What authority regulates BC Hydro? If they self regulate, then you have no recourse besides their concern for fish, or else the court system.
In the US, non-federal dams are regulated by FERC (Federal Energy Regulatory Commission). Dam owners are licensed by FERC, and fish agencies recommend downramping rate restrictions as license conditions. This seems to work pretty well. Federal dam operations have gotten better, although that might be due to so many Endangered Species Act restrictions.
Downramping mortalities cannot be eliminated, but they can be drastically reduced. We have lots of studies and reports on this in WA if it would be helpful to you.