There's an inherent cost to foraging, so a high-quality food item that requires little effort is a much greater net energy benefit. When we're talking about an endangered species whose margins are quite slim to begin with, this can be a big difference maker. A couple dead trout reduces the population size, increases inbreeding depression, things aren't looking so good. I certainly oversimplified the mechanisms here, but a change in 60% of an organism's diet is not easily dismissed.