9. Biodiversity issues in relation to stock enhancement in inland waters

Relatively unregulated tropical rivers and their floodplains support a high biodiversity, rivalling that of the most diverse marine systems. This is due, in part, to extreme ecosystem complexity. Such rivers traditionally support very important, but often under-valued, fisheries (Coates et al. 2003).

Although surface freshwaters account for only a very small proportion of all waters on earth (see Figure 6), they are estimated to contain 2.4 percent of all known living species, and per unit area, are slightly richer in species than is the land (3.0 vs 2.7), and about ten times richer than are the oceans (3.0 vs 0.2) (McAllister 1999). They also account for a relatively richer ichthyofauna, an estimated 41 percent of the approximately 25 000 species of fish occurring in freshwaters. On the negative side, 20 to 35 percent of freshwater fish species are thought to be either threatened or extinct, and 43 percent of crocodilians and 59 percent of freshwater mammals are threatened (McAllister 1999). An analysis of fishes under threat in the 1996 International Union for the Conservation of Nature (IUCN) Red List indicated that species that depend on freshwater at any stage of their lifecycle are 10 times more likely to be threatened than marine and brackishwater species (Froese and Torres 1999). These authors, who confined themselves to only those species listed in FishBase (Froese and Pauley 2005), observed that 547 of the 637 threatened species (nearly 85 percent) had a link to freshwater. All these facts show the need to consider the impacts of stock enhancement on biodiversity if fisheries in inland waters in developing countries are to be sustainable.

There seems to be a common perception that, apart from recent developments such as dam building and a general deterioration of the quality of natural waters, the deliberate and/or accidental introduction of species has had a significant affect on biodiversity. However, De Silva et al. (2004), in reference to tilapias, which have had a major impact on fisheries and aquaculture in the Asia-Pacific region, concluded that there is no objective evidence to show that these introductions have significantly affected biodiversity in the region. Indeed, these authors went on to demonstrate that most of the evidence that has been brought forth previously has been misinterpreted and misconstrued.

Most stock enhancement practices in Asia, except perhaps in PR China and India, tend to use exotic species, mostly Indian and Chinese carps, which are known to grow fast and reach large sizes. There has not been a concerted attempt to assess the influences of these species on biodiversity in any nation, except for the preliminary study by Hossain et al. (1999) on stock enhancement in beels in Bangladesh described in Section 9.1. Even more disconcerting is that translocations of some of the above species within national boundaries are a common practice. More often than not, such translocations are not considered as "introductions", and any affects they may have on biodiversity receive little or no attention.

9.1 Biodiversity issues associated with floodplain fisheries stock enhancement

Floodplain waterbodies are neither riverine nor lacustrine, exhibiting features of both of these categories at some stage of their annual watercycle. The importance of floodplains as nursery, breeding and feeding grounds for many riverine fish species has been well documented. It is within this context that human interventions through physical changes to the floodplains and/or biological modifications such as through stock enhancement and introductions and transfers can impact biodiversity.

In the floodplain examples cited in the previous sections, stock enhancement activities have almost without exception involved exotic species, e.g. the use of Chinese carps and Java barb in Bangladesh and tilapia in Myanmar. Appropriate assessment of the biodiversity of floodplain fisheries is problematic, as they do not always have a permanent/resident fauna and the cyclical effects of flooding mean that this fauna changes according to the effects of inundation. To assess changes in biodiversity under such conditions is difficult, and to attribute the impacts of stock enhancement to any of these changes is even more problematic.

Hossain et al. (1999) conducted a study in three floodplain beel fisheries in Bangladesh from 1992 to 1995 that involved catches for 23 gear types (11 of which were selective). In this study, 41 species belonging 19 families (Table 30) were recorded (not taking into account species groups and two species for which the family status was not clear). Most importantly, only six species (including stocked species) were common to all three fisheries, clearly indicating the diversity of the fish fauna of the different floodplains. The authors used the Shannon-Weaver Index as a measure of diversity and concluded that (Table 31):

• in one beel (BSKB), fish diversity increased during the study period;

• the diversity index for all three beels varied from year to year;

• the stocked species dominated (by number) in the catches only once and in one beel only;

• the diversity index in two of the beels (Chanda and Halti beels) declined on termination of stocking with carp fingerlings; and

• overall, fish diversity declined significantly in one beel, remained unchanged in another and showed a small increase in the other beel.

In general, it is accepted that fish biodiversity of rivers and their associated wetlands is under severe threat. Welcomme (2000) suggested that perhaps the current threats make them the most endangered ecosystems on earth. Although the extent of stock enhancement of floodplain fisheries, and in particular, the use of exotic species for their enhancement, is not widespread in Asia, we know very little of the influence of this practice on biodiversity.

As the fragility of river and associated wetland ecosystems is increasingly perturbed through direct and indirect human intervention, the threat to the biodiversity of these systems is likely to increase. Stock enhancement offers opportunities for sustaining the productivity of some waterbodies - particularly those waterbodies whose fisheries have been impacted by environmental modifications or man-made structures such as reservoirs. This must be balanced against potential negative effects on biodiversity in other waterbodies that are still in a relatively unperturbed state and which still provide fisheries services through natural, unenhanced recruitment processes.

