Otolith Age Validation and Growth Estimation from Oxytetracycline-Marked and Recaptured American Shad

Abstract

This study validated a whole-otolith aging method using known-age American shad Alosa sapidissima from the Delaware River system. Although scale ages are commonly used in autecological and assessment studies of American shad, scale ages from the same fish could not be validated. New data reported here used known-aged otoliths and scales available from shad marked by oxytetracycline as larvae in a hatchery and recaptured as adults on or near their spawning ground. A subset of whole otoliths were examined and annuli—defined as a pair of translucent and opaque bands—were counted using males and females ranging in age from 3 to 9 years. The reading and interpretation of annuli by the more experienced reader were accurate with respect to the known age, whether measured as the percent agreement (PA = 80%) or Chang's coefficient of variation (CV = 3.11). The use of otoliths provided more accurate results than scales obtained from the same fish (PA = 46%; CV = 7.84). A second reader, who had no previous experience with this species, had lower performance scores but also performed better with otoliths; this demonstrated the need for training and testing when using the whole-otolith aging method. Growth modeling using ages of known-age juveniles and adults confirmed dimorphic growth. Females grew larger than males (von Bertalanffy L_∞ = 552 and 495-mm FL, respectively). The maximum age observed for females was only slightly older than males (9 versus 8 years). The superiority of the otolith-based age method makes it difficult to compare our results with older, scale-based demographic studies, but it represents an improved method for generating ages for future stock assessments.

Received February 15, 2012; accepted July 31, 2012