Estimating abundance without recaptures of marked pallid sturgeon in the Mississippi River

Abstract

Abundance estimates are essential for assessing the viability of populations and the risks posed by alternative management actions. An effort to estimate abundance via a repeated mark-recapture experiment may fail to recapture marked individuals. We devised a method for obtaining lower bounds on abundance in the absence of recaptures for both panmictic and spatially structured populations. The method assumes few enough recaptures were expected to be missed by random chance. The upper Bayesian credible limit on expected recaptures allows probabilistic statements about the minimum number of individuals present in the population. We applied this method to data from a 12-year survey of pallid sturgeon (Scaphirhynchus albus) in the lower and middle Mississippi River (U.S.A.). None of the 241 individuals marked was recaptured in the survey. After accounting for survival and movement, our model-averaged estimate of the total abundance of pallid sturgeon ≥3 years old in the study area had a 1%, 5%, or 25% chance of being <4,600, 7,000, or 15,000, respectively. When we assumed fish were distributed in proportion to survey catch per unit effort, the farthest downstream reach in the survey hosted at least 4.5-15 fish per river kilometer (rkm), whereas the remainder of the reaches in the lower and middle Mississippi River hosted at least 2.6-8.5 fish/rkm for all model variations examined. The lower Mississippi River had an average density of pallid sturgeon ≥3 years old of at least 3.0-9.8 fish/rkm. The choice of Bayesian prior was the largest source of uncertainty we considered but did not alter the order of magnitude of lower bounds. Nil-recapture estimates of abundance are highly uncertain and require careful communication but can deliver insights from experiments that might otherwise be considered a failure.

Entrainment of endangered sturgeon by a large water diversion: Rescue, enumeration, and conservation opportunities

“…This suggests that the number captured in the spillway after closure is an underestimate of the total number entrained. A conservative estimate of Pallid Sturgeon age 3+ population size in the 1,931 km reach of the Mississippi River below the confluence of the Missouri River ranged from 4.5 to 15 fish per river kilometer or a total of 4,600–15,000 (Friedenberg, Hoover, Boysen, & Killgore, 2017). Hintz et al (2016) estimated the population size of Pallid and Shovelnose Sturgeon in the Middle Mississippi River, a 322‐km reach between the confluences of the Missouri and Ohio Rivers, at 1,516 (5 individuals/rkm) and 82,336 (266 individuals/rkm), respectively.…”

Entrainment of Endangered Sturgeon by a Large Water Diversion: Rescue, Enumeration, and Conservation Opportunities

“…This suggests that the number captured in the spillway after closure is an underestimate of the total number entrained. A conservative estimate of Pallid Sturgeon age 3+ population size in the 1,931-km reach of the Mississippi River below the confluence of the Missouri River ranged from 4.5-15 fish per river kilometer or a total of 4,600 to 15,000 (Friedenberg et al 2017). Hintz et al (2016) estimated population size of Pallid and Shovelnose Sturgeon in the Middle Mississippi River, a 322-km reach between the confluences of the Missouri and Ohio Rivers, at 1,516 (5 individuals/rkm) and 82,336 (266 individuals/rkm), respectively.…”