies, 1993). Inorganic fertilisers are generally applied at levels of 10 kg/ha of ammoniated polyphosphate or orthophosphate, 18 kg/ha of diammonium phosphate or a combination of 18 kg/ha triple superphosphate and 24 kg/ha of ammonium nitrate.

Fertilisation has usually been limited to inland waters. However, there are current efforts to fertilise coastal areas around Norway and even the open ocean in areas such as the Atlantic Gulf Stream. Iron has been found to be limiting in the open ocean, whereas the addition of nitrogen and phosphorus to coastal areas increases productivity. That fertilisation of the marine ecosystem can be effective is indicated by the increased production of parts of the Mediterranean Sea that is believed to have been caused by nutrient inflow from land-based agriculture activities (FAO, 1995a). Conversely, the high productivity of the North Sea has decreased after reducing the inputs as a result of the increased number of water treatment facilities on the inflowing rivers. Whether fertilising the open sea will prove economically viable is still not clear.


Gains in production of favoured species can be achieved through the elimination of other elements of the naturally occurring fish assemblages. These may be either competitors, usually small, abundant, fast growing species which are of no commercial value, or predators which affect the survival rate of the target species. Removal of unwanted fish is common in both recreational and commercial operations. It is a keystone of Chinese and Russian management practice but is also attempted in most countries where intensification is reasonably advanced (Berka, 1990; Cowx, 1994; EIFAC, 1991; Li and Xu, 1995; Sugunan, 1995).

A detailed analysis of fish control projects in the United States, Canada and Mexico (Meronek et al., 1996) classified control methods into four categories:

  1. chemical applications typically using rotenone or Antimycin;
  2. physical removal using nets, traps, electric fishing or by drawing down the reservoir even to the point of complete desiccation;
  3. stocking with predatory or competing species; and
  4. combinations of the above.

Control efforts were successful in 43% of the cases and attempts at complete elimination were generally more successful than effort to partially control selected species. Chemical control was more successful than physical removal which in turn was more successful than stocking with conflicting species. Combinations of physical and chemical methods may add to the efficiency. In the marine environment where chemical treatment may not be feasible or appropriate, removal of predatory starfish has proved effective at increasing the production of scallop beds in Japan (Kitada, 1996).

It is clear from experiences around the world that one hundred percent control is not easy and ideally the water body should be emptied completely and sealed against subsequent invasions from upstream. However, this is only possible in the smallest of reservoirs and more commonly mechanical and chemical controls are attempted.


Chinese, Russian, Indian and Cuban fisheries managers place great importance on polyculture whereby a balanced community of species exploits various trophic levels and spatial niches (Table 3). This permits the maximisation of the use by target species the food resources available. It also allows gains to be made through interactions such as the release of nutrients by bottom feeding carps which fertilise the pond for increased phytoplankton production. Balanced mixtures usually include bottom feeders, higher vegetation feeders and pelagic zoo- and phytoplankton feeders. Benthic filter feeders such as freshwater molluscs may also be added, but this is not common at present. Occasionally a low-grade predator may be included. Favoured species are usually Chinese and Indian carps, eels, tilapias and coregonids, with the occasional inclusion of predators such as pike perch or mandarin fish (Siniperca chautsi) which have a higher market value.

Table 3. Combinations of species used for stocking in extensive and semi-intensive systems.

AreaPlanktonophage PelagicOmnivore-detritivore BenthicPiscivore General
ChinaAristichthys nobilisCtenopharyngodon idella 
Hypophthalmichthys molitrixCyprinus carpio 
 Cirrhinus molitorella 
IndiaCatla catlaLabeo rohita 
Cirrhinus mrigalaLabeo calbasu 
Russia (cold water)Coregonus peledCyprinus carpioPerca fluviatilis
 benthic coregonidsStizostedion lucioperca
Russia (warm water)Aristichthys nobilisCtenopharyngodon idella 
Coregonus peledCyprinus carpio 
AfricaOreochromis niloticusClarias gariepinus 
Latin America (temperate) Cyprinus carpioMicropterus salmoides
Latin America (warm water)Oreochromis niloticusCyprinus carpio 
Oreochromis aureusCtenopharyngodon idella 

Another aspect of artificial fauna creation is the introduction of food organisms. This practice has been mostly confined to the former USSR where a number of invertebrate species have been transferred from the east to the west of the country. Berka (1990) claimed impressive increases in the production of fish from reservoirs into which larger, faster growing and more productive mysid and amphipod species were introduced.

