Validation of an otolith ageing method for American shad Alosa sapidissima

The accuracy of scale-based age information for American shad Alosa sapidissima has come into question over the last decade.  This study explored the use of otoliths as an ageing structure for American shad, using otoliths from known- aged fish from the Delaware River.  Fifty-four whole otoliths were placed in mineral oil and viewed with a dissecting microscope under reflected light.  Alternating hyaline and opaque bands were considered a full year, and the hyaline bands were counted to determine the age.  Because the shad were recaptured on their spawning grounds, the edge of the otoliths was considered the final annulus. The reading and interpretation of annuli was highly accurate with respect to the known age.  The percent agreement (PA) was 79.2, and Chang’s (1982) coefficient of variation (CV) was 2.72.  The same otoliths were read a second time and a precision test was conducted; (PA = 76.5; CV = 3.15).  The use of otoliths was also more accurate than scales obtained from the same fish (PA = 52.0; CV = 6.93).  Because these otoliths are from known-aged fish, these results validate the otolith-based ageing method for American shad from the Delaware River for ages 4, 5, 6, 7, and 9.