Colonial Waterbird Predation and Angler Catch of Hatchery Rainbow Trout Stocked in Southern Idaho Fisheries

Abstract

The abundance of piscivorous colonial waterbirds such as double‐crested cormorants Phalacrocorax auritus and American white pelicans Pelecanus erythrorhynchos has increased dramatically in recent decades in North America, resulting in increased conflict between these birds and fisheries resources, which raises the need to quantify the effects of waterbird predation on fisheries. We estimated avian predation and angler use of two sizes of hatchery‐reared Rainbow Trout Oncorhynchus mykiss (250 mm and 300 mm on average; termed standard catchables and magnum catchables) at 15 Idaho waters with known cormorant and pelican presence. Fish were tagged with passive integrated transponder (PIT), radio, and T‐bar anchor tags prior to release; the tags were subsequently recovered from bird nesting, roosting, and loafing sites. Estimated angler use, which included fish harvested as well as those that were catch‐and‐release, averaged 17%, while avian predation averaged 35%. Angler catch at study waters declined exponentially as avian predation increased. Predation specifically attributed to double‐crested cormorants and American white pelicans was related to their relative abundance at the waterbodies where the fish were stocked. Avian predation rates were similar for standard and magnum catchables, suggesting that larger fish were not able to escape predation any better than smaller ones. Our results add to existing evidence indicating that in some southern Idaho fisheries, piscivorous birds, rather than anglers, are the dominant consumers of hatchery trout. This study further contributes to the larger issue of competition between humans, wildlife, and fisheries populations.

Relative Cost and Post‐Release Performance of Hatchery Catchable Rainbow Trout Grown to Two Target Sizes

“…The observed difference in angler catch between standard and magnum catchables was unlikely to have been caused by other confounding factors. For example, individuals from both target length groups tested were presumed to be invulnerable to predation by piscivorous fishes (Yule et al 2000), and any effect of predation by avian predators was assumed to be equal between length groups (Walters et al 1997;Chiaramonte et al 2019). We found no difference in tag reporting rate between length groups, indicating that the difference in angler catch can be attributed to the angling process itself.…”

“…Depending on management objectives, multiple stocking events may be warranted in select fisheries to maintain adequate catch rates throughout the angling season. Catch rates of Rainbow Trout by recreational anglers may also be affected by the abundance of aquatic predators (Baldwin et al 2003), the presence and abundance of avian predators (Walters et al 1997;Chiaramonte et al 2019), and prey availability (Haddix and Budy 2005). In general, larger trout are better suited to avoid predation by piscivorous fish (e. g. Yule et al 2000), but growing hatchery fish to a larger size is accompanied with increased rearing costs (e.g., feed expenditures).…”

Fisheries Volume 46 Number 8 August 2021

“…The observed difference in angler catch between standard and magnum catchables was unlikely to have been caused by other confounding factors. For example, individuals from both target length groups tested were presumed to be invulnerable to predation by piscivorous fi shes (Yule et al 2000 ), and any effect of predation by avian predators was assumed to be equal between length groups (Walters et al 1997 ;Chiaramonte et al 2019 ). We found no difference in tag reporting rate between length groups, indicating that the difference in angler catch can be attributed to the angling process itself.…”

“…Depending on management objectives, multiple stocking events may be warranted in select fi sheries to maintain adequate catch rates throughout the angling season. Catch rates of Rainbow Trout by recreational anglers may also be affected by the abundance of aquatic predators (Baldwin et al 2003 ), the presence and abundance of avian predators (Walters et al 1997 ;Chiaramonte et al 2019 ), and prey availability (Haddix and Budy 2005 ). In general, larger trout are better suited to avoid predation by piscivorous fi sh (e. g. Yule et al 2000 ), but growing hatchery fi sh to a larger size is accompanied with increased rearing costs (e.g., feed expenditures).…”

Evaluating Target Levels of American White Pelican Abundance to Achieve Management Objectives for Adfluvial Yellowstone Cutthroat Trout

“…2016; Chiaramonte et al. 2019), including the Blackfoot River Yellowstone Cutthroat Trout fishery. Although pelican populations are robust throughout the United States, they remain of conservation focus and are federally protected under the Migratory Bird Treaty Act of 1918.…”