Comparing abundance estimates from closed population mark-recapture models of endangered adult Atlantic sturgeon

Atlantic sturgeon Acipenser oxyrinchus oxyrinchus were listed as 5 distinct population segments under the US Endangered Species Act in 2012. At that time, only 2 abundance estimates of the Atlantic sturgeon population were available: one from commercial fisheries landings in the Hudson River ending in 1995 and one from mark-recapture research in the Altamaha River, Georgia, in 2004 and 2005. In 2013, we verified spawning in the York River, Virginia, system and initiated a multiple-year mark-recapture study focusing on spawning-run abundance. We used a Schumacher-Eschmeyer model and Program CAPTURE to produce estimates of annual spawning abundances from 2013 to 2018. The Schumacher-Eschmeyer estimates of spawning-run abundance with 95% confidence intervals from 2013 to 2018 were 75 (31-190), 157 (115-244), 184 (150-238), 222 (137-576), 212 (157-328), and 145 (89-381), respectively. Because Atlantic sturgeon do not spawn every year, the trends in estimates do not suggest a recovering or declining population, but rather variability in proportions of the adult population that return to spawn each year. The estimates produced in Program CAPTURE using M0 (null), Mt (Chao Mt and Darroch), Mh (Chao Mh and Jackknife), and Mth (Chao Mth) models all produced similarly reliable estimates. The models that consider a behavioral response to initial capture (Mb, Mbh, and Mtb) failed to produce reliable estimates for this data, likely because as an endangered species, the dataset for Atlantic sturgeon was sparse. The Jackknife equation (model Mh) was the most precise every year with reliable accuracy and therefore is recommended