The study of Hossain et al. (1999) is perhaps the only attempt in Asia to discern a relationship between stock enhancement and fish faunal biodiversity, and it brings to focus the complexity of the problem and the serious lack of information relating to the issues concerned. With the current state of knowledge, it would be difficult, if not impossible, to draw general guidelines for future developments of stockenhanced fisheries of floodplains. Indeed, the only way this would become possible is through the commissioning of some relatively long-term studies on some of the significant floodplain fisheries of the region where stock enhancement is currently (or intended to be) undertaken.

9.2 Biodiversity issues related to stock enhancement in large lacustrine waters

The impoundment of rivers and streams brings about a reduction in biodiversity of the fish species of the impounded waters. This is due to the changes in flow regimes, barriers to spawning migrations, stratifications and altered trophic interactions. A recent study by Li (2001) on four representative reservoirs in PR China clearly showed a reduction in fish species biodiversity resulting from reservoir impoundment (Table 32). Another feature of impoundment is that, with the decrease in diversity, a few fish families tend to dominate the ichthyofauna. For example, in Danjiangkou Reservoir members of the family Cyprinidae account for 64.2 percent of all the species, whereas in the original river they accounted for only 44.2 percent. Similarly, only 12 families occur in the reservoir as opposed to 49 in the original river (Li 2001).

Table 30. Fish species recorded from three floodplain, stock-enhanced fisheries in Bangladesh. Based on data from Hossain et al. (1999); only data on identifications to the specific level are included

¹ introduced species.
² CB - Chanda Beel, 10 870 ha; HB - Halti Beel, 16 770 ha; BSKB Beel, 26 040 ha.
³ ++ recorded among top five species at least once; + recorded at least once.

Table 31. Summary results of the Shannon-Weaver Index on fish species diversity, in different years, including (A) and excluding (B) stocked species, of the three floodplain beels (modified after Hossain et al. 1999)¹

¹ Note the number of species includes some species groups and hence, the discrepancy from Table 30.
² na = not available.

Table 32. Selected features of four reservoirs and the status of the fish fauna in comparison to the original river and the principal river system (modified from Li 2001)

¹ na = not available.

Comparable reductions in the biodiversity of the ichthyofauna have been demonstrated in other countries. For example, in Hoa Binh Reservoir in northern Viet Nam, only 21 species have been recorded, whereas the river basin is purported to have 108 species (Ngo and Le 2001).

The loss of biodiversity in reservoirs cannot be avoided; it is an inevitable consequence of change from a riverine to a lacustrine habitat and the dam acting as a barrier to upstream movement of species. Conversely, it has been clearly shown that reservoirs have enabled increases in aquatic reptilian fauna and in fish-eating birds. Reservoirs are man-made habitats, and the more important question is whether the construction of a reservoir has resulted in a loss of biodiversity in the river that has been dammed and/or the principal river system of which the river is a part. The study of the effects of water management structures on river fisheries has been largely ignored during the initial environmental impact assessments, and it has only more recently been acknowledged that there have been wide ranging changes in the fisheries that were part of the systems affected.

More recent constructions have attempted to assess the effect of impoundments, but these have largely targeted the economic impact and/or overall production rather than the wider issue of species diversity. There is still a very poor understanding of how to balance the impacts of water management structures on fisheries and local livelihoods against the more wildly perceived benefits of income from electricity generation and irrigation. The reduction of the issues to gross economic returns often fails to capture relevant issues relating to people and their homes and the critical issue of long-term sustainability. A good example of this is the inappropriate application of aquaculture as mitigation for lost fishing livelihoods.

It is worth mentioning at this point that (typically cage) aquaculture is occasionally suggested as a means of offsetting the impacts on fisheries caused by dam closure or other changes to inland fisheries. The assumption is that the fishers can merely shift their activity to aquaculture. This simplistic approach is critically flawed for the following reasons:

• The aquaculture operation may often be geographically remote from the original fishing location (i.e. the loss of river fishing and requirement to move activities into the headpond behind a dam).

• The cost of the cages for aquaculture limits the ability of the great majority of fishers to change over to this activity.

• Fish in cages must be permanently guarded against theft, requiring a time investment that may have previously been used for other income generating/livelihood activities (this is rarely taken into economic calculations of the viability of aquaculture).

• The technical complexity of aquaculture is something that must be learned, making the activity highly risk prone in its early years.

• The establishment of aquaculture in a reservoir where there is a fishery means that marketing of the aquaculture product is in direct competition with the fish from the fishery, which may be lower priced or have a higher consumer preference (the result is that the aquaculture product may be more difficult to market profitably).

What is more relevant to the present study, however, is to assess whether stock enhancement in large inland waterbodies has been or is responsible for directly or indirectly influencing biodiversity. With the exception of PR China and Thailand, it can be generalized that the species used for stock enhancement of large waterbodies are almost always exotics. The issue then becomes more complex, as there are the double effects of the introduction of exotic species coupled to their effects on the existing species. This makes it very difficult to demonstrate cause and effect.

