4.3.4.2 — Fishes: Migration in freshwater — potamodromous species

Fish moving entirely within freshwater exhibit seasonal return movements to spawning areas, usually located upstream. Within rivers, this is called potamodromous. If feeding areas or spawning areas are located in lakes, this is called limnodromous. Besides bony fish (Osteichthyes), this migration pattern is observed in Cyclostomes (Lampreys: Petromyzodontidae) and even freshwater sting rays (Potamotrygonidae: Chondrychthyes) that migrate in the Amazon, the Mekong and the Niger river. Up to now, around 100 limno- and potamodromous species have been identified (see Table 4.9), but the scattered literature about migration in tropical river systems has not yet been evaluated. For example, in the middle Indus there seem to be "upstream migrations of three major carps Labeo rohita, Catla catla and Cirrhinus mrigala, the two carnivorous species Mystus aor and Mystus seenghala, the catfish Wallago attu and the snakehead Channa marulius. The large catfish Bagarius bagarius, a species that is widely distributed in the subcontinent, may also [...] move upstream" (Reeves & Smith 1997, p. 13). While this information needs careful examination, the disastrous impacts of dams on a great variety of highly productive tropical species is now a sad fact: major spawning areas of mahseers (Tor putitora and T. tor), holy carps in the eyes of Hindus, have been destroyed by dams in the upper reaches of rivers in the South Asian subcontinent (Nautiyal 1984, 1989).

The complexities of migration within the Mekong river were elucidated by an extensive field study in 1993 at Ban Hang Khone, an important fishing village on Khone Island just below the great waterfalls of the Mekong River at Lee Pee, southern Laos (Roberts 1993). Roberts identified three migratory periods, late January-February, May-July, and November-December. The study diagnosed a gradual decline of fisheries for all observed 93 species to about 20% of what it was in 1970. The endemic Mekong freshwater herring "pa mak pang" (Laotian shad: Tenualosa thibaudeaui), formerly one of the most abundant species, had almost disappeared, and is now red-listed as "Endangered" (Table 4.11). The decline is attributed to deterioration of flooded forest habitat, overfishing and destruction of spawning grounds by upstream dam construction. The Mekong river is also home to the only fish species listed in CMS Appendix I, the endangered giant catfish (Pangasius gigas, listed under Pangasianodon gigas). The Mekong river is still considered "a pristine river, free of dams and large-scale water diversion projects, and the water quality is generally good" (Hill & Hill 1995, p. 1). But in the light of huge development projects, co-ordinated by the Mekong River Commission, the authors conclude that "preservation and management of Mekong fish faunas will depend upon incorporating spatial information into decisions about [...] anthropogenic change" (l.c., p.1).

Probably the worst conservation status is observed among the sturgeons (Acipenseridae), which are actually driven to the brink of extinction by illegal fishing, habitat degradation and pollution. Following a proposal of the Federal Republic of Germany (1999), 6 potamodromous sturgeon species (Acipenseridae) have been added to CMS Appendix II, together with 18 anadromous sturgeons (see below). Further endangered potamodromous species are listed in Table 4.11. Research on the present conservation status is needed for the six additional species listed as "Data Deficient" (Table 4.12). In addition, it is necessary to keep in mind that dam construction can affect entire populations or whole species by irreversible blocking of upstream migration.

This document should be quoted as part of the publication "Riede, K. (2001): The Global Register of Migratory Species ­ Database, GIS Maps and Threat Analysis. Münster (Landwirtschaftsverlag), 400 pp." + CD

 by Klaus Riede