Polyculture has not been as widely applied to the marine environment. However, there are efforts to improve production while also reducing harmful effluents by incorporating multiple organisms, such as fish (mullets and milkfish)/crustaceans (shrimps and crabs)/molluscs/algae, in culture systems (Csavas, 1993). Filter feeders such as oysters and clams have been used in effluent raceways or settling tanks to take advantage of micro-algae that grow in response to nutrient loads (Shpigel et al., 1991, see also review in Barg, 1992). Macro-algae, especially commercially-important red algae (e.g. Gracilaria spp.), have also been grown in conjunction with coastal shrimp ponds to utilise nutrients in effluent water (Sinha, 1993).


In addition to stocking, interventions are made into the stocked lakes and rivers to improve shelter, feeding and breeding grounds. Weed cutting is common, especially in lakes where high levels of eutrophication encourage the growth of emergent vegetation. Accumulated bottom sediments must be cleared in intensive systems to avoid the build-up of anoxic material. Bottoms may be graded to increase production of benthic organisms and to facilitate harvesting. Spawning gravels and shelter devices may be installed and aerators may be deployed in intensively stocked systems. In the case of floodplain pools, bunds and sluice systems can be installed to retain water and to control the inflow and outflow of water.

Engineering in the marine environment involves construction of artificial reefs and breakwaters to protect growing areas. Artificial reefs provide substrate for the colonisation of food organisms that will hopefully attract commercially important species. The reefs provide refuge, as well as food, so in theory they should increase productivity. However, the efficacy of artificial reefs has been questioned and many believe they are simply fish aggregating devices that do not increase overall productivity. If true, artificial reefs may still be a useful means to increase the efficiency of coastal fishing.

Not surprisingly, the developed countries, and especially the Japanese, are involved with artificial reef construction. The Japanese are also experimenting with the placement of large concrete blocks in sub-tidal coastal areas that deflect nutrient-rich deep water towards the shallow fishing grounds to increase productivity. This programme is only now starting and results have not been determined (Morikawa, 1996).


Intensified culture may be practised in areas of the reservoir cut off from the main body. This usually involves isolation of bays and side arms by bunds, fences or block nets, after which the isolated area is treated as an intensive aquaculture pond either to produce food fish or stocking material for the main reservoir. This practice is particularly developed in China (Lu, 1992). Coastal ponds may be similarly isolated by constructing dikes or berms along tidal inlets. Often fish, crustaceans, and other organisms are allowed into the ponds during tidal inflows or specific times of year by removing or opening part of the dike/berm (Csavas, 1993).


The whole water body may be treated as a fish pond with full control of all processes. At present this applies mainly to smaller inland water bodies, for example the Chinese 16 ha ponds which are managed by stocking and fertilisation to attain yields of 5000–6000 kg/ha (Lu, 1992). As technologies improve the degree of control over larger reservoirs and lakes may also be increased.


Cage culture in reservoirs and offshore marine areas is often developed as an independent process parallel to the enhancement of capture fisheries. In many ways it represents a short-cut to aquaculture and as such, cage culture associated with capture fisheries is expanding rapidly. The practice originated in China both for the culture of table fish and for the rearing of juvenile fish later used for stocking. Subsequently other countries have begun using this method as a cheap way to expand aquaculture facilities without making inroads into scarce land resources. For example, Malaysia increased its area of freshwater cages from 2.14 to 4.87 ha and its brackishwater cages from 24.29 to 66.82 ha between 1990 and 1993, whereas the area of ponds, although much greater, has increased only slightly (Ferdouse, 1995). Cages deployed in Saguling reservoir in Indonesia for rearing common carp have proved highly cost effective despite stocking densities that at 1.4 kg/m3 are lower than could be supported by such systems (Rusydi and Lampe, 1990). Cage culture in the lakes of the Pokhara Valley, Nepal has proved similarly attractive as the most direct way to increase the normally low natural production (Swar and Pradhan, 1992). In Latin America, Chile, Colombia, Brazil and Mexico have begun to use cages to increase the productivity of their reservoirs and lakes.

