In view of these concerns and the special importance of
migratory fish in the Mekong, we conducted an assessment
of the potential impact of mainstem dams on fish migration
and recruitment in the Mekong and on the fisheries that
depend upon migratory species (Dugan 2008). We did so
under the auspices of the Mekong River Commission and
brought together expertise covering dams and fisheries in
over 20 river systems in Asia, Africa, Australia, Latin
America, North America and Europe. We reviewed available
information on ecological and population characteristics for
important fish species in the Mekong and comparable rivers,
as well as experience in designing and operating hydroelectric
dams to minimize impacts on migratory fish populations.
We concluded that the dams currently planned for the
Mekong will have a major impact on the fisheries of the
basin. In particular, we concluded that the barriers created by
the dams will disrupt upstream spawning migration of economically
and biologically important species (Dugan 2008).
In addition, the downstream drift of fish eggs and larval
stages that sustain fisheries recruitment will be compromised,
mainly because juvenile life stages will be trapped in
the impoundments. Dams in the middle and lower reaches of
the lower Mekong basin, including in the major tributaries,
will stop the longest migrations and disrupt recruitment to
the lower reaches of the river. Although the impacts of dams
higher in the basin and on individual tributaries will be
restricted to the fish populations that use these reaches, these
populations contribute substantially to fish production along
large stretches of the river (Poulsen et al. 2002).
In other regions of the world, a suite of fish passage
technologies has been developed to partially mitigate the
impacts of dams. However, our assessment concluded that
existing mitigation technology in the form of fishways, locks
and lifts cannot cope with the scale of fish migration on the
Mekong mainstream, which involves over 50 species, many
tens of millions of individuals, and biomass that is much
greater than that found today in the rivers of Europe and
North America (Dugan 2008; Baran et al. 2001; Halls 2009).
In addition, fish passage mitigation measures for dams in
North America and Europe necessitated research and
development conducted over decades, and relied on teams of
experienced biologists and fish passage engineers. Similar
investments would be needed in the Mekong before any level
of certainty on their effectiveness could be determined.
Furthermore, specific mitigation measures adapted to the
species and hydrological conditions of the Mekong would
need to be designed from the start and integrated into dam
engineering and operation. Given the lack of investment so
far, it is unlikely that any substantial mitigation measures
will be available in the foreseeable future.