In short the age of the fish can be determined by which fin (or no fin) has been clipped. Before stalking F&G Clips the fins of each year class stalked.
EX. This year (2006) every fish stalked in Winni will have adipose fin clipped so if in 2 years you catch a Salmon with an adipose fin clip you can examine the fish and determine the age based on weight and length using the chart...