Large dams vs large otters

8 October 2014



Researchers have been studying the effects of dam construction on the giant otter population at the Balbina hydroelectric lake in Central Brazilian Amazonia. Report by Ana Filipa Palmeirim, Carlos A. Peres and Fernando C. W. Rosas


In our study entitled "Giant otter population responses to habitat expansion and degradation induced by a mega hydroelectric dam", we were looking at the impacts of mega dams on this apex predator. For this, we assessed the rates of giant otter population changes ~25 years after dam construction. Our research was carried out within and around the 443,772-hectare Balbina hydroelectric lake of central Brazilian Amazonia between 2001 and 2012, corresponding to 14 and 25 years after dam construction.

Giant otters are top predators that are thought to be sensitive to habitat modification and they had not been previously seen using other novel habitat created by vast hydroelectric impoundments. This further encouraged the beginning of our research in Balbina reservoir and its undisturbed surroundings.

In this research, we hypothesised that giant otter population growth should scale to rates of habitat expansion induced by dam construction, if additive habitat area were of similar habitat quality. Such scaling patterns should be expected unless population densities are limited by trophic or denning resources in the newly altered area.

The Balbina dam created ~3,525 islands and increased the open-water surface and total reservoir perimeter available to otters by a factor of 62.7 and 8.9 respectively. Some 25 years after damming, however, the estimated post-filling giant otter population size was only twice greater than that estimated before filling, and 4.5 times smaller than would be predicted given the total available habitat area and density of dens quantified at a neighbouring undisturbed area used as a surrogate of the pre-filling phase. The observed mismatch between the proportional increase in otter population size and the much greater newly available habitat area was not a surprise for us. It is likely due to low habitat quality in terms of low fish prey productivity and scarcity of suitable sites for denning and territory demarcation.

The transition from background conditions of a fast-flowing river into an artificial lake leads to major shifts in fish species composition and abundance. In the aftermath of dam closure the initial community-wide fish abundance tends to increase. However, within a few years after the rise of the floodwater, fish diversity begins to decline - as has been observed in all Amazonian hydroelectric reservoirs studied (eg Samuel, Tucuruí and Curuá-Una).

At Balbina such decrease coincided with a generalised decline in fish biomass, most likely due to the extinction of the migratory fish as well as a sharp decline in predatory fish abundance.

Currently, fish biomass at Balbina reservoir is thought to be stable but lower than levels observed prior to damming, thereby representing a further decline in prey biomass density available to a large apex predator such as giant otters, which harvests some 10% of its body mass in fish biomass every day. Additionally, suitable sites for giant otters' denning and territory demarcation, particularly located on steep slopes, appear to be highly heterogeneously distributed along the total islands and mainland perimeter of the reservoir. Therefore, finding the optimal spatial mosaic, including both productive foraging habitat and favourable denning terrestrial habitat, might be a challenge for breeding packs of giant otters at Balbina, further constraining the reservoir population density.

Caution

Such findings should be considered in strategic environmental assessments of planned hydroelectric dams and in managing existing and future development in lowland tropical forest. As giant otters are top predators it is necessary to consider lower trophic levels in the food web. Therefore, after damming, not only otter populations, but also fish communities should be monitored and the habitat managed to ensure otter habitat requirements. This may include building adequate fish passages so that migratory fish can persist upstream. Moreover, we urge caution when designing new hydropower projects and in deciding when, where and how to develop new hydroelectric dams. Strategic-planning and environmental impact assessments of new proposed dams that have yet to be sanctioned should also take into account the cascading effects of dams on both terrestrial and aquatic food webs.

This research shows that although giant otters can persist in the Balbina hydroelectric lake, they occur at a much lower density (4.5 times) when compared with the pre-filling scenario, considering the island perimeters available to otters after damming the river. Bearing in mind that giant otters have already vanished from a major part of its original geographic range and are currently classified as endangered by the International Union for Conservation of Nature, future hydropower developments should take into account the impacts of dam construction on otter habitat degradation in order to maintain necessary habitat requirements needed to sustain viable populations of this species. Furthermore, we urge caution in extrapolating the impacts of the Balbina Dam to giant otter populations at other planned hydroelectric reservoirs under different landscape scenarios.

About the authors
Ana Filipa Palmeirim, PhD student, Laboratório de Vertebrados, Departamento de Ecologia, Universidade Federal do Rio de Janeiro, CP 68020, Rio de Janeiro, RJ CEP 21941-590, Brazil, email: [email protected]

Carlos A. Peres, Professor, School of Environmental Sciences, University of East Anglia, Norwich, NR4 7TJ, UK.

Fernando C. W. Rosas, Professor, Instituto National de Pesquisas da Amazônia, caixa postal 2223, Manaus, AM, Brazil.

The original paper entitled Giant otter population responses to habitat expansion and degradation induced by a mega hydroelectric dam by Ana Filipa Palmeirim, Carlos A Peres and Fernando C. W. Rosas, was published in Biological Conservation. Volume 174 June 2014. PP30-38.

Otters Giant otters at Balbina Lake


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