How do you know it will have a negative impact? Honest question. Seems like you're making assumptions. Here's some more from the linked-to-proposed changes, that provides evidence that there will not be a negative impact.
"To look at the populations I used a student’s t-test to compare the number of wild STW crossing Winchester Dam while there was a 1/5 harvest (2000/01 – 2006/07) to the number of wild STW when the harvest was closed 2007/08 – 2010/11). There was no significant difference in the number of wild STW crossing the dam (P = 0.629, SD 2138.45 during harvest years, SD 996.55 non-harvest years). During 2004/05 through 2006/07 the Main stem was open to harvest in addition to the harvest on the North. With an ANOVA, the years when both the Main and North were open to harvest, had the highest significance level (P = .084). When both rivers were open, the Winchester Dam counts did stay above intrinsic productivity needs (1,664 to 5,757 returning adults) noted in the 2004 BA of Impacts to Wild Winter Steelhead from the Recreational Fishery report. When the Main was open to a 1 and 5 harvest, from 49% to 66% of the wild STW harvested were harvested from the Mainstem. Wild STW counts across Winchester Dam from 1990 – 1998 were relatively low when both the Main and the North were open to a 2 per day, 20 per year wild STW harvest. Under this regime there were years when less 5,757 wild adults returned. However, during the last 10 years the average wild counts have been 8,723."
From 2000 to 2007 there were an average of 8,504 wild fish across the dam each year, when harvest was allowed.
From 2007 to 2011 there were an average of 8,979 wild fish across the dam each year, when harvest was not allowed.
Lean up on some population dynamics Brandon. It's not so simple that 8,000 wild fish - 500 fish = 7,500 returning down the road. Populations have different growth rates under different densities.
