Avian Predation on Juvenile and Adult Lost River and Shortnose Suckers: An Updated Multi‐Predator Species Evaluation

Abstract

Previous research suggests that predation by piscivorous colonial waterbirds may negatively influence the survival of Lost River Suckers (LRS) Deltistes luxatus and Shortnose Suckers (SNS) Chasmistes brevirostris in the Upper Klamath Basin (UKB), USA. However, estimates of predation from past studies, which were based on suckers with PIT tags, represent minimum estimates of sucker mortality because analyses did not account for the proportion of tags that were consumed by birds and deposited beyond their breeding colony. To address this uncertainty, we fed PIT‐tagged suckers to American white pelicans Pelecanus erythrorhynchos to estimate deposition probabilities. A hierarchical Bayesian model was then used to estimate predation rates (percentage of available tagged fish that were consumed) on juvenile suckers that were released as part of the Sucker Assisted Rearing Program (SARP) and on wild juvenile and adult LRS and SNS during 2009–2020. Pelican deposition probabilities were estimated to be 0.47 (95% credible interval = 0.36–0.60), indicating that for every 100 tags consumed, 47 tags on average were deposited on breeding colonies by birds. Deposition‐corrected estimates of predation rates were approximately two times greater than those previously reported and ranged annually from 4.3% (95% credible interval = 2.9–6.7%) to 8.5% (6.3–12.7%) on SARP juvenile suckers, from 4.3% (0.9–13.2%) to 10.5% (3.8–24.5%) on wild juvenile suckers, and from 0.1% (<0.1–0.3%) to 7.2% (2.8–16.4%) on adult suckers, depending on species and location. Results suggest that predation by colonial waterbirds, although not the original cause of sucker declines, was a substantial source of sucker mortality in some years. Future studies should consider models that jointly estimate both predation and survival and models that include environmental factors that potentially influence sucker susceptibility to avian predators in the UKB.

Estimating Cause‐Specific Mortality and Survival of Juvenile Fall Chinook Salmon: An Investigation of Avian Predation across Large Spatial Scales

“…The colony‐specific PIT tag detection probabilities and predator‐specific PIT tag deposition probabilities (collectively referred to as “recovery probabilities”) used in this study were those previously reported by Evans et al. (2019, 2022a).…”

“…(2015) for CATE, DCCO, and LAXX and were taken from Evans et al. (2022a) for AWPE. is a 15‐length vector, where represents the probability of recovering a tag deposited on nesting area a for . We let represent the lack of recoveries of fish that died from all other unspecified causes.…”

“…Unlike deposition probabilities, results of detection probability studies indicated that detection varied within and between years, necessitating empirically derived estimates of detection probability at each colony during each year (Hostetter et al 2015;Payton et al 2019). To estimate detection probabilities, as part of independent studies, PIT tags were intentionally sown in nesting areas by researchers prior to, during (when possible), and after the breeding season each year at each of the colonies included in this study (Evans et al 2012(Evans et al , 2019(Evans et al , 2022a(Evans et al , 2022b. Recoveries of sown tags during scanning efforts after the nesting season were then used to model the probability of detecting a tag that was deposited on the colony during the breeding season (see also Predation and Survival Estimation below).…”

“…To estimate detection probabilities, as part of independent studies, PIT tags were intentionally sown in nesting areas by researchers prior to, during (when possible), and after the breeding season each year at each of the colonies included in this study (Evans et al. 2012, 2019, 2022a, 2022b). Recoveries of sown tags during scanning efforts after the nesting season were then used to model the probability of detecting a tag that was deposited on the colony during the breeding season (see also Predation and Survival Estimation below).…”

“…Recoveries of sown tags during scanning efforts after the nesting season were then used to model the probability of detecting a tag that was deposited on the colony during the breeding season (see also Predation and Survival Estimation below). The colony-specific PIT tag detection probabilities and predator-specific PIT tag deposition probabilities (collectively referred to as "recovery probabilities") used in this study were those previously reported by Evans et al (2019Evans et al ( , 2022a.…”