Date of Graduation
Davis College of Agriculture, Natural Resources and Design
Forest Resource Management
Sturgeon first appear in the fossil record in the Triassic Period just over 200 million years ago and are among the most primitive of the bony fishes. Despite their large size and historic presence along the East Coast, Atlantic sturgeon were not targeted for their meat and caviar as a commercial fishery until 1880. By 1905 they had declined to less than one percent of their pre-fishing abundance but the fishery continued. Prior to 1980, there had been very little research on Atlantic sturgeon, primarily limited to documenting landing location and poundage, maximum longevity, or weight of eggs per fish. By 1990 most research into Atlantic sturgeon population dynamics had been focused on fisheries, and specifically, on when, where, and in what condition they were available for capture. The first true efforts at understanding population dynamics of Atlantic sturgeon led to the closure of all U.S. state and federal fisheries in 1998 and 1999, respectively.
Much of the initial research on Atlantic sturgeon population dynamics focused on larger river systems with the largest relative populations: primarily the Hudson River, New York, and to a lesser extent, the Altamaha River, Georgia. While this initial work suggested longitudinal differences in age at first reproduction and longevity, for topics like spawning return frequency, only generic estimates from northern populations were available. There was no historic or contemporary estimate of population sex ratio. Survival rates have been estimated for a number of southern populations and for distinct population segments in the U.S.
By 2007, Atlantic sturgeon were thought to have been extirpated from large portions of their range and where they were known to still persist, abundance was believed to be significantly depressed from historic levels. Estimates of adult abundance had been made for two populations and both suggested fewer than 1,000 individuals. This led the National Marine Fisheries Service (NMFS) to list Atlantic sturgeon as endangered from Florida to Connecticut and threatened in the Gulf of Maine in 2012. The determination that Atlantic sturgeon may be extirpated in the foreseeable future was based on the threats each population faced and a general understanding that abundance of all populations was limited. With the exception of the Hudson River, NMFS estimated that most Atlantic sturgeon populations had a total abundance of fewer than 300 individuals.
This research found a previously unidentified population of Atlantic sturgeon and then produced estimates of annual spawning run abundance, apparent annual survival rate, spawning return frequency, sex ratios, and total adult population abundance for what appears to be one of the
smallest populations of Atlantic sturgeon. Annual spawning run abundance estimates were made using the Schumacher-Eschmeyer model as well as 10 models in Program CAPTURE that estimate abundance using the assumption that capture probability is constant (M0 model, null), varies by individual heterozygosity (Mh models; Chao Mh and jackknife), varies by time (Mt models; Chao Mt and Darroch), varies by individual and time (Mth model; Chao Mth), varies by behavioral response to capture (Mb model; Zippin), varies by behavioral response to capture and heterozygosity (Mbh models; Generalized Removal and Pollock and Otto), or varies by behavioral response to capture and time (Mtb model; Burnham). Annual survival estimates were made using the Cormack-Jolly-Seber (CJS) model. Spawning return frequencies were calculated using acoustic transmitter detections of males and females and a subsequent ratio estimator for each sex. Sex ratio was calculated by comparing the observed and calculated sex ratios on each spawning run to determine the overall sex ratio in the population. The super-population estimate was produced by the POPAN model as well as a novel estimator using annual closed population abundance estimates.
The Schumacher-Eschmeyer estimates with 95% confidence limits (CL) of annual spawning run abundance 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. 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 similar 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 was sparse. If using an estimator from Program Capture, I recommend the Jackknife equation (model Mh), which produced point estimates with 95% CLs of 52 (35-85), 152 (102-264), 182 (145-243), 219 (166-298), 215 (167-292), and 154 (112-222), from 2013 to 2018.
Theoretic annual survival based on maximum life expectancy for sturgeon from the Chesapeake Bay was estimated to be approximately 89.6% per year. For telemetry-derived survival studies, detections are recorded monthly within a known abundance (number of transmitters at large) to calculate survival in a CJS model. Given all transmitters implanted in sturgeon at the start of this study, the York River population survives at approximately 89.0% per year. However, I was able to recapture 45% of telemetered adult Atlantic sturgeon, revealing an 11% transmitter failure rate. When these failed transmitters were removed from the study, adult York River Atlantic sturgeon appear to survive at a rate of 94.8% per year (95% CL, 83.1-98.9%). Females are also less likely to be detected each month. When detections were assessed on an annual basis so each sex was equally likely to be detected, adult York River Atlantic sturgeon appear to survive at a rate of 97.9% per year (95% CL, 88.9-99.8%), males at a rate of 99.3% per year (95% CL, 96.1-99.9%), and females at a rate of 95.0% per year (79.0-99.5%). By identifying these two common violations of the CJS model assumptions and then removing failed transmitters and using detection periods with equal probability of detection, Atlantic sturgeon appear to survive at considerably higher rates than are theoretically predicted, suggesting a different survival curve than for many other fish species.
Spawning return frequencies for males and females were calculated using ratio estimators, but also reported as the maximum length of time between spawning runs, relying on telemetry detections on spawning grounds. Male York River Atlantic sturgeon returned to the Pamunkey River 85 times and to the York River system 92 times of a possible 104 possible spawning runs. Therefore, males return to the Pamunkey River once every 1.23 years and to the York River system once every 1.13 years. No male ever skipped more than two consecutive spawning seasons. Female York River Atlantic sturgeon returned to the Pamunkey River 17 times and to the York Rive system 19 times of a possible 41 spawning runs. Therefore, females return to the Pamunkey River once every 2.4 years and to the York River system once every 2.16 years. No female ever skipped more than two consecutive spawning seasons. These spawning return frequencies for both males and females are more frequent than has been estimated generically for northern populations of Atlantic sturgeon.
The observed sex ratio on spawning runs in the Pamunkey was 2.8 males to every female. However, of the 239 adult sturgeon captured during this project, 31% could not be sexually identified. Assuming all unidentified fish are either male or female gives a range of as few as 2.1 males to female to as many as 3.4 males to female. The calculated sex ratio, based on the spawning return frequencies of males and females, would anticipate a ratio of approximately 1.76 males for every female on the spawning grounds. The ratio between the observed and calculated sex ratios, which estimates the total adult population sex ratio, suggests there are approximately 1.59 males for each female in the York River population. Therefore, males account for approximately 61% of the adult population estimate. While no estimates of adult population sex ratio existed for Atlantic sturgeon, because males mature earlier than females, a skewed sex ratio may be expected. The sex ratio of adults and juveniles combined may still be equal but has yet to be investigated.
The POPAN model super-population estimate produced a range of 95% confidence limits between 245 and 361 individuals. My model relying on spawning frequency and sex ratio produced 95% confidence limits based on the underlying mean annual jackknife abundance estimates of between 239 and 386. When Atlantic sturgeon were listed in 2012, NMFS estimated that most U.S. populations had an abundance of fewer than 300 fish. However, assuming a proportional relationship between effective population estimates and census population estimates, the York River population is one of the smallest on the Atlantic Coast, suggesting most, if not all, Atlantic sturgeon populations have more than 300 individuals.
Kahn, Jason E., "Adult Atlantic sturgeon population dynamics in the York River, Virginia" (2019). Graduate Theses, Dissertations, and Problem Reports. 4042.