There is some interesting ongoing work in AK that shows salmon getting smaller with age. Some appears to be due to either being outcompeted by other species (Pinks) or switching to less nutritious food.

The second, less nutritious food, has a real hidden pitfall. In order to survive in warmer water, salmon need to burn more calories-being poikilothermic. With less energy per bite, a full stomach supports smaller size because a large fish does not have the energy for metabolism.

So, if our waters are warming, the salmon need more nutritious food to maintain (or recover-see Elwha and Kenai) "lost" size. Which means if we want more and larger salmon we need to leave more of the lower levels of the food chain in the ocean.