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:

• compensation - to mitigate a disturbance to the environment from human activities;

• maintenance - to compensate for recruitment overfishing;

• enhancement - to maintain fisheries productivity of a waterbody at the highest possible level; and

• conservation - to retain or replenish stocks of a species that is threatened or vulnerable.

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

• Stock enhancement of wild fisheries - The enhancement of stocks of an existing wild, open-access fishery with species that may or may not be self-recruiting. This category includes the stocking of relatively large inland waterbodies where there are no property rights to the stock. Generally the recapture rate of stocked fish is low and repeated enhancement is not always necessary to maintain the fishery.

• Culture-based fisheries - The stocking of small waterbodies is a form of enhancement that is typically undertaken on a regular basis and the stocking activity is the only means of sustaining the fishery. Typically, a person or a group of persons and/or an organization will have property rights to the stock. The source of stock for the enhancement may be derived from capture, but more typically is obtained from a hatchery operation. These features collectively amount to a form of aquaculture that according to the FAO definition (FAO 1997), is referred to as culture-based fishery.

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:

• Use as a seeding mechanism for replenishing depleted "breeding stocks" - e.g. the case of Indonesian reservoirs particularly using species that are indigenous to the country, but not necessarily to a particular waterbody (See Section 4.1.2 and Table 6).

• Replacement of existing, self-recruiting species/stocks, with species/stocks with more desirable traits (such as higher growth rate, reduced tendency to stunt etc.) - e.g. the endeavours to replace Oreochromis mossambicus with O. niloticus, a practice that has become increasingly popular over the last three to four decades in most Asian lakes and reservoirs. More recently, there have been attempts to replace the original stocks of O. niloticus with the "GIFT" strain of O. niloticus (GIFT-genetically improved farmed tilapia).

• Regular stocking of species with a view to sustaining a fishery - In most instances, such stocked species are unlikely to form breeding populations in the waterbodies concerned. This is due to the stocked species requiring migration to a riverine habitat for breeding. Typical species used for this purpose include the Chinese and Indian major carps.

• Regular stocking of floodplains - as a form of compensation for reduction in recruitment resulting from developments related to flood control (e.g. in Bangladesh) and to increase fish yields, and/or develop new fisheries to enhance fish supplies (e.g. in Myanmar).

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

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)

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:

• The fisheries in large reservoirs in the region are usually "open access" and are not typically dependent upon stocking. These fisheries are based upon self-recruiting populations that may be "seeded" by stocking at irregular intervals.

• In small and medium reservoirs, the fisheries are almost always dependent on stocking, as natural recruitment is too small to sustain a fishery, even on a very small scale, and fishing pressure on the stocks is typically high.

• Fishery activities in small and medium reservoirs that are based entirely on a stocking and recapture strategy often also have well defined ownership. In such cases they are referred to as culture-based fisheries.