Mercury concentration is due to fish eating crustaceons and then eating each other. The smaller fish (stocked trout) primarily feed on insects, so there is much less problem with them.
The mercury in Cottage Grove Lake primarily came from discontinued mining practices of the late 1800s. Sadly, it just doesn't go away. Currently the largest source of mercury in Oregon is the Boardman coal-fired electric generator.
Symptoms of mercury poisoning usually begin with numbness and tingling in the fingers and toes, irritability, tremors. High-level, long-term exposure can lead to memory loss, vision loss, hearing loss, psychosis, birth defects, and death. All forms of mercury affect the central nervous system. High levels of elemental, inorganic, or organic mercury can permanently damage the brain, kidneys, and developing fetuses. Organic forms of mercury, which are found in the edible parts of fish, are the most toxic.
Follow the advisory and you'll be fine. Catch & release if you're more concerned. The fish survive just fine with mercury, but the accumulation & concentration is a problem once mammals & birds eat the fish.