A rough estimate (according to Art Lingren) is the length minus 20 rule. I recall reading this in the Westcoast Fly Fisher under Lingren's chapter on flyfishing for steelhead. Anyway, take the length in inches and subtract 20 and that should give you a rough estimate in weight for the "average" girth steelhead. So - 37 inches minus 20 gives you 17 pounds.
As an example, I landed a 35 inch hatchery buck last year and it weighed exactly 15 pounds. Also - 35 minus 20 = 15. Sounds pretty close to me...
Mind you - any steelhead is a beautiful fish no matter what the weight happens to be. I'm just as happy with a 5 - 7 pound fish as a 15 - 20 pound fish! So - a 37 inch fish is a beauty no matter how you look at it! Nice job!
