The response of otters to environmental changes imposed by the construction of large dams

The number of dams is progressively increasing, but nonetheless there are few examples of long-term studies involving otters and these structures.This study assessed how Eurasian otter Lutra lutra responded over time to environmental changes imposed by the construction of a large reservoir, the Alqueva Dam (250 km2, SE Portugal).Otter distribution was monitored over four phases (pre-deforestation/flooding, deforestation, flooding, and post-flooding) from 2000 to 2006 in both the flooded and surrounding areas. During each phase, presence was assessed at two resolutions: 25 km2 (76 survey sites in wet and dry seasons) and 1 km2 (39 survey sites, every three months). In addition, otter spraints were collected at eight survey sites in 2000 (pre-dam) and 2006 (post-dam) to assess and compare otter diet with prey availability.Otters were widespread before dam construction (>82.4% positive cells), decreased during deforestation (68.5%), and particularly during the flooding phase (33.3%), recovered during the post-flooding phase, although not at the level recorded before dam construction (61.5–83.3%). Otter diet was dominated by fish and American crayfish Procambarus clarkii during the pre-deforestation/flooding phase (56.7% fish, 35.3% crayfish) and at the end of the post-flooding phase (60.7% fish, 33.2% crayfish). Species richness of fish prey decreased with flooding (16 to 8), as did the ratio of native/non-native fish species (1.7 to 0.3).Changes in the availability of the main ecological requirements of otter were similar to the changes in the observed otter distribution in the flooded area throughout the impact phases, and decreased during deforestation and flooding with some recovery in the post-flooding phase.The results emphasize the importance of long-term monitoring studies that include several post-impact phases, to evaluate species response to impacts. This will allow better planning of mitigation and compensation measures