Cage culture in the marine environment is also expanding and involves finfish such as Pacific and Atlantic salmon, seabream, and mullet (Dahle, 1995). Shellfish are also cultured in cages, but these cages are typically smaller and suspended from ropes. Clams, scallops, oysters and abalone may be raised in this manner.

Inevitably installation of cages in a water body will lead to a parallel enhancement of harvest from the wild stock as nutrients and excess food become available to the resident fish. Stocking with selected species will be needed to benefit fully from this so cage culture may act as a trigger for other forms of enhancement. Eutrophication from cages may eventually exceed the self-purification capacity of the water body and will then lead to diminished productivity and to the disappearance of species. Strict adherence to guidelines regulating culture densities and water quality will then be necessary.


An emergent technology for fisheries management is the use of genetically improved strains. Natural water bodies tend to accumulate populations adapted to the climatic, abiotic and biotic conditions. Increasing human intervention, including the introduction of exotic species (Williams et al., 1989), tends to change the selection pressures of a habitat and may weaken or eliminate local populations. Furthermore, native strains may not be the best adapted to the altered conditions of the water body under intensive management regimes. There is now a trend to develop new strains for aquaculture having desirable characteristics for growth (Ecknath et al., 1993), temperature tolerance and disease resistance (Jhingran and Pullin, 1985). It is possible that in future similar strains will be bred for intensive management.

Genetic modifications are starting to be used in intensively managed systems. Grass carp used for aquatic weed control are rendered sterile by triploidization (Wynn, 1992) so the chance of them establishing sustaining populations is reduced. Mono-sex populations of tilapia are being produced to reduce unwanted reproduction and the associated reduction in growth rate. Similarly, hybrid crappie (Pomoxis annularis x P. nigromaculatus) are being used on an experimental basis to avoid their uncontrolled reproduction when stocked as bait-fish in small impoundments (Hooe et al., 1994). Hybrid fishes are also stocked for direct harvest. The saugey, a walleye x sauger cross, is very adaptable, displays significant improvement over both of the parent species, is reproductively viable and is being stocked in rivers, to support recreational fisheries (White and Schell, 1995). Although specific selection programmes for fish used for inland stocking is not common at present, the genetic structure of many populations in less intensively managed systems is being conserved through hatchery management, reducing stock transfers and reducing transfers of eggs and fingerlings among hatcheries (Philipp et al., 1993).


The increases in productivity associated with the various steps of the intensification process are described in Figure 1 and Table 4.


12.1 Ownership

The development and extension of more intensive systems for managing inland and coastal waters for food production from fisheries can only occur within an appropriate social and economic setting. Increased human control of the fishery requires investment of time, funding and other resources which will only occur when the investor is guaranteed a due return. As human inputs increase, the control of the aquatic system and the rights to the fish tend to be transferred from the public to the private domain so that those investing in the fishery can protect their interests and are in a position to negotiate with other users of the resource. This means an assignment of ownership or rights to individuals or groups of individuals of what was previously an open access resource. In many areas of the world existing ownership patterns are inappropriate and impede the adoption of enhancement methodologies. Typically in “western” countries, fishery resources are perceived as common resources and the public feels all have a “right” to fish. This open access regime has been implicated as one cause of the decline in capture fisheries. However, in many “eastern” countries and small island states fishing rights are granted to specific communities or families, thus reducing the tragedy of the commons and facilitating management, including enhancement. Action is required both at community and government level. In the first instance private, communal or corporate owners of waters which are used for purposes other than fisheries should be encouraged to cede the rights to develop the fishery to interested parties. In the second, governments should legislate for the protection of those developing fishing waters.

Table 4. Summary of stocking and ancillary enhancement practices at various intensities of input.

Supported natural< 500Small - largeNil
Extensive500 – 1000Small - largeNil
occasional fertilisation
artificial reefs
Semi-intensive1000 – 2000Small - mediumsome fertilisation
species eradication
artificial fauna
strain selection
hatchery management
habitat modification
Intensive2000 – 3000Smallintense fertilisation
species eradication
artificial fauna
system modification
genetic modification in the short term
Aquaculture> 3000SmallIntense fertilisation
Intense feeding
System modification
Modification of water body
Genetic modification in the short and long term

12.2 Cost effectiveness

Analysis of the cost effectiveness of enhancements can be carried out at two different levels, that of the individual fishery and that of the society as a whole.

