Population viability of stream-resident salmonids after habitat fragmentation: a case study with white-spotted charr (Salvelinus leucomaenis) by an individual based model

Throughout the world, rivers are increasingly fragmented by artificial barriers, such as damming and road crossing. In Japanese mountain streams, most populations of white-spotted charr, a salmonid fish, are highly fragmented by numerous small dams. Most of these dams were constructed after 1970. In this study, we conducted a population viability analysis of fragmented white-spotted charr populations using a simple individual based model. The constructed model is age-structured and density dependent that incorporates both demographic and environmental stochasticity. The probability of population persistence decreased substantially between after 30 years and after 100 years, especially for populations having a small carrying capacity. Even though white-spotted charr occupies many fragmented habitats now, our results suggest small fragmented populations are not viable. A sensitivity analysis showed that adult survival, not juvenile survival and egg number, was the most important criterion for the prospects of population persistence. Installations of efficient fish ladders or dam removal are necessary as soon as possible to restore dammed-off populations.