Estimating the Annual Spawning Run Size and Population Size of the Southern Distinct Population Segment of Green Sturgeon

Abstract

The Southern Distinct Population Segment of Green Sturgeon Acipenser medirostris spawns in the Sacramento River, California, and is listed as a threatened species under the U.S. Endangered Species Act. We estimated the spawning run size and population size in 2010–2015 by using dual‐frequency identification sonar (DIDSON) sampling, underwater video camera species identification, and acoustic tag detections. Spawning run size varied from 336 to 1,236 individuals. We estimated the total population size to be 17,548 individuals (95% confidence interval [CI] = 12,614–22,482). The estimated number of adults was 2,106 (95% CI = 1,246–2,966), the estimated number of juveniles was 4,387 (95% CI = 2,595–6,179), and the estimated number of subadults was 11,055 (95% CI = 6,540–15,571). This study provides the first estimate of Sacramento River Green Sturgeon run size and initiates a time series of abundance that can inform Endangered Species Act recovery processes. Furthermore, these absolute abundance estimates provide a context for evaluating the significance of impacts, such as bycatch in coastal fisheries or entrainment in water diversions, where the number of impacted individuals is known.

Using Side‐Scan Sonar and N‐Mixture Modeling to Estimate Atlantic Sturgeon Spawning Migration Abundance

“…Our methods are novel in that we were able to produce a range of abundance estimates for ATS on each sampling occasion. Previous studies using sonar to estimate sturgeon abundance have focused on enumerating all individuals within the system (Flowers and Hightower 2015;Hughes et al 2018), whereas our methods are meant to estimate only adults within the uppermost reach of the Savannah River expected to be contributing to spawning (sensu Mora et al 2018;Kahn et al 2019). We repeatedly sampled putative spawning grounds on each occasion over the entire fall run to identify key times in which spawner abundance increased.…”

“…Coupling this approach (or modified versions of it) with traditional abundance estimation approaches outside of the spawning season will help to better calibrate the accuracy of SSS counts and N ‐mixture modeling. Conducting sonar‐based approaches in conjunction with a robust telemetry program will also help to validate their accuracy as well as provide important insight into the influence of reproductive periodicity on estimates (Mora et al ). Furthermore, accurate spawner abundance estimates paired with year‐1 abundance estimates will greatly increase our understanding of ATS population dynamics.…”

“…; Mora et al. ) or SSS (Thomas and Haas ; Flowers and Hightower , ; Seesholtz et al. ; Hughes et al.…”

“…As an alternative, hydroacoustic tools (e.g., dual-frequency identification sonar [DIDSON] and side-scan sonar [SSS]) provide a noninvasive alternative for enumerating sturgeon populations. Fisheries researchers typically use split-beam sonar (Auer and Baker 2007;Crossman et al 2011;Mora et al 2018) or SSS (Thomas and Haas 2002;Hightower 2013, 2015;Seesholtz et al 2014;Hughes et al 2018). Hydroacoustic data can be easily recorded in the field and then processed and analyzed in a lab setting, during which sturgeon can be enumerated and measured.…”

Postrelease survival of green sturgeon (Acipenser medirostris) encountered as bycatch in the trawl fishery that targets California halibut (Paralichthys californicus), estimated by using pop-up satellite archival tags

“…Additional studies and modeling are needed to understand the effects of the estimated level of postrelease mortality on the threatened Southern DPS. Population estimates based on adult abundance have recently been developed for the Southern DPS (Mora et al, 2018), allowing evaluation of population-level impacts of bycatch mortality on status and recovery. The California halibut bottom-trawl fishery primarily interacts with immature individuals of the Southern DPS (Anderson et al, 2017;Richerson et al 3 ).…”