12.2.1 Economics of individual fisheries

Contemporary policies in developed countries would maintain that ideally each fishery should be financially self-supporting. In such cases the financial benefit derived from the. fishery (B) should equal or exceed the costs of producing the catch (C). Thus


In its simplest form, when applied to a stocked fishery B would consist of the price of the fish produced and C would have the following components:

C1the cost of the stocking material, which often amounts to between 40 and 70% of total costs;
C2costs of harvesting.
 As the fishery is intensified other components are added to C:
C3cost of fertilisers;
C4cost of removal of unwanted species;
C5cost of physically intervening to maintain the environmental quality (draining reservoir, dredging, weed removal, liming, etc.);
C6costs of physically modifying the environment (creation of bunds, embankments, creation of spawning and shelter habitats, construction of artificial reefs, etc.);
C7costs of genetic manipulation and genetic resource management (selective breeding, hybridisation, polyploidization, gene transfer, or sex manipulation) will increase C1, cost of stocking material.

In each case as an additional term is added to C the benefit B should rise to cover the costs involved. This implies that the increasing costs involved in production from enhanced systems can only be met firstly by increases in the productivity of the system with respect to the target species and secondly by increasing stringency in financial management with the minimisation of support costs to both commercial and recreational fisheries and the elimination of inefficient or unnecessary practices.

Evaluating recreational fisheries has always proved particularly intractable to standar d cost-benefit analyses because of the difficulty in assigning an accurate price to the value of a “recreational fish”. The cost per fish caught in a recreational fishery is high relative to the cost of the same fish caught commercially because the recreational fishermen's sport equipment and “willingness to pay” values, i.e. contingent evaluation, are factored into the price, as well as the cost of maintaining recreational fisheries through stocking and environmental enhancement programmes.

For example the difference in “value” between the same fish caught in a recreational fishery or a commercial fishery can be substantial. A 2–3 kg barramundi Lates calcarifer caught in Queensland, Australia would yield 1–1.5 kg of fillets worth about A$ 6–15/kg in a commercial fishery with about half this amount going to the fishermen; the recreational fishermen, however, probably spent about A$ 150 to catch such a fish (Rutledge et al., 1991; Bartley, 1995a).

12.2.2 Economics in the wider context

It is often difficult to consider the fishery in isolation in considering its cost effectiveness. Clearly at the level of the individual entrepreneur failure to make a profit would prove an obstacle to the continuance of the fishery. However, there are larger factors that have influenced planning at several levels. Firstly the component B, while it contains only the price of the fish produced, will lead to strict cost-effectiveness approaches, but in many cases B contains other values for instance:

B1price of fish produced;
B2benefit to society of not having to support unemployed fishermen;
B3benefit to society of supporting indigenous communities;
B4benefit to manufacturing and tourism sectors supporting recreational and commercial fisheries;
B5support to the fish farms producing stocking material;
B6when enhancement involves endangered species there may be a benefit in that restrictive regulation on fisheries and on water use in order to protect the endangered species may be avoided;
B7benefits to society by allowing alternate uses of water, e.g. hydroelectric generation, transportation.

In these cases societies may, and often do, subsidise the enhancement process for the benefit of the larger sector. Such subsidies may take the form of assistance in supply of stocking material, in public works for construction of bunds or restoration of substrates, weed cutting, maintenance of water quality, etc. With the existence of such subsidies the individual entrepreneur can still make a profit and continue the fishery.

12.2.3 Examples of cost effectiveness

Formal equations have not been used to pre-evaluate proposed stocking programmes. Rather estimates are made on a more empirical basis as a programme develops. Empirical results of this type indicate that stocking and fertilisation can be a highly cost-effective ways in which to increase production.

In Southeast Asia stocking is considered highly profitable and the fisheries are based on good recapture rates (Sreenivasan, 1988). For example, some 42.2 million exotic carps are stocked per year into Thai reservoirs at a value of $118,000. This material contributes to the 26,827 t of these species caught every year at a value of over $2 million. However, it has proved difficult to evaluate the economic return of the stocking programme as it is impossible to separate stocked fish from natural production (Bhukaswan, 1988). Stocking of oxbow lakes in Bangladesh has also proved profitable, giving income over expenditure ratios of 1.78 (PIU/BRAC/DTA, 1995) whereas profits from stocked beels rose by 266.35%, equivalent to about $700 ha/yr.

