Testing the validity of Cating's (1953) method for age determination of American shad using scales

Cating's method of using scales to age American shad (Alosa sapidissima) has been the standard for more than 50 years. However, the only validation of this method is for ages 4–6 in the Connecticut River. To test the method for these—and older—age classes in another river, we obtained scales from 52 known‐age fish from two Pennsylvania rivers and had 13 experienced biologists estimate ages using Cating's method. Each biologist read the scale impressions twice, and these readings were then assessed in terms of precision, accuracy, and bias. Percent agreement between estimates for the same scale set (precision) ranged from 50.0 to 76.5%. Percent agreement between estimated age and known age (accuracy) was highest for ages 3–6 (33.7–48.5%), markedly lower for age‐7 (12.1%), and lowest for age‐8 fish (3.9%). Ages of the youngest fish were often overestimated, and those of the oldest fish were typically underestimated (bias). Therefore, Cating's method is not applicable to American shad in these Pennsylvania rivers. In fact, this scale‐ageing method has never been validated across all ages for any American shad stock. Thus, we recommend against using age‐based techniques to assess stocks of American shad until further age‐validation studies have been completed.