There are a ton of different ways to measure trout. You can weigh them in a net, measure the length, or do the length/girth calculation to get an estimated weight. Now it seems that the Nørre company has come up with another method with a product called the Tail Scale. I've yet to see the device, but I'll say that if measuring fish is important to you, this might be the best way to do it, as it keeps them in the water. The company states on its kickstarter page: "The Tail Scale is a measuring tool that estimates the fish length and weight based on the width of the tail fin...Fish appendages, like the tail, dorsal, and pectoral fins, grow in remarkable proportion to the fish body size. This growth pattern has been researched and documented by various studies over the years. After confirming the research with our own testing, we found a rate that would be compatible for our tool."