Effects of a low-head dam and water abstraction on migratory tropical stream biota

Migration of large-bodied "macroconsumers" (e.g., fishes, shrimps, and snai ls) is an important functional linkage between many tropical rivers and the ir estuaries. Increasingly, this linkage is being severed by darns and wate r abstraction. The ecological impacts of these activities are poorly unders tood and are largely being ignored by dam operators. We investigated the di rect effects of a water intake and low-head dam on the migration of amphidr omous freshwater shrimps between the headwater streams and estuary of the R io Espiritu Santo, Puerto Rico, USA. Both downstream migratory drift of lar vae and upstream migration of postlarvae had strong diel patterns, with mos t activity occurring at night. Unlike large dams on the island, this low-he ad dam did not act as a complete barrier to the upstream migration of metam orphosed postlarvae. However, the dam did cause large numbers of postlarval shrimps to accumulate directly downstream of the structure. Mortality of d rifting first-stage larvae by entrainment into the water intake during down stream migration averaged 42% during the 69-d study period. During low disc harges, 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 held data and a 30-yr discharge record to mo del the long-term impacts of different intake management strategies on the entrainment mortality at this dam. The simulation model estimated long-term mean daily entrainment mortality at 34-62%, depending on the amount of wat er extracted from the river. Monthly differences in mean daily entrainment mortality (27-76% depending on estimates of abstraction) were caused by sea sonal variation in discharge. Modeling of mitigation options suggested that daily entrainment mortality of larvae could be reduced to 11-20% if water abstraction was halted for 5 h during evening periods of peak drift. Impact s of the dam and operations can be significantly ameliorated by 3-5 h stopp ages in water abstraction during peak nocturnal larval drift, upkeep of a f unctional fish ladder, and maintenance of minimum flow over the dam. Since the impacts of dams depend on the hydrology and design of specific water in take systems, mitigation strategies must be tailored to individual dams and intakes. However, our approach and results are likely to apply to low-head dams throughout the range of amphidromous species.