Three things.
1) Remember that your data is only of the fish that you can catch. There will be fish that you may not be able to catch. That's where periodic electroshock surveys help.
2) When weighing and measuring fish, have all the stuff ready before the fish comes out of the water, and make sure that you take all the precautions necessary to minimize handling of the fish to minimize the stress/damage to the fish. I see people using a hook in the gill area to weigh the fish. not good.... for the smaller fish, here's a thought. Look at getting a bag with handles, fill it with a quart or 2 of water, hang the bag, zero the scale. Put the fish in the bag and you can weigh the fish. Get a bump board, wet it and then measure the fish. If you can get the fish back into the water within 30 seconds, great! but shoot for less than a minute out of water.
3) I'd initially cull any fish that weigh under 80% RW BUT not take the RW until a month or so post spawn for that species. The more fish you get a RW on the more accurate your data will be.
There are two ways to check the age of a fish. One way allows the fish to live another day, the other requires killing the fish and using a microscope. You can age fish by looking at a single scale and reading the rings like reading the rings on a tree stump. Ditto for reading the rings in the otolith of the fish, but you have to kill the fish to do that, crack/slice the otolith, then look at it under a microscope to count the rings.
