Low levels of straying are crucial, since the process provides a source of novel genes and a mechanism by which a location can be repopulated should the fish there disappear.
Yet high rates of straying can be problematic because misdirected fish may interbreed with the existing stock to such a degree that any local adaptations that are present become diluted.
Straying rates remain relatively low when environmental conditions are stable, but can increase dramatically when streams suffer severe disturbance.

E.
the extinction of the salmon populations that spawn in polluted streams is accelerated

请问答案确实是d吗？为什么？b哪里错了?

什么叫should the stream recover????