Life history movements and spawning of Sao Francisco River fishes, Brazil

The Sao Francisco River (SFR), a floodplain river southeast of the Amazon River, provides commercial and recreational fishing for thousands of people, but fisheries have collapsed in the last decade. During 32-months, I radio-tracked 37 curimbatas (Prochilodus argenteus , Characidae, 0.8--2.9 kg) and 24 surubims (Pseudoplatystoma corruscans, Pimelodidae, 9.5--29.0 kg), two important commercial and recreational fishes. I did the study to characterize their migratory style and to determine if the cool hypolimnetic discharge from an upstream dam (Tres Marias Reservoir, TMR) would block their upriver migration. I also made a conceptual model of the biological; river, dam, reservoir, and economic factors for a supplemental water release (SWR) to enhance SFR fisheries using a release from TMR. Migratory style of both species was dualistic with resident and migratory individuals. Home range was 1--127 km for curimbata and 1--210 km for surubim. The most important spawning area for curimbatas was the SFR mainstem at its juncture with the Abaete River mouth, and for surubims, the most important spawning area was the mainstem at Pirapora Rapids. Pre-spawning adults of both species staged at or near the spawning grounds. Some curimbatas and surubims homed to pre-spawning staging areas and to spawning areas. Some curimbatas also homed to non-spawning areas. The movements and thermal fluctuations experienced by both fishes showed they are eurythermal with a broad short-term temperature variation tolerance. Also, water discharged from dams that is <5°C cooler than ambient river water will not disrupt their migrations. The best date for a SRW is when there is a natural flood, which triggers spawning. The SWR will intensify the natural flood, cover a greater floodplain area, and increase survival of early life stages of fish. The TMR frequently impounded enough water for SWR only in the second half of the fish spawning season (January--March). Lost revenue at TMR depended on release volume and ranged from US{dollar} 0.493 million to US{dollar} 3.452 million for the actual power rate. However, SWR could increase commercial fisheries income an estimated US{dollar} 4.468 million. Planned construction of 16 dams in the mainstem and tributaries downstream from TMD will greatly affect curimbata and surubim migrations and spawning habitat, extirpate populations, and eventually reduce their abundance.