Identify the weight of a fish based on fish length If you know the length of your fish and simply want to know the fish weight, the app "Fishing Scale" will identfiy the weight for you without hanging the fish by its jaw to weigh it. That can certainly damage the fish, and doing this also means you are handling the fish far longer out of water, which isn't good if doing catch and release. On the fishing scale app you simply select what style of fish you caught, select the length of the fish in inches, and it will give you a very good estimate on the weight of many popular types of game fish. Talk about fishing