Practice catch and release with all fish. Take a photo of the fish. Record a measurement of the fish's length and girth. Keep the fish in the water as much as possible during measurement. Be sure to take a few extra minutes to revive and release all fish after measurement. Use this calculator for figuring the weight of your fish.
The algorithm used is W=(L * (G^2))/R, where:
W = Weight in decimal pounds L = Length in inches G = Girth in inches (default is .58 * L) R = 900 for steelhead, 800 for Salmon