Post‐release predation mortality of age‐0 hatchery‐reared Chinook salmon from non‐native smallmouth bass in the Snake River

Abstract

Release of age‐0 hatchery‐reared fall Chinook salmon, Oncorhynchus tshawytscha (Walbaum), in the Snake River resulted in up to 30‐fold increases in salmon consumption by non‐native smallmouth bass, Micropterus dolomieu Lacepѐde. In an upper river reach, smallmouth bass fed intensively during a release in May, but Chinook salmon consumption returned to pre‐release levels within 1–2 days as hatchery‐reared fish quickly emigrated downstream. The predation response during a June release located farther downstream was dissimilar. Chinook salmon consumption increased to a lesser extent (11‐fold), lasted several days (~4) and no changes in feeding intensity were evident. Estimated numbers of age‐0 hatchery‐reared Chinook salmon lost to short‐term predation varied by year and study reach and ranged from 12,007 (6.03% of those released) to 210,580 (14.6% of those released) fish. Short‐term, intense feeding by smallmouth bass can contribute significantly to mortality of hatchery‐reared fish and should be considered when supplementing populations with hatchery juveniles.

Impact of smallmouth bass predation on subyearling fall Chinook salmon over a broad river continuum

“…Smallmouth bass were collected from 2013 to 2018 to describe diet changes and estimate subyearling loss to predation in each reach, but not all reaches were sampled every year. Data for this study were collected and combined with data from previously published studies (Erhardt et al 2018a ; Erhardt and Tiffan 2018 ) to derive monthly estimates of predation metrics for each study reach. Study designs varied between reaches because of potential differences in habitat-related sampling efficiencies, safety, and logistical reasons (e.g., limited access points within Hells Canyon).…”

“…Most sites covered multiple habitat types (i.e., pool, riffle, glide, fan, bar), and sampling occurred on both banks of the river. The start and end points of each site typically occurred at habitat breaks (see Erhardt and Tiffan 2018 for details on habitat delineation) and when possible extended between two rapids. Preliminary analyses showed that the proportions of habitat surveyed in the fixed sampling sites, along with smallmouth bass relative abundances, were representative of the entire reach.…”

“…Smallmouth bass abundances, which were necessary to estimate predation losses, were estimated with methods described by Erhardt and Tiffan ( 2018 ) and Erhardt et al ( 2018a ). In brief, for Hells Canyon reaches, collected bass were tagged with anchor tags and recapture data were used to estimate abundance for the fixed sites in each reach for each biweekly sampling interval using open-population models (Jolley-Seber type; Schwarz and Arnason 1996 ).…”

“…Predation is a significant source of mortality for juvenile salmon and is commonly cited as a contributing factor to the decline of anadromous salmonids in the Pacific Northwest of the United States (Sanderson et al 2009 ; Carey et al 2011 ). Predation on juvenile salmonids by both piscine and avian predators has been studied extensively in anadromous fish-bearing waters along the west coast of the United States and Canada (Rieman et al 1991 ; Gregory and Levings 1998 ; Furey et al 2016 ; Grossman 2016 ; Sabal et al 2016 ; Evans et al 2012 ), but past efforts have generally focused on specific areas like river transitional areas (Tabor et al 1993 ; Shively et al 1996 ; Erhardt et al 2018a ), river reaches (Fritts and Pearsons 2004 ), or to address ecological questions [e.g., binge feeding (Collis et al 1995 ; Furey et al 2016 ; Erhardt and Tiffan 2018 )]. One reason for this is likely the cost and effort required to estimate predation losses at larger spatial scales.…”

“…Their relatively small size and extended shoreline rearing makes subyearlings vulnerable to predation by smallmouth bass, which are present throughout the river system. Although several past studies have examined smallmouth bass predation on juvenile salmon in specific locations in the Snake River (Nelle 1999 ; Anglea 1997 ; Naughton et al 2004 ; Erhardt and Tiffan 2018 ; Erhardt et al 2018a ), none has estimated predation losses over a broader landscape to identify how changes in river habitats and prey assemblages affect predation losses. Furthermore, these and other studies typically report numbers of juvenile salmon lost to predation but rarely is any effort made to put those losses in a population-level context.…”