The lesser of two evils: seasonal migrations of Amazonian manatees in the Western Amazon

Abstract

We investigated the paradox of why Amazonian manatees Trichechus inunguis undergo seasonal migrations to a habitat where they apparently fast. Ten males were tracked using VHF telemetry between 1994 and 2006 in the Mamirau´a and Amana˜Sustainable Development Reserves, constituting the only long-term dataset on Amazonian manatee movements in the wild. Their habitat was characterized by analysing aquatic space and macrophyte coverage dynamics associated with the annual flood-pulse cycle of the River Solimo˜es. Habitat information came from fieldwork, two hydrographs, a three-dimensional model of the water bodies and classifications of Landsat-TM/ETM + images. We show that during high-water season (mid-May to end-June), males stay in várzea lakes in association with macrophytes, which they select. We then show that, during lowwater (October-November), the drastic reduction in aquatic space in the várzea leads to the risk of their habitat drying out and increases the manatees' vulnerability to predators such as caimans, jaguars and humans. This explains why males migrate to Ria Amana˜. Based on data on illegal hunting, we argue that this habitat variability influences females to migrate too. We then use published knowledge of the environment's dynamics to argue that when water levels are high, the habitats that can support the largest manatee populations are the várzeas of white-water rivers, and we conjecture that rias are the species' main low-water refuges throughout Western Amazonia. Finally, we warn that the species may be at greater risk than previously thought, because migration and low-water levels make manatees particularly vulnerable to hunters. Moreover, because the flooding regime of Amazonian rivers is strongly related to large-scale climatic phenomena, there might be a perilous connection between climate change and the future prospects for the species. Our experience reveals that the success of research and conservation of wild Amazonian manatees depends on close working relationships with local inhabitants.

Wetlands of the Lowland Amazon Basin: Extent, Vegetative Cover, and Dual-season Inundated Area as Mapped with JERS-1 Synthetic Aperture Radar

“…Management-related applications of the data set include those relating the abundance of fish or aquatic mammals to floodplain habitats (Arraut et al 2010;Lobón-Cerviá et al 2015), or reviewing the extent and conservation status of Amazonian wetlands (Castello et al 2013;Junk 2013).…”

What do Local People Know About Amazonian Manatees? Traditional Ecological Knowledge of Trichechus inunguis in the Oil Province of Urucu, AM, Brazil

“…The local people also use those areas to fish, and therefore the probability of encounters between manatees and humans is higher. During the high water season manatees forage is more abundant in the lakes (Arraut et al 2009), and feeding tracks of manatees are frequently observed, especially in the marginal communities of grasses. Manatee hunters explained that they use the evidence of feeding to detect and poach the manatees.…”

“…The ecological processes that drive migration often seem to be related to habitat variability. A telemetry study showed that Amazonian manatees are subject to challenging habitat conditions during part of the year, and that they will migrate to areas that offers them the most suitable conditions under the difficult circumstances (Arraut et al 2009). …”

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

“…This would be advantageous because it would allow them to forage for longer within H h prior to withstanding the scarce-forage low-water season (Arraut et al 2010;Guterres-Pazin et al 2014). The risk of reaching the refuge with low fat reserves is exemplified by manatees in Lake Amanã having been reported as dying in distinctly emaciated condition (thin and lacking internal and external fat reserves) at the end of the low-water season (Best 1983).…”

“…The migration of this exclusively aquatic mammal seems to be a behavioral adaptation to the seasonal changes of the aquatic environment that are brought about by the floodpulse (Arraut et al 2010 and Figure 2). As explained in Arraut et al (2010), during the high-water season, when rivers and their floodplains form an interconnected water body, manatees remain within floodplain lakes, where water current is weak and aquatic macrophytes, their main forage, abound. In the low-water season, when most floodplain lakes become shallow and isolated and macrophytes die or go through a terrestrial phase, manatees aggregate in Lake Amanã.…”