4.3.4.3 — Fishes: Migration between fresh- and saltwater — diadromous species

A considerable number of fish species cross the barrier between salt- and freshwater, either to spawn in saltwater (catadromous), or to spawn in rivers (anadromous). Eels are catadromous, and the spawning journey of the European eel (Anguilla anguilla) to the mysterious spawning grounds deep in the Sargasso Sea is a well-known example. The area was found indirectly, by painstaking mapping of larvae of distinct sizes, thereby circling in the breeding area (Schmidt 1923). The upstream migrations of anadromous salmon are easier to observe. Huge numbers of fishes attracted fishermen and predators in many temperate river systems. The productivity was enormous, and during the U.S. depression, Pacific salmon sold as "poverty steak" for 10 cents a pound. In 1993, prices were up to 8 dollars a pound, due to damming of the Sacramento river (Dingle 1996). Winter runs up the Sacramento sustained a harvest to over a million fish, but dropped to a pitiful 191 fish in 1991. This decline is clearly caused by the Red Bluff Diversion Dam, and survival of the salmon populations is now totally dependent on the amount of water allotted for the spawning runs (l.c., p. 408). Similar declines are observed throughout industrialised countries, some of which have now started expensive rehabilitation programs (Neumann et al. 1998). The most recent dramatic decline is observed in sturgeons (Acipenseriformes). Within the former Soviet Union, a system of hatcheries compensated partially the losses caused by large river dam projects. Today, the operation of hatcheries is ineffective, and illegal fisheries are driving several species towards the brink of extinction. 18 anadromous sturgeon species have now been listed on Appendix II of CMS, and a recent total catch ban of three nations bordering the Caspian Sea will hopefully help to save the species at the last minute (see references in Figures A2.84 and A2.85).

Finally, there is amphidromous migration; this refers to fish that migrate from freshwater to the sea or vice versa. This migration is not for breeding, but mainly associated with feeding or because of climatic reasons. Examples are the endangered sawfishes (Pristis spp.), which are now a target species for industrial fisheries (see Table 4.11).

All species depending on freshwater are suffering from degradation of river habitats and pollution. The "Living Planet Report" identified freshwater systems among the most endangered habitats (Loh et al. 1998). Finally, it needs to be kept in mind that marine fisheries may jeopardise conservation efforts: The bycatch of industrial fisheries targetting small species for fishmeal (known in German as "Gammelfischerei") will affect any juveniles of anadromous species trying to grow in the sea.

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