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WOS:000630750300001; 1. The Amazon basin hosts the Earth's highest diversity of freshwater fish. Fish species have adapted to the basin's size and seasonal dynamics by displaying a broad range of migratory behaviour, but they are under increasing threats; however, no study to date has assessed threats and conservation of Amazonian migratory fishes. 2. Here, the available knowledge on the diversity of migratory behaviour in Amazonian fishes is synthesized, including the geographical scales at which they occur, their drivers and timing, and life stage at which they are performed. 3. Migratory fishes are integral components of Amazonian society. They contribute about 93% (range 77-99%) of the fisheries landings in the basin, amounting to -US\436 million annually. 4. These valuable fish populations are mainly threatened by growing trends of overexploitation, deforestation, climate change, and hydroelectric dam development. Most Amazonian migratory fish have key ecological roles as apex predators, ecological engineers, or seed-dispersal species. Reducing their population sizes could induce cascading effects with implications for ecosystem stability and associated services. 5. Conserving Amazonian migratory fishes requires a broad portfolio of research, management, and conservation actions, within an ecosystem-based management framework at the basin scale. This would require trans-frontier coordination and recognition of the crucial importance of freshwater ecosystems and their connectivity. 6. Existing areas where fishing is allowed could be coupled with a chain of freshwater protected areas. Management of commercial and subsistence species also needs fisheries activities to be monitored in the Amazonian cities and in the floodplain communities to allow assessments of the status of target species, and the identification of management units or stocks. Ensuring that existing and future fisheries management rules are effective implies the voluntary participation of fishers, which can be achieved by increasing the effectiveness and coverage of adaptive community-based management schemes.

Species-specific disturbance calls of five commercially-important characiform species are described, the Curimatidae commonly called branquinhas: Potamorhina latior, Potamorhina altamazonica and Psectrogaster amazonica; Prochilodontidae: jaraquí Semaprochilodus insignis and curimatã Prochilodus nigricans. All species have a two-chambered swimbladder and the sonic mechanism, present exclusively in males, utilises hypertrophied red muscles between ribs that adhere to the anterior chamber. The number of muscles is unusually plastic across species and varies from 1 to 4 pairs suggesting considerable evolution in an otherwise conservative system. Advertisement calls are produced in river confluences in the Madeira Basin during the high-water mating season (January-February). Disturbance calls and sampling allowed recognition of underwater advertisement choruses from P. latior, S. insignis and P. nigricans. The advertisement calls of the first two species have largely similar characteristics and they mate in partially overlapping areas in the Guaporé River. However, P. latior sounds have a lower dominant frequency and it prefers to call from river confluences whereas S. insignis shoals occur mostly in the main river channel adjacent to the confluence. These results help identify and differentiate underwater sounds and evaluate breeding areas during the courtship of commercially important characids likely to be affected by two hydroelectric dams.

Monthly (April 2009 to May 2010) bottom-trawl sampling for Brachyplatystoma species along the rapids stretch of the Madeira River in Brazil revealed that Brachyplatystoma rousseauxii larvae and juveniles were present in low abundances in all areas and during all hydrological periods. The presence of larvae and juveniles throughout the hydrological cycle suggests asynchronous spawning in the headwaters of the Madeira River.

The middle stretch of the Madeira River has supported an intensive commercial fishery for several decades. Developed in an area of rapids and waterfalls, this fishery focuses primarily on catfish stocks, mainly pimelodids. Data from fish landings collected in the years immediately prior to the start in operations of two large hydroelectric dams were used to estimate growth and mortality rates for Pirinampus pirinampu (Spix & Agassiz) and Brachyplatystoma platynemum Boulenger. In addition, stock assessment was carried out for both species, and the results used to evaluate the status of the fishery. Mean population parameters were L∞ = 80.85 cm (total length), k = 0.52 year−1, ϕ = 3.53, A0.95 = 6 year, M = 0.87, F = 1.41–1.64 and Z = 2.28–2.51 for P. pirinampu; and L∞ = 95.55 cm (total length), k = 0.28 year−1, ϕ = 3.40, winter point = 0.41, A0.95 = 11 year, M = 0.55, F = 0.16–0.35 and Z = 0.71–0.90 for B. platynemum. Yield-per-recruit analysis indicated P. pirinampu was overfished and that the stock of B. platynemum was under-exploited. Moreover, overfishing of the P. pirinampu stock and the effects of fragmentation caused by the construction of the dam should be monitored.

Amazonian rivers are characterized by a strongly seasonal flood pulse, which is being altered by ongoing and planned hydropower development across the basin. Changes in hydrology have a cascade of physical, ecological, and social effects, and some ecohydrological changes in the Amazon are expected to be irreversible. To better understand these linkages, we investigated shared trends and causal factors driving fish catch in a major Amazonian tributary before dam construction to derive relationships between catch and natural hydrologic dynamics that can be used to assess likely post-dam fisheries impacts. We applied a time series-based dimension reduction technique (dynamic factor analysis) to investigate dynamics in fish catch across 10 commercially important species using daily fish landings and hydrological data. We found a 4-trend dynamic factor model to best fit the observed data, with fitted trends exhibiting variation representative of seasonal and longer-term hydrologic variation. We next considered 11 candidate explanatory time series and found the best dynamic factor model used 4 explanatory variables: maximum water level, flooding duration, previous year's flow, and rate of change in flow; however, each species showed a unique response to this set of hydrological variables. Species-specific responses suggested that future dam operating rules need to closely mimic the natural hydrologic regime in order to maintain the biological dynamics of this and similarly diverse systems. In particular, high flows associated with increased catch the following year are important to maintain, but may be difficult to achieve in the context of reservoir regulation and energy production goals.

We examined responses from shore fish assemblages and bottom channel fish assemblages before and after two cascade run-of-river dams in the Madeira River, in the Amazon. Those dams were installed in a river corridor where historical factors, represented by the presence of the Teotonio and Jirau falls, were the best predictors of the fish fauna assemblage composition. Jirau and Santo Antonio were the first dams constructed with Kaplan horizontal bulb turbines in the Amazon. These bulb turbines have the ability to operate by forming small reservoirs known as run-of-river, which should minimize impacts on fish assemblages. Overall, there were major short-term impacts in shore fish assemblages by increasing species richness, abundance and biomass in experimental catches. Shore fish assemblages dissimilarities before and after the dams’ closure were also noticed and were related to an increase in water temperature and dissolved oxygen. However, the historical factor represented by Teotonio Fall is still the best predictor of fish assemblage dissimilarities, combined with effects of dams’ closure. Bottom channel fish assemblage dissimilarities can be explained only by dams closure and are related especially to changes in fish species abundance inside the reservoirs. Our study revealed an increase in native opportunistic fish species and changes in fish assemblage structure at local scale. Kaplan horizontal bulb turbines employed in run-of-river dams seem to be less deleterious than vertical axis turbines typically used in accumulation reservoirs, and should be preferred in the face of burgeoning new hydroelectricity development plans for rivers across the Amazon basin.