Comparing scale and otolith age estimates to understand the potential bias of long-term American shad age data in the Penobscot River

Wilson, Camille, Environmental Sciences, University of Virginia
Castorani, Max, AS-Environmental Sciences, University of Virginia

American shad (Alosa sapidissima) are indigenous to the Atlantic Ocean, but despite their long history in this range, they currently represent only a fraction of their historical abundance. Since they are an ecologically and commercially important species that has experienced an extensive decline, it is important to understand the demographics of the populations to best manage stocks and promote sustainable harvest. One key aspect of demography is population age structure. Fish age can be estimated by reading the annuli found on scales or otoliths. For roughly a decade, scales have been collected from shad within the Penobscot River to estimate the age structure of this population over time. While studies have found that otoliths provide a more accurate estimate of shad age, it is impossible to retrieve otoliths while keeping the fish alive, so this study has relied on scales alone for past age estimates. In 2020 and 2021, 185 dead shad were retrieved from the Milford Dam fish lift for this study, and both scales and otoliths were taken from each individual fish. For each fish, I compared the otolith age estimate to the scale age estimate, as well as the precision of age determinations among readers for each sample. I concluded the otoliths were the easiest to read with the most precision among both readers, and the scales will underestimate the age of older fish and overestimate the age of younger fish. This information on the exact extent of these differences might be useful in understanding the bias of previously recorded scale data in order to get a more accurate depiction of the age structure of American shad in the Penobscot river over time.

BS (Bachelor of Science)
American shad, otolith, scale, aging
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