Data from: Predation probabilities and functional responses: How piscivorous waterbirds respond to pulses in fish abundance

How predators respond to changes in prey abundance (i.e. functional responses) is foundational to consumer-resource interactions, predator-prey dynamics, and the stability of predator-prey systems. Predation by piscivorous waterbirds on out-migrating juvenile steelhead trout (Oncorhynchus mykiss) is considered a factor affecting the recovery of multiple Endangered Species Act-listed steelhead populations in the Columbia River basin. Waterbird functional responses, however, may vary by predator species and location, with important implications to predator management strategies. We used a 13-year dataset on waterbird abundance across seven breeding colonies (three Caspian tern [Hydroprogne caspia], two double-crested cormorant [Nannopterum auritum], and two California and ring-billed gull [Larus californicus and L. delawarensis] colonies) and steelhead tag-recovery data (>645,000 tagged and >32,000 recovered steelhead) to quantify weekly predation probabilities and functional responses across waterbird species, colonies, and years. Weekly predation probabilities were highly variable, ranging from 0.01–0.30 at tern colonies, 0.01–0.20 at cormorant colonies, and 0.03–0.13 at gull colonies. Per capita predation probabilities were an order of magnitude higher at inland tern and cormorant colonies relative to estuary colonies of the same species. Terns displayed Type II functional responses across colonies and years, where predation probabilities peaked at low steelhead abundances and declined as steelhead abundance increased (i.e., predator swamping). Cormorants nesting at the large estuary colony (several thousand birds) displayed a Type III functional response, but cormorants nesting at the smaller inland colony (several hundred birds) displayed a Type II response. Consumption probabilities of steelhead by gulls remained consistent across a large range of steelhead availability, suggesting a Type I or a Type III functional response, but a lack of colony abundance data prevented quantifying functional responses. The level of tern predation combined with Type II functional responses indicate possible population-level impacts that could destabilize small or declining prey populations. Conversely, the apparent Type III functional responses of gulls and estuary nesting cormorants are indicative of prey switching behaviors targeted at periods of high steelhead abundance. Our results illustrate the complexity of predator-prey interactions and the importance of quantifying predator- and location-specific functional responses when predicting the efficacy of management strategies to enhance prey populations.Contact NJ Hostetter for additional data or usage requests.Data files and R scripts refelct the final formatted data to run the predation probablity and functional response models presented in: Hostetter, NJ, Q Payton, DD Roby, K Collis, and AF Evans. Predation probabilities and functional responses: How piscivorous waterbirds respond to pulses in fish abundance. PIT tag detection data were retrieved from the PIT Tag Information System (PTAGIS), a regional salmonid tag database maintained by the Pacific States Marine Fisheries Commission, during January 2020 (PSMFC 2020). Detailed information on PIT tag recovery data are in the associated manuscript and Evans et al. (2012; 2019) and Hostetter et al. (2015). Literature Cited Evans, A. F., N. J. Hostetter, D. D. Roby, K. Collis, D. E. Lyons, B. P. Sandford, R. D. Ledgerwood, and S. Sebring. 2012. Systemwide evaluation of avian predation on juvenile salmonids from the Columbia River based on recoveries of passive integrated transponder tags. Transactions of the American Fisheries Society 141:975–989. Evans, A. F., Q. Payton, B. M. Cramer, K. Collis, N. J. Hostetter, D. D. Roby, and C. Dotson. 2019. Cumulative effects of avian predation on Upper Columbia River steelhead. Transactions of the American Fisheries Society 148:896–913. Hostetter, N. J., A. F. Evans, B. M. Cramer, K. Collis, D. E. Lyons, and D. D. Roby. 2015. Quantifying avian predation on fish populations: integrating predator-specific deposition probabilities in tag recovery studies. Transactions of the American Fisheries Society 144:410–422. PSMFC (Pacific States Marine Fisheries Commission). 2020. PIT Tag Information System (PTAGIS): A regional database. www.ptagis.org