Stocking in the ex-USSR has also increased the revenue from reservoir fisheries. Normally the biological efficiency of stocking decreases as the size of water body increases. However, in reservoirs of the ex-USSR (Berka, 1990) stocking was considered more economically efficient in terms of worker productivity in large reservoirs than in smaller ones. Stocking and other forms of enhancement proved successful in many countries with centrally planned systems such as China, Cuba, the USSR or Poland where much of the cost was borne by subsidies. These usually took the form of state-financed hatcheries which rarely recuperated the cost of the stocking material. More recently the transition to free market economies has removed subsidies, causing the collapse of many such fisheries and of the hatcheries supporting them. In an increasing number of countries of Central Europe both recreational and commercial fisheries are being stocked by the private sector.

Although stocking in many inland water systems is demonstrably cost-effective, the situation with regard to marine ecosystems is less clear. Ungson et al. (1993) claimed that the Japanese experience has been positive and cited the results with red sea bream in Kagoshima Prefecture where about $3 million net profit were generated by a stocking and catching exercise costing about $4.7 million. Stocking with salmon was regarded as equally profitable but the flounder stocking programme was not regarded as economically viable at a benefit-cost ratio of only 0.4 Kitada (1996) analysed several Japanese ranching operations and found that the most profitable were for scallops and chum salmon. Experiments by European and North American countries on the same lines have had dubious results (Bartley, 1995) on the basis of the low rates of recapture and the poor cost-benefit ratios. For example, Winton and Hilborn (1994) evaluated the cost per fish in the Canadian salmon enhancement programme as ranging from $40–$340 depending on the hatchery. However, the motivation for such programmes often tends to be political and they represent a subsidy to a sector which would otherwise prove uneconomic, increasing the burden of unemployment to the society as a whole. The burden of out-of-work fishermen can be substantial as demonstrated by the 40,000 Newfoundland cod fishermen that are idle following the collapse of the cod fishery. Each fisherman is entitled to CAN$ 225–460 per week for a planned 4-year cod-fishing closure at a potential cost to the Canadian Government of over CAN$ 3.6 billion (Bartley, 1995a).

The Japanese Government acknowledges that their substantial ranching programmes are subsidised and that the subsidy is justified because of the importance of marine fish in the diet and culture of the Japanese and because of past government policies that promoted land-based industrialisation that lead to the degradation of the marine and coastal environment and their fisheries. It is viewed as a justified function of the Japanese Government to provide important services, such as the construction of hatcheries and artificial reefs, for the benefit of its constituency, the fishing communities and fish consumers.

12.3 Financing

The transition to more intensified systems has also had impacts on the way in which fisheries are funded. Enhancement of fisheries implies increased costs to the fishery managers in fish seed, fertilisers and labour for species elimination, system modification, etc. One problem here is that the original fishermen are unlikely to be able to mobilise sufficient funds and funding of stocking tends to shift to external financing agencies. The greater predictability, more controlled harvesting season and more concentrated harvest attract investment from businessmen who then take the major part of the profit. This means that the intensification process is frequently accompanied by a loss of independence by the fishermen and a growing dependence on credit or other external forms of funding. As a partial solution to these problems governments should consider subsidies to enhancement either through direct grants or through the creation of state hatcheries at least in the earlier phases of the adoption of the technology. Governments should also encourage the setting up of rural financing institutions to assist fishermen and rural communities in developing the sector without the intervention of third-party financiers.

One approach to the social-economic management of fisheries is to form co-opera tives where the entire sector is vertically integrated. Thus, habitat improvement, hatchery operation, harvest, processing, distribution, marketing, and sales are all performed by different members of the co-operative. Losses in one section can be compensated for by large profits in another. Japan is currently employing this strategy in a formal way, whereas hatchery enhancement of white seabass in California is approaching such a system informally by requesting sport fishing clubs to help with the culture and release of fish hatched in a state-supported hatchery that receives much of its funding from special licenses sold to commercial and sport fishermen (Kent and Drawbridge, 1996).

