Estimating Fish Abundance in a Large Reservoir by Mark–Recapture

Abstract

We conducted a large-scale mark–recapture study to estimate the population size of blueback herring Alosa aestivalis in the J. Strom Thurmond Reservoir, a 28,329-ha reservoir on the Savannah River. The reservoir was divided into four strata, and 100,619 blueback herring (140 ± 5.0 mm TL; mean ± SD) were tagged and released between April 26 and May 24, 1996. Blueback herring were tagged with 1-mm sequentially coded wire tags injected into the nasal cartilage. In a controlled field study conducted concurrent to tagging, we estimated a combined survival and tag retention rate of 90% and used this estimate to adjust the number of marked fish. In all, 155 tags were recovered from 144,227 fish examined from May 29 to August 14. Chi-square analysis indicated that recaptured fish from the lower stratum of the reservoir were not represented in proportion to tagging effort. A stratified approach was taken in an effort to eliminate bias due to incomplete mixing. When we disregarded fish marked in the lower three strata, we estimated the population size of blueback herring in the upper area of Thurmond Reservoir as 58,268,586 (95% confidence interval, CI = 48,550,118–69,932,033). Using a stratified population estimation procedure, we estimated the population size of adult blueback herring in Thurmond Reservoir to be 89,393,566 (95% CI = 74,483,842–107,287,206). This study demonstrates that coded wire tag technology allows the marking and examination of large enough numbers of fish to develop mark–recapture population estimates in large bodies of water.