Tests on the Accuracy of Ageing Chinook Salmon (Oncorhynchus tshawytscha) from their Scales

The accuracy with which several readers determined the ages of adult chinook salmon from their scales averaged 75% in a test that used scales from fish of known ages. Their consistency in assigning the same age to a scale when presented with the same selection of scales on two occasions averaged 76%. In these tests the readers derived age composition arrays which were similar to the true age compositions of the two samples. When several pairs of readers assigned ages to a large numbers of scales from fish of unknown ages agreement within pairs was good (roughly 75%), and among the pairs the derived age compositions were very similar. The levels of both accuracy and consistency achieved in these tests indicate that experienced personnel, using good scale-reading equipment, are able to produce age composition data that could be used as reliable parameters in chinook salmon population analyses.