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4. Stock enhancement: general considerations

The term "stock enhancement" is often broadly used to describe most forms of stocking, irrespective of purpose. From a fisheries view point, this can be somewhat misleading, even though the ultimate goal of every enhancement practice is to increase stock size and thereby, the fishable stock. Welcomme and Bartley (1997) recognize four major types of stocking intervention based on the objective of the intervention:

For the purpose of this review, "enhancements" are separated into two types:

Apart from the above differences, the objectives for stock enhancement may differ markedly between developed and developing countries. Welcomme (1996) characterized the differing strategies with regard to management of inland waters for fish production, and these are equally applicable, with minor modification, to stock enhancement in inland waters (Table 5). The primary purpose of stock enhancement of floodplains, large reservoirs and lakes in Asia is to increase the foodfish supplies and is in contrast to that in developed countries, where it is to enhance recreational fisheries and for conservation purposes (Cadwallader 1983, Welcomme 1997, Miranda 1999).

Stock enhancement in developing countries may be based on one of four broad strategies, or combinations thereof:

Table 5. Differing strategies for management of inland waters for fish production through stock enhancement (modified from Welcomme 1997)

Developed countries

Developing countries

Main objectives

· Conservation

· Provision of food

· Recreation

· Employment

· Political


· Sports fisheries

· Food fisheries

· Habitat restoration

· Enhancement through intensive stocking (and management of ecosystems?)

· Environmentally sound stocking

· Extensive/semi-intensive (+ integrated) rural aquaculture; culture-based fisheries

· Intensive, discrete, industrialized aquaculture


· Capital intensive

· Labour intensive

4.1 Stock enhancement of inland waters in Asia

4.1.1 Rivers and floodplains

The enhancement of riverine stocks for fisheries development in Asia is relatively rare compared with developed countries. Stocking programmes for the Mekong giant fish species - the giant barb (Catlocarpio siamensis), the giant catfish (Pangasianodon gigas), isok barb (Probarbus jullieni), thicklip barb (P. labeamajor) and thinlip barb (P. labeaminor) are some of the few instances of riverine stock enhancement in Asia. Stocking of these species, some of which are endangered, is planned and/or in progress as a component of an integrated management strategy for improving the status of wild stocks (Mattson et al. 2002).

In developed countries, there are few (if any) remaining artisanal freshwater river or floodplain fisheries. Riverine stock enhancement in these countries is carried out primarily for sport fishery development and for the conservation of indigenous stocks. A secondary purpose in some rivers is stocking for the control of aquatic weeds.

Riverine stocking with exotic species for the purpose of developing recreational fisheries, often associated with the promotion of tourism, has taken place in some Asian countries. These exotic species are typically salmonids, and the countries where this has taken place include India, Pakistan and Sri Lanka. This activity is still continuing to some degree, despite its potential negative affects on indigenous flora and fauna. Some enhancements have had negative impacts on native flora and fauna, such as the introduction of brown trout (Salmo trutta) and rainbow trout (Oncorhynchus mykiss) into New Zealand in the late 1800s, which is purported to have negatively impacted the native galaxiid stocks (McDowall 2003).

It is disheartening to note that the riverine stocking of exotic species has not been objectively evaluated and has attracted very little attention from the scientific community in the region. Indeed, attempts to justify culturing such exotics (Nepal et al. 2002) in the mountainous areas in the region have generally ignored the availability of local species that are equally or even better suited for this purpose (e.g. some species of Tor are excellent sportfish) and the increasing body of evidence of negative impacts on native fauna (Petr 2002).

Stock enhancement of the giant river prawn, Macrobrachium rosenbergii, has been attempted in some rivers, large waterbodies and reservoirs in Thailand over a fairly long period of time; however, reliable data on stocking are available only since 1998. This is one of the relatively uncommon examples of stock enhancement with a non-finfish species. During the period 1998-2003, 15 Thai rivers were stocked in one or more years, with nearly 70 million postlarvae. The most intensely stocked river was Pak Panang in southern Thailand, which was stocked with 26 million postlarvae in 1999. Songkhla Lake has also been repeatedly stocked (in 2002, 11 million tiger prawn postlarvae, 7 million banana prawn postlarvae and 14 million giant river prawn postlarvae were stocked) (Choonhapran et al. 2003). Regrettably, however, there are few statistics available on the returns from these stock enhancement attempts. Although Choonhapran et al. (2003) report some increased production following the stocking activities, no evaluation as to whether this was a direct result of the stocking activity could be made. Stock enhancements for giant river prawn have also been conducted in reservoirs (see Section 6.7.2).

