Looks like a
saprolegnia infection to me, or possibly
furunculosis, so fingal and bacterial infections respectively. Although it's a good pic, I can't make my mind up which is the more likely fairly..... possibly
furunculosis.
Both are common infections in salmonids (and indeed most cold-water species) at ths time of the year as the water warms up.
However, when was the photo taken and where