Effects of a low-head dam and water abstraction
on migratory tropical stream biota.
ABSTRACT: Migration of large-bodied
'macroconsumers' (e.g., fishes, shrimps, and snails) is an important functional
linkage between many tropical rivers and their estuaries. Increasingly,
this linkage is being severed by dams and water abstraction. The ecological
impacts of these activities are poorly understood and are largely being
ignored by dam operators. We investigated the direct effects of a water
intake and low-head dam on the migration of amphidromous freshwater shrimps
between the headwater streams and estuary of the Rio Espíritu Santo,
Puerto Rico. Both downstream migratory drift of larvae and upstream migration
of postlarvae had strong diel patterns, with most activity occurring at
night. Unlike large dams on the island, this low-head dam did not act as
a complete barrier to the upstream migration of metamorphosed postlarvae.
However, the dam did cause large numbers of postlarval shrimps to accumulate
directly downstream of the structure. Mortality of drifting first-stage
larvae by entrainment into the water intake during downstream migration
averaged 42% during the 69-d study period. During low discharges, 100%
of the drifting larvae were entrained by the intake. The rate of nocturnal
entrainment-induced mortality averaged 233 larvae/s and peaked at 1167
larvae/s. We used our field data and a 30-yr discharge record to model
the long-term impacts of different intake management strategies on the
entrainment mortality at this dam. The simulation model showed that long-term
mean daily entrainment mortality ranged between 34% and 62%, depending
on the amount of water extracted from the river. Monthly differences in
mean daily entrainment mortality (27% to 76% depending on estimates of
abstraction) were caused by seasonal variation in discharge. Modeling of
mitigation options suggested that daily entrainment mortality of larvae
could be reduced to between 11% to 20% if water abstraction was halted
(5 h) during evening periods of peak drift. Impacts of the dam and
operations can be significantly ameliorated by 3 to 5-h stoppages in water
abstraction during peak nocturnal larval drift, upkeep of a functional
fish ladder, and maintenance of minimum flow over dam. Since the impacts
of dams depend on the hydrology and design of specific water intake systems,
mitigation strategies must be tailored to individual dams and intakes.
However, our approach and results likely apply to low-head dams throughout
the range of amphidromous species.
Benstead, J.P., J.G. March, C.M. Pringle, and F.N. Scatena. 1999. Effects of a low-head dam and water abstraction on migratory tropical stream biota. Ecological Applications 9: 656-668.
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