Is Cating's Method of Transverse Groove Counts to Annuli Applicable for all Stocks of American Shad?

Abstract

A scale aging method was reported by Cating in 1953 for American shad Alosa sapidissima in the Hudson River and subsequently validated by recapturing fish marked and released in the Connecticut River. However, American shad spawn in all major rivers from Canada to Florida and their scales record growth events occurring in three distinct biogeographic provinces. Thus, a single scale aging method may not be applicable across the latitudinal range of this species. To address this concern, scales from American shad from one southern river (the St. Johns), three Middle Atlantic rivers (the Delaware, Hudson, and Connecticut), and one northern river (the Merrimack) were examined. Scales were cleaned, impressed in acetate, and analyzed by the same reader using a digital imaging system. The transverse grooves, the key morphological character used in Cating's method, were counted to the distal edge of the freshwater zone and the first three annuli. In most instances, these groove frequencies were statistically different from Cating's data for the Hudson River. Moreover, our data showed enough overlap in groove frequencies that they cannot be relied on as diagnostic characters for the freshwater zone and first three annuli in fish with difficult-to-interpret scales. Scale size explained more of the variance in groove frequencies than fish age did. Regardless of the specific process creating transverse grooves, we provide evidence that Cating's method should not be used to age American shad.

ACKNOWLEDGMENTS

We thank the following state agencies for their assistance in collecting samples: the Pennsylvania Fish and Boat Commission; Massachusetts Division of Marine Fisheries; New Hampshire's Fish and Wildlife; Florida's Fish and Wildlife Conservation Commission; and the New York State Department of Environmental Conservation Bureau of Marine Resource. Funding was provided by NOAA's Advanced Study Program. We would like to thank anonymous reviewers with prior experience aging American shad, who took the time to look at scales from multiple rivers to ensure that annuli and transverse grooves were consistently identified between rivers. We would also like to thank L. Natanson, R. H. Castro, and two anonymous reviewers whose suggestions were helpful in strengthening this manuscript. Reference to trade names does not imply endorsement by the U.S. Government