Critical to the issue of financing is the issue of access, i.e. who can harvest the enhanced fishery. A strong rational for Japan's efforts at marine enhancement comes from the fact that with the entry into force of the UN Convention on the Law of the Sea and the recognition of countries EEZ, Japanese access to foreign fishing grounds has been reduced, while at the same time, Japan has much more control over the fishing activities on its own extensive coastal shelf area. In Oregon (USA) private salmon ranchers shut down because there was unrestricted access to their “ranched salmon” in the Pacific Ocean. Iceland allows no ocean fishery in its EEZ for stocked Atlantic salmon so that those financing the hatcheries can preferentially benefit from the harvest as the salmon return to their natal rivers (see discussion and references in Bartley, 1995). Enhancement of marine fisheries in the Persian Gulf is hampered by the suspected high amounts of illegal fishing (poaching).

12.4 Education

The management of high input, enhanced systems implies a greater level of knowledge than that which is needed for a simple extractive activity. In some countries the development of the new intensified systems of management has been accompanied by the emergence of research, often empirical, on the most appropriate ways to manage the systems and there is already a wealth of information in countries such as China, Cuba, India or Poland on stocking, feeding and fertilisation ratios. Research alone is not enough as the extension of the results and the application of the formula by the fishermen also require a higher level of preparation at their level. This trend reinforces the drift from simple fishermen to a more complex organisation of the fishery. Governments can support the spread and more effective application of enhancement strategies through research into the most effective ratios for stocking, fertilisation, etc. They can also promote extension and training to upgrade the capacity of fishermen to use this approach to management.


Capture fisheries in inland and marine waters, which are based on natural productivity have generally reached levels at which the fish communities are fully exploited or are overexploited. River and lake systems which are unmodified are now rare as are inland fish faunas. The marine environment has also been impacted, especially in coastal areas where development and land-based sources of pollution have degraded valuable nursery/spawning habitat. The trend for inland waters therefore is towards faunas which are supplemented or even constructed to maximise benefits from the modified systems. Inland capture fisheries are maintained increasingly by stocking and managers now frequently aim to increase productivity beyond natural levels by fertilisation, ecosystem manipulation and the elimination of competitors and predators. How this approach will work for many marine species is still controversial. Further gains in production in both inland and marine areas are made by incorporating cage and cove culture systems into the environment.

There remains a considerable divergence of opinion as to the value of enhancement programmes in fisheries. These often result from different perceptions of the aims of the enhancement and, where pure market criteria are applied, often result in conclusions that lead to doubt as to the effectiveness of the methodologies. Furthermore, concerns are frequently expressed as to the environmental consequences of the various enhancement methods. However, unspoilt habitats are now rare and most have already undergone substantial modification either of their form and function or of the fish assemblages they contain. Indeed many of the aquatic systems manipulated in this way are completely artificial. Environmental consequences will be judged by the particular society involved. Some societies may value native species over exotics, or naturally spawned fish over those produced in a hatchery, whereas other societies will be content with any form of fish protein. There will be increased competition from agriculture, industry, recreation, and transport sectors for the use of “water”; enhancement and intensive management may be one means to allow multiple uses of the water resource if society can accept a less than 100% natural product.

Present evidence indicates that stocking is a valid management tool which can substantially increase yields in specific circumstances and provide for food and social security in others. Fertilisation, habitat enhancement and other techniques supplement the effectiveness of stocking and appear to be particularly valuable in smaller water bodies. The adoption of some or all of these techniques indicates a trend to convert small dams and reservoirs into aquaculture installations in many parts of the world. The rapid adoption of cage and pen culture in water bodies that have otherwise remained unmanaged also indicates the success of this approach which appears to develop in parallel to other attempts to increase the food producing potential of inland and inshore marine ecosystems.

Given current technology, it will nearly always be possible to increase production from marine, coastal and inland areas. However, the cost and real benefit of this increase will depend on biological, ecological, social and economic factors. An accurate assessment of past programmes and monitoring and evaluation of ongoing programmes will be necessary to put the variety of options for fishery enhancement into proper perspective; such an assessment must also involve biological, ecological, social and economic factors. What are the tools needed for such an assessment? How can fishery managers decide where and when to apply what technologies? The tasks for the future are to address these questions and discover where our gaps in knowledge and technology lie and where fishery enhancement can be utilised effectively to improve the quality of life for humankind.


Any review paper of this type is the result of a communal effort in discussing and digesting the contributions of many workers in the field. This is particularly the case as we move towards a more complete review of techniques for intensified management and prepare for a number of consultations and symposia on these topics. The authors would therefore like to express their appreciation of all who have participated in the discussions on enhancement techniques, particularly our colleagues in the Department of Fisheries.


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