Bottlenecks in the migration routes of Amazonian manatees and the threat of hydroelectric dams

Abstract

In a particular region within western Amazonia, Amazonian manatees (Trichechus inunguis) live in a floodplain environment that becomes inhospitable for them during the annual low-water season. To flee from it, they undergo a dangerous migration to a refuge while water levels are dropping fast. Our aim was to better understand the role of depth variation in this migratory process. We analyzed the sole tracking data on wild manatees (n=10 males), 30 years of Landsat images, a 14-year hydrograph and a 3-D bathymetric model. Migratory routes contained shallower segments, here called 'migratory bottlenecks', which dried out at the end of most lowering-water seasons, blocking the passage to the refuge. Manatees began migrating just in time to traverse the bottlenecks furthest away, suggesting they fine-tuned their departure so as to maximize time within the foraging home range without compromising safety. They apparently achieved this by estimating depth at the bottlenecks. Moreover, a bottleneck was created in >15 years, illustrating the environment's dynamism and the challenge this imposes upon manatees. Our results are probably generalizable to most of the species' range. We contend manatees possess an updatable cognitive map of their environment and are behaviorally plastic. Current dam-building plans, if implemented, would create more bottlenecks and make flooding less predictable, increasing manatee mortality from unsuccessful migrations. It would also partition the species into small populations, each prone to short-term extinction. The natural outcome would be the second species-level collapse. Economic growth should not come at the expense of the extinction of the iconic manatee. KEYWORDS: Sirenia, behavioral plasticity, Mamirauá, Amazon development Gargalos na rota migratória do peixe-boi amazônico e a ameaça das barragens hidrelétricas RESUMO Em uma região particular da Amazônia ocidental, peixes-boi amazônicos (Trichechus inunguis) vivem em um ambiente que se torna inóspito para eles durante a água-baixa anual. Para fugir dele, realizam uma migração perigosa para o refúgio enquanto o nível da água desce rapidamente. Nosso objetivo foi compreender melhor o papel da variação da profundidade neste processo migratório. Analisamos os únicos dados de rastreamento de peixes-boi selvagens (n=10 machos), 30 anos de imagens Landsat, 14 anos de hidrógrafa e um modelo batimétrico 3-D. As rotas migratórias possuíam trechos mais rasos, denominados gargalos migratórios, que secaram no final da maioria das vazantes, bloqueando o acesso ao refúgio. Os peixes-boi começaram a migração em tempo justo para atravessar os gargalos mais distantes, sugerindo que a sintonizaram para maximizar o período se alimentando sem comprometer a segurança. Para tal, parecem ter estimado a profundidade nos gargalos. Adicionalmente, um gargalo foi criado em <15 anos, ilustrando o dinamismo do ambiente e o desafio que isto impõe aos peixes-boi. Esses resultados provavelmente valem para boa parte da área de distribuição da espécie. Argume...

In‐Water Bridge Construction Effects on Manatees with Implications for Marine Megafauna Species

“…Many potential effects of bridge construction reviewed here are transferable to other types of in-water construction activities such as land reclamation, energy exploration and production, ship channel construction and maintenance, and development of coastal and inland waterway infrastructure (e.g., docks, ports, dams). For example, dam construction and other types of hydrologic engineering can cause direct mortality of aquatic species and sustained or permanent habitat alteration (Marmontel et al 2016, Arraut et al 2017, He et al 2017. At least 7 mortalities and 10 additional rescues of African manatees have been caused by entrapment in dam control gates or turbines or behind dam structures (Keith-Diagne 2015).…”

“…At least 7 mortalities and 10 additional rescues of African manatees have been caused by entrapment in dam control gates or turbines or behind dam structures (Keith-Diagne 2015). Similarly, extensive dam construction in the Amazon River Basin has increased vulnerability of Amazonian manatees to strand in floodplain areas dependent on seasonal waterflow (Arraut et al 2017). African manatees in Nigeria have been documented to strand in borrow pits that are used to excavate sediment for roadway and other construction projects and then are abandoned following project completion (E. A. Eniang, personal observation).…”

“…African manatees in Nigeria have been documented to strand in borrow pits that are used to excavate sediment for roadway and other construction projects and then are abandoned following project completion (E. A. Eniang, personal observation). Immediate effects result in individual mortalities, but population-level effects are possible if construction projects obstruct migration routes and fragment or destroy habitats (Marmontel et al 2016, Arraut et al 2017, He et al 2017.…”

“…Although immediate direct effects are more easily quantifiable, indirect effects of construction activities that result in long-term habitat degradation or sublethal stressors may be more difficult to detect (Zainal et al 2012, Sundblad andBergström 2014). Cumulative effects through time may result in short-term extinction of isolated populations or total species loss in affected areas (Arraut et al 2017, Avila et al 2018. Effective assessment and mitigation of potential effects must consider in-water construction activities within the complex framework of co-occurring anthropogenic and natural threats and include future predictions for effects of climate change and sea level rise (Marsh et al 2017, Avila et al 2018, Lewison et al 2018.…”

Conservation of Amazonian manatee (Sirenia: Trichechidae): the case of Extractive Reserve Verde para Sempre, Brazil

“…As observed in the literature, the installation of hydroelectric power plants in the Amazon region can make the migration difficult and increase the mortality of the Amazonian manatee (Arraut and Marmontel 2016;Arraut et al 2017). The Belo Monte hydroelectric plant on the Xingu river, the largest in operation in the Amazon region, may have secondary effects, facilitating the mining of bauxite, nickel, copper and gold (Lees et al 2016).…”