 The causes of this decline are not fully understood, but alteration and degradation of its freshwater habitat undoubtedly plays a central role.The negative impacts humans have on rivers and streams come from a wide range of activities such as river regulation, drainage, sewage disposal, dredging, and water pollution, including the introduction of excess nutrients.Anything that affects the abundance of the fish hosts will also affect the freshwater pearl mussel; for example, the introduction of exotic fish species, such as the rainbow trout, reduce the number of native fish hosts.