The strains of hatchery stocks, usually. They're typically different builds. Chambers Creek stock as we see them now, typically lack any semblance of shoulders as most wild fish have and some of the other hatchery strains.

In terms of two salt vs. three? Not really ... obviously, 99% of the really small ones will be twos, 99% of the big ones will be threes. The above average fish can often be smaller threes or larger two's.

You can tell by reading the annuli on the scales. That's why WDFW checkers will ask for scale samples from your fish.

You can get number of freshwater years and salt years and any spawning runs in the mix by reading them ... much like looking at the rings on a tree

Remember that there are other influences too, and we may actually be seeing some of it now. Some west coats streams that had a lot of warm water off the coast the last go round of El Nino saw diminished returns that year, and then higher returns of three the next year ... almost seemed like the fish wouldn't push through it and said the heck with it!