I used to do spawner surveys in a couple creeks. We'd do 100% of the anadromous zone weekly from September through June, counting salmon and steelhead. We had the advantage of knowing how many fish were in the creeks (2-4m wide) because we counted them above a rack. We saw less than 10% of the steelhead and 20-30% of the coho on the really clear days. They were there, just hidden.