Mathematically speaking, the creation of the massive Balbina Hydroelectric Dam in Brazil should have been a boon to the giant river otters (Pteronura brasiliensis) that live in the area. The dam, which went online in 1989, created a reservoir more than 443,000 hectares in size. As the land behind the dam flooded, more than 3,500 islands were created from the former hilltops in the region. All of this new water and coastal territory should have established vastly expanded habitats for endangered giant otters and allowed their populations to boom.

But that hypothesis proved to be only half true. According to research published in the June 2014 Biological Conservation, giant otters did get 67 times more potential habitat after the reservoir flooded (habitat that other land-based species lost). And yes, their population did grow—from about 140 individuals to around 280—but that growth does not match the amount of new water habitat that became available. The researchers calculate that the giant otter population should have grown by a factor of nine during this time to closer to 1,250 individuals.

What went wrong? Well for one thing, the waters created by the Balbina Dam were not the same type of waters in which the otters previously lived. Giant otters typically live in fast-moving rivers; Balbina created a vast, slow-moving lake. This difference created a major shift in the fish species living in the lake and available to the otters, which must consume about 10 percent of their body weight in food every day.

These changes in fish abundance and species population occurred in a few different steps. When the reservoir was first flooded it created a brief "nutrient pulse" as more than 160 million trees were drowned and began to decompose. This, in turn, allowed a few species of predatory fishes such as peacock bass and piranha to flourish. Otters love to eat both of those types of fishes, so that was good for a while, but it didn't last.

Within a few years the nutrient pulse had subsided, as had the populations of migratory fishes that once swam through the region's now-nonexistent rivers. As migratory fishes disappeared, the predatory fishes lost their prey and they, too, began to decline—some by as much as 50 percent. This drop has reduced the amount of food available to the otters.

Meanwhile the mostly shallow artificial lake also contains numerous very deep areas away from the shoreline. These areas of the lake often reach depths of more than 15 meters and waters that deep are oxygen-poor and inhospitable to almost all fish species. This means that much of the massive lake is also incapable of supporting otter populations.

As the authors wrote, all of these developments have been particularly damning for giant otters: "the giant otter population size at Balbina is unlikely to be able to adjust proportionally to the landscape-scale habitat even decades into the future." Given the time frame since the region's flooding, the otters may have already approached their maximum population density for the new lake.

The story of the otter and the dam poses a cautionary lesson for the future. The paper points out the fact that 21 hydroelectric dams are currently being built in the electricity-starved Amazon Basin and another 277 are planned. Undoubtedly, some of these projects will have an impact on other giant otter populations, and that could pose quite a risk for a species that has already lost 80 percent of its range. The authors wrote that future dams should be monitored so populations of both otters and fishes can be properly managed, perhaps by building passages that would allow migratory fishes to pass through the dams so they could continue swimming upstream. This approach would benefit not just the fishes but also the otters that need them.

Main photo: Giant otter in Brazil by Alastair Rae. Balbina Dam photo by rmaranhao. Giant otter with fish photographed at Chester zoo by Tambako the Jaguar. All via Flickr and used under Creative Commons license

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