Humans Often Build Dams on Rivers for Hydroelectric Power or Other

Essay

Humans often build dams on rivers for hydroelectric power or other purposes. Dam-building obstructs fish movement and alters fish populations above and below the dam, sometimes leading to declines in threatened species. One effect is delayed movement of fish to new locations. This increases fish densities in certain locations, resulting in greater competition for space and likelihood of spread of diseases. Fish may be slower in reaching spawning or feeding grounds, and have less energy available for reproduction and for later attempts to reach the new location. What unexpected or unintended ecological consequences would occur in this scenario?