Effects of habitat fragmentation by damming on the persistence of stream-dwelling charr populations

Dam construction has serious consequences for aquatic ecosystems, and one of the most serious is the barrier effect, the prevention of organism migration throughout a system. We assessed the effect of habitat fragmentation by damming on the population persistence of a stream-dwelling fish, the white-spotted charr (Salvelinus leucomaenis), in streams of southwestern Hokkaido, Japan. We sampled for charr at 52 dammed-off sites by electrofishing or snorkeling and measured five habitat characteristics: isolation period, watershed area, gradient, elevation, distance from sea. Of the 52 study sites above dams, white-spotted charr were absent at 17 sites and were present at 35 sites. Because the charr occupied all undammed upstream reaches, the damming would cause the absence of charr upstream. Among five habitat characteristics examined, stepwise logistic-regression analysis showed that disappearance was promoted with increasing isolation period, with decreasing watershed area (i.e., habitat size), and with decreasing gradient. The resulting logistic model explained 82.7% of the present white-spotted charr occurrence and forecasted that 12 of 35 extant populations will disappear after 50 years. Our findings imply that extirpation of small, dammed-off populations is inevitable unless efficient fish ladders are installed or dams are removed.