Trophic Feasibility of Reintroducing Anadromous Salmonids in Three Reservoirs on the North Fork Lewis River, Washington: Prey Supply and Consumption Demand of Resident Fishes

Abstract

The reintroduction of anadromous salmonids in reservoirs is being proposed with increasing frequency, requiring baseline studies to evaluate feasibility and estimate the capacity of reservoir food webs to support reintroduced populations. Using three reservoirs on the north fork Lewis River as a case study, we demonstrate a method to determine juvenile salmonid smolt rearing capacities for lakes and reservoirs. To determine if the Lewis River reservoirs can support reintroduced populations of juvenile stream‐type Chinook Salmon Oncorhynchus tshawytscha, we evaluated the monthly production of daphnia Daphnia spp. (the primary zooplankton consumed by resident salmonids in the system) and used bioenergetics to model the consumption demand of resident fishes in each reservoir. To estimate the surplus of Daphnia prey available for reintroduced salmonids, we assumed a maximum sustainable exploitation rate and accounted for the consumption demand of resident fishes. The number of smolts that could have been supported was estimated by dividing any surplus Daphnia production by the simulated consumption demand of an individual Chinook Salmon fry rearing in the reservoir to successful smolt size. In all three reservoirs, densities of Daphnia were highest in the epilimnion, but warm epilimnetic temperatures and the vertical distribution of planktivores suggested that access to abundant epilimnetic prey was limited. By comparing accessible prey supply and demand on a monthly basis, we were able to identify potential prey supply bottlenecks that could limit smolt production and growth. These results demonstrate that a bioenergetics approach can be a valuable method of examining constraints on lake and reservoir rearing capacity, such as thermal structure and temporal food supply. This method enables numerical estimation of rearing capacity, which is a useful metric for managers evaluating the feasibility of reintroducing Pacific salmon Oncorhynchus spp. in lentic systems. Received April 24, 2016; accepted July 28, 2016 Published online October 11, 2016

New Insights into the Ecology of Adfluvial Bull Trout and the Population Response to the Endangered Species Act in the North Fork Lewis River, Washington

“…The interannual stability of lacustrine environments and Bull Trout populations in reservoirs and lakes ) suggests that the higherthan-expected mortality is unlikely to be driven by interannual climatic differences. It is also possible that food resources have shifted in Swift Reservoir, where Bull Trout are the top predator, and in situ diet studies suggest that adults predominately feed on species strongly tied to benthic resources (i.e., Coastal Cutthroat Trout, Mountain Whitefish, Rainbow Trout, sculpin, Largescale Suckers; Sorel et al 2016). Existing trend data for these important forage fishes is limited, but declines in abundance through natural fluxes or resulting from anadromous species reintroductions (see below) may be driving the lower-thanexpected survival for adult Bull Trout.…”

“…1110 research to consider the effects of angling within the reservoirs may be warranted. Specifically, the warm epilimnetic temperatures in Swift Reservoir (Sorel et al 2016) suggest the potential for handling and postrelease effects on the survival of Bull Trout (sensu Gingerich et al 2007), which have narrow thermal regimes (Selong et al 2001).…”

Bioenergetics model for the nonnative Redside Shiner

“…Our simulations and other studies suggest that some salmonid species may be more affected by warming surface waters than others (Wenger et al 2011; Crozier et al 2019). The higher thermal optima of the Redside Shiner may allow it to dominate the warm productive surface waters during stratified periods, limiting resource access for Sockeye Salmon and possibly Chinook Salmon when most prey production occurs in the warmer, shallow depths of the pelagic and littoral habitats (e.g., Tunney et al 2012; Sorel et al 2016a). Under such scenarios of high prey productivity and thus high consumption rates, steelhead appear to have a similar thermal tolerance to Redside Shiner on the warm end of the spectrum, suggesting that they may have comparable access to prey resources.…”

“…As such, these models can be used to estimate growth from consumption or vice versa, while accounting for size‐ and temperature‐dependent effects on these physiological processes. Applications of these models range widely (see Deslauriers et al 2017 for review) but can include quantitative assessments of predation mortality (Lowery and Beauchamp 2015; Sorel et al 2016b) or consumption demand and carrying capacity (Sorel et al 2016a; Taylor et al 2020), estimating habitat quantity and growth potential (Weber et al 2014; Carmichael et al 2020), or evaluating possible distributional (Lawrence et al 2015) or trophic shifts due to climate change (Breeggemann et al 2016).…”

Pelagic food web interactions in a large invaded ecosystem: Implications for reintroducing a native top predator

“…We estimated the mean monthly density (organisms/L), standing stock biomass (MT, metric tonnes), and production (MT) of adult Epischura and Diaptomus based on the sample counts and volumes interpolated among sampling months, BLs, mean temperatures in each depth layer, and depth‐specific estimates of lake volume to characterise the predominant food supply available to kokanee and Mysis (Hansen et al, 2016; Schoen et al, 2015; Sorel et al, 2016). For biomass (B), live wet masses were estimated from mean BLs using a joint BL to dry weight (DW in μg) relationship developed from Bottrell et al (1976) for Epischura (DW = 7.05·BL 2.40 ) and McCauley (1984) for Diaptomus (DW = 3.56·BL 2.15 ) and a DW to wet weight (WW in μg) ratio of 9.6%.…”