Seasonal Diet and Habitat Use of Large, Introduced Rainbow Trout in an Ozark Highland Stream

Abstract

Stocking of Rainbow TroutOncorhynchus mykiss commonly provides seasonal or mitigation fisheries; however, these fish are usually small and ecosystem effects are spatially or temporally limited. Yet agencies receive requests to stock Rainbow Trout in relatively natural settings (i.e., not tailwater or mitigation fisheries), where introductions may have greater ecosystem consequences. The size of introduced fish is an important factor in determining biotic interactions with native species; therefore, our objectives were to assess the seasonal feeding ecology and microhabitat use of large ( 265-530 mm TL) nonnative Emmerson strain Rainbow Trout in a relatively unaltered, groundwater-influenced, warmwater stream of the Ozark Highlands. Rainbow Trout consumed a variety of prey; however, diets differed between cool (winter and spring) and warm (summer) seasons. Cool-season Rainbow Trout exhibited a mixed feeding strategy, with individual specialization on crayfishes and fishes and generalist feeding on Ephemeroptera and Diptera, but Gastropoda were the dominant prey. Feeding strategy in the warm season switched to individual specialization on numerous prey types. Overall, larger prey resources were important components of Rainbow Trout diets. Piscivory was relatively high in both seasons, and crayfishes were one of the most important prey types across seasons. Selection of coarse substrates and deeper-water microhabitats (>0.95 m) was similar between seasons. Rainbow Trout selected the lowest-velocity microhabitats available during the warm season and moderate velocities in the cool season. Rainbow Trout were five times more likely to be associated with cover in the warm season. Due to their higher temperature tolerance, Emmerson strain Rainbow Trout may persist in Ozark Highland streams, where they disrupt local food webs and occupy habitat otherwise selected by native fish, such as Neosho Smallmouth Bass Micropterus dolomieu velox. If native species conservation is a priority for agencies, then caution regarding Rainbow Trout stockings may be warranted.

Seasonal changes in abundance and feeding ecology of an endemic, keystone predator following the introduction of large rainbow trout Oncorhynchus mykiss

“…Prey collected from stomachs of Neosho bass were classified into genus for fish and crayfish, family for mammals, order for amphibians, class for aquatic snails and order for all other macroinvertebrates. However, for diet overlap comparisons with rainbow trout (see below), prey groupings matched those of Rodger et al (2021) by classifying fish prey into habitat guilds (i.e. benthic or pelagic).…”

“…To assess effects of large rainbow trout in Spavinaw Creek, 10,800 rainbow trout were stocked in one reach of Spavinaw Creek between autumn 2018 and winter 2019 (Rodger et al, 2021). Historically, a rainbow trout fishery existed in Spavinaw Creek, Arkansas, but stocking ceased in 2013 (Christy Graham, Arkansas Game and Fish Commission, oral communication, 2021).…”

“…Neosho bass data collected up to autumn 2018 served as a baseline for comparison with data collected after rainbow trout stocking, when rainbow trout were present at considerably higher densities and spread throughout the system. Survival, movement, habitat use and diets of this introduced rainbow trout population were studied by Rodger et al (2021) and Wolf and Brewer (2021).…”

“…diet composition, feeding strategy, diet breadth, trophic position, prey size selection) of Neosho bass? Recent interest by trout anglers of an Ozark Highlands stream led to the stocking of large (maximum size >500 mm TL) rainbow trout in a stream occupied by Neosho bass (Rodger et al, 2021; Wolf & Brewer, 2021). The disparity in size among endemic Neosho bass and introduced rainbow trout presented an opportunity to study the response of a keystone predator to the introduction of a novel invasive that has the potential to usurp their ecological role.…”

“…We predicted that estimates of Neosho bass abundance would decline in reaches with high rainbow trout density (Healy et al, 2020; Shelton et al, 2015b; Simon & Townsend, 2003). We also expected considerable diet overlap between rainbow trout and Neosho bass and shifts in Neosho bass feeding ecology because of high rates of piscivory and consumption of crayfish by rainbow trout (Rodger et al, 2021). Smallmouth bass predator–prey interactions with crayfish are known to be size‐specific (Probst et al, 1984; Roell & Orth, 1993; Stein, 1977), so we also hypothesised a shift in Neosho bass crayfish size selection after stocking.…”