6.5 VERTEBRATES

Amphibians / Reptiles / Riverine birds / Riverine mammals

Fishes .
Fish are one of the most obvious elements of the freshwater fauna. They are very varied in size, form and behaviour and provide the essential trophic link between the primary producers and detritus feeders and the large carnivorous organisms. They are the basis for valuable fisheries and are of great economic importance. Because of this, the fish of the Mekong River are dealt with in detail in Section 12. However, some general information on the diversity of Asian river fishes in general, especially those of the Mekong, is given here.

Over 3,500 species of freshwater fishes occur in Asia (Kottelat and Whitten 1996). When the countries of Asia are ranked in terms of relative species richness of freshwater fishes, they constitute half of the 10 top-ranked nations.

The top 10 countries in the world, in terms of numbers of fish species (KOTTELAT & WHITTEN 1996). Survey data for Asia are incomplete so totals are conservative.

If we restrict the analysis to species richness per river (thereby omitting fishes confined to lakes), 11 out of 20 (55%) of the top-ranked rivers in the world are in Asia. This result does not reflect a tendency for larger river basins, which are likely to support more species, to occur in Asia. There are no Asian rivers among the ten largest river basins; the species-rich Mekong ranks 25th by area but third by fish species. Given their relatively small basins (ranking 88th or lower), the Cauvery, Kapuas, Sittang and Mahakam support disproportionately rich fish faunas (ranking 16th or higher). When family richness is considered, only five Asian rivers appear among the top 20 ranks, the Mekong peaking at 10. This is surprising: Asia has 105 families of freshwater fishes compared to 74 in Africa and only 60 in South America. However, fish communities in most Asian rivers are overwhelming dominated by the family Cyprinidae, which is very species rich, and there is a relatively low numbers of families in each river.

River Basin	Approx. number of species	Global rank: species richness	No. fish families	Global rank: family richness
Mekong (25)	450	3	37	10
Ganges (16)	350	4	32	20
Chang Jiang (11)	320	5	23	53
Cauvery (98)	265	7	27	39
Kapuas (88)	250	8	32	20
Chao Phraya (69)	222	10	36
Brahmaputra (22)	200	11	32(?)	20(?)
Sittang (132)	200	11	31	25
Krishna (56)	187	13	29	35
Song Hong (64)	180	15	24	51
Mahakam (108)	174	16	33	20
Indus (19)	147	21	24	52
Salween (57)	143	22	34	16
Zhujiang (138)	106	33	21	62
Fly (74)	101	34	13	94

Global ranking of Asian rivers (basins >22,500 km²) according to fish biodiversity (based on raw data from Kottelat and Whitten 1996; Groombridge and Jenkins 1998). Ranks of drainage basin area for each river are given in parentheses. (?) is an estimate based on a figure for the combined Ganges-Brahmaputra drainage (Groombridge and Jenkins 1998).

Data on fish biodiversity in river basins are uneven in quality. For example, species totals for the Kapuas River (250 species) cited by Groombridge and Jenkins (1998) do not match those of Kottelat and Whitten (1996: 320 species). Rainboth (1996) estimates that the Mekong drainage may support 1,000 species – more than twice the total in Table 2 – but most scientists working on the Mekong believe that the final total may be 1,500 or more species (see Section 12). Whatever figure is finally accepted, there is no doubt that the Mekong Basin is unusually rich in fishes.

Differences in estimates of species richness illustrate our incomplete knowledge of Asian river fishes, reflecting inadequate fieldwork, differing approaches to fish taxonomy, and other constraints. Recent experience suggests that where fish faunas are evaluated, more species than expected turn out to be threatened or cannot be re-recorded (Groombridge and Jenkins 1998). Many species might be extinct, and knowledge of some faunas is so incomplete that species could have been lost before scientists discovered them.

The disproportionately rich fish communities of Asia are disproportionately endangered, as is evident from a list of the 30 river basins warranting global conservation priority given by Groombridge and Jenkins (1998). This assessment was based on fish family richness, river vulnerability (related to remoteness from settlements, road access, and man-made structures), and pressure for water resource development (i.e., existing or projected problems with water supplies for humans). Seventeen (57%) of the 30 rivers are in tropical Asia: Ca, Cauvery, Chao Phraya, Ganges-Brahmaputra, Godavari, Indus, Irrawaddy, Krishna, Ma, Mahanadi, Mekong, Narmada, Pahang, Perak, Salween, Sittang, and Song Hong. Had the analysis had been based on fish species richness, rather than family richness, the list would have contained many more Asian rivers.

page 1 of 2