Perhaps the most controversial issue has been the introduction of exotic tilapias, most notably into Lake Lanao in the Philippines and their purported adverse affects on native cyprinids. Another example was the near extinction of a small endemic goby (the "sinarapan", Mistichthys luzonensis) in Lake Buhi, which is also suggested to be the result of the introduction of tilapia. De Silva et al. (2004) critically examined the available evidence on both cases and concluded that the exotic tilapia was not a primary factor in the decline of these indigenous species. This is evidenced by the fact that, with better management of the fishery activities in Lake Buhi, "sinarapan" is staging a recovery. More recently, Guerrero (1999) considered the influence of tilapias on the biodiversity of finfish in lakes and reservoirs in the Philippines and concluded that there had not been any adverse affects on the endemic fish fauna.

In Indonesia, the decline of the indigenous cyprinid Lissochilus spp., considered to have cultural importance, in Lake Toba, was attributed to the introduction of Oreochromis mossambicus to the lake (Baluyut 1999), although supporting evidence for this observation is not available.

Perhaps the primary reason that most stocked species (particularly the Chinese and Indian major carps) do not tend to influence the biodiversity of large inland, lacustrine waterbodies is that they are generally unable to reproduce in such waters and thereby form large populations that would compete for common resources. The accidental introduction of silver carp into Gobindasagar Reservoir (16 867 ha), Himachal Pradesh, is reported to have resulted in a marked decline in the fishery for indigenous major carps and other indigenous species and a concurrent dominance of silver carp, followed by grass carp and common carp. The contribution to the fishery from the latter group increased to 87.4 percent in 1989 from a meagre 14.2 percent in 1974-1975 (Sugunan 1995).

During this change, the importance of indigenous species, in particular Labeo dero, L. dyocheilus, L. bata, L. ariza and Barbodes sarana drastically declined, not necessarily a reduction in biodiversity per se, but a change that could lead to such a status in the future.

The exception to this is the tilapia, which readily establishes in large waterbodies and forms large self-sustaining populations. The case may be clearer for natural waterbodies; however, it is difficult to resolve the issue for large man-made waterbodies. In this case, it is necessary to determine the extent to which the tilapia is exploiting vacant niches within the artificial waterbody and the extent to which it is directly displacing indigenous species that may otherwise have established following the creation of the waterbody.

9.3 Biodiversity issues in culture-based fisheries

Except perhaps for the oxbow lakes in Bangladesh, all culture-based fisheries in the region are conducted in quasi-natural habitats. These are most commonly man-made waterbodies, some ancient and some relatively recent. Needless to say, these habitats are colonized to varying degrees by indigenous flora and fauna. However, the fish populations are not generally sufficiently large to support subsistence or artisanal fisheries on their own, and hence the secondary use of the waters for culture-based fisheries. From the earlier sections and previous reviews on the subject (De Silva 2003, Lorenzen 2003), it is evident that one other characteristic feature of these fisheries, with the exception of PR China, is that the practices depend either wholly or partially on exotic species. For example, the culture-based fisheries of Sri Lanka, Thailand and Viet Nam and to a lesser extent, India and Bangladesh, are based almost entirely on exotic species (Indian and Chinese major carps, common carp etc.).

As enhancement activities are most frequently conducted in quasi-natural waters, the apparent negative impacts on the biodiversity of the ‘indigenous’ flora and fauna of such waters cannot be strictly considered to be invasive as the environment has been artificially created. Indeed, the interactions and potential competition between exotic and native species in small waterbodies in the region have been barely studied. In one such study, in Sri Lanka, Wijeyaratne and Perera (2001) concluded that although some exotic and indigenous species shared common food resources, because of the nature of these food resources and their great abundance, there was no foreseeable competition per se between the two groups. This conclusion is supported by the study of Piet (1996).

Conversely, negative effects of culture-based fisheries could arise from exotic escapees invading the natural habitats of indigenous species, but there is no evidence yet that this has occurred. Since exotic species have been used for both fisheries enhancement and also aquaculture, it will be difficult, if not impossible to determine the specific effect of an enhancement activity. Despite this uncertainty, it is still important is to ensure that no new exotics are introduced for culture-based fisheries development per se and to make do with those species that are currently available. Additionally, it is preferable to explore the possibility of using other indigenous species, although economic considerations such as yield reductions that may result from this will have a strong influence on decisions.

There is an urgent need to address biodiversity issues in Asian waters, particularly in relation to fish species usage in stock enhancement practices. Not only are the direct affects of such practices important, but also their affects on the genetic diversity of the enhanced species brought about through generations of inbreeding. Unlike in aquaculture operations where there are deliberate efforts to prevent escapees of species with reduced genetic diversity, stock enhancement activities deliberately introduce hatchery-bred stocks (which may have similarly narrow genetic diversity to aquaculture stocks) to open environments where there is a greater probability of mixing with wild stocks, thereby increasing risks to the biodiversity of natural systems.