This is about protecting a natural food source or opening up land for mining resources. It's one resource vs. another. Gold and copper can be found elsewhere. Salmon are not. It would harm pristine habitat and release dangerous chemicals into the fragile ecosystem. Also, on a side note, remember that heavy metals remain in fish bodies and render them inedible. The two resource operations cannot coexist. Here's a great article from the Washington Post that explains it all in pretty clear terms.
http://www.washingtonpost.com/national/health-science/bristol-bay-mining...