A theoretical study of river fragmentation by dams and its effects on white sturgeon populations

Most of the world's large rivers are fragmented by dams. Fragmentation of t he river ecosystem alters migration patterns among fish populations and con verts free-flowing river to reservoir habitat. In this study, we used an in dividual-based genetic metapopulation model to study the effects of fragmen tation on the population viability and genetic diversity of a large-river f ish, the white sturgeon, Acipenser transmontanus. In the first of two simul ation experiments, we fragmented a 200 km river reach by building 1 to 20 v irtual dams. Increased fragmentation produced an exponential decline in the likelihood of persistence, but no extinction threshold to suggest a minimu m viable length of river. Compounding isolation with the loss of free-flowi ng habitat did not further reduce viability until free-flowing habitat was nearly eliminated, at which point extinction was certain. Genetic diversity within (among) populations decreased (increased) as we 'built' the first s everal dams. Adding more dams caused the number of persisting populations t o decline and eroded genetic diversity within and among populations. Our se cond simulation experiment evaluated the effects of different levels of ups tream and downstream migration between river segments. The results of these migration experiments highlighted the importance of balanced migration rat es. We found that extinction risk was high for populations linked by high d ownstream, and low upstream, migration rates, as is often the case in impou nded rivers. Our results support the view that migration patterns will play a significant role in determining the viability of riverine fishes, such a s the white sturgeon, in river ecosystems fragmented by dams.