Floodplains are wetlands that retain an association with the parental river and are typically inundated for part of the year during annual floods. These wetlands are very productive ecosystems and also provide crucial habitats for the spawning of some riverine species. The inundated parts of floodplains also provide important feeding grounds for fry and fingerlings of most riverine species. The floodplains of Bangladesh, Cambodia, Myanmar and several other Asian countries support substantial artisanal fisheries. In Bangladesh and Myanmar, some of these floodplain fisheries have stock enhancement strategies. In some cases, parts of the floodplain have been cut off from the parental river by damming for fishery enhancement and management. These fisheries are obviously managed in a more intensive manner and are more akin to "culture-based fisheries" (see Section 8).

4.1.2 Lakes and reservoirs

There are relatively few natural lakes in the Asian region, and the emphasis of stock enhancements has been mostly directed at reservoir stocking. Most of the natural lakes are not stocked regularly, and the available evidence indicates that stocking has been confined to self-recruiting exotic species such as tilapias (Oreochromis spp.) and common carp (Cyprinus carpio). In some countries such as Thailand, there may be an increasing trend towards the stocking of indigenous species capable of forming self-sustaining populations in large waterbodies. Details on such introductions and stock enhancements in Indonesian lakes are given in Table 6. It is evident from Table 6 that in most instances, translocated indigenous species were not successful in establishing self-recruiting populations. In contrast, introduced exotic species such as Mozambique tilapia (O. mossambicus) and common carp were able to establish self-recruiting populations in almost all the lakes and have subsequently become the dominant species in their respective fisheries.

Due to their limited numbers, lakes are less significant in terms of fish production (except Tonle Sap Lake in Cambodia) and are not regularly stocked or enhanced. The stock enhancement of reservoirs is a major management strategy adopted for increasing fish production in these man-made waterbodies.

Table 6. Stock enhancement/introductions into Indonesian Lakes (extracted from Sarinita 1999)


Size (ha)

(Year introduced)




L. Toba

112 000

Cyprinus carpio(1905)


Contributes about 2%

Oreochromis mossambicus (1940s)


Dominant in the fishery


L. Tondano

5 600

Trichopterus trichopterus (1925)

Yields of 340 kg/ha/yr; T. trichopteruscontributed about 10% to the yield but declined after the latter introductions

C. carpio


O. mossambicus

L. Limboto

7 000

O. mossambicus (1944)

30% of the yield (330 kg/ha/yr) in 1985-1991

L. Lindu

3 500

O. mossambicus


Yields of 120 kg/ha/yr; contributes about 75-80% of the yield

L. Tempe

10 000 to 30 000

Trichogaster pectoralis


Dominated the fishery until about 1948 Established

Clarias batrachus(1939)
H. temmincki(1925)

H. temminckidominated the fishery for a few years but declined rapidly with the introduction and repeated stocking of B. gonionotus, which accounted for most of the production (900 kg/ha/yr); but since 1982 yields are about 600 kg/ha/yr

Barbonymus gonionotus (1937)

Repeated since 1937

Irian Jaya

L. Sentani

9 360

Osphronemus gouramy(1937)


Apart from O. mossambicus, other species have not established self-recruiting populations and do not contribute significantly to the fishery, which yields about 42 kg/ha/yr

T. pectoralis(1937)


H. temmincki(1937)


C. carpio (1937)


O. mossambicus(1951)


B. gonionotus(1966)


L. Ayamaru

2 200

O. gouramy(1937)


All introduced and/or translocated species, except O. gouramyare established in the Lake and C. carpiois the dominant species in the fishery (60%)

T. pectoralis(1937)


H. temmincki (1937)


C. carpio(1957)


1 na - not available

The range of sizes of Asian reservoirs that are used for fishery activities requires that stock enhancement strategies of large (>600 ha), medium (<600 to >100 ha) and small (<100 ha) reservoirs are best considered separately. This is justified for the following reasons:

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