Linking functional response and bioenergetics to estimate juvenile salmon growth in a reservoir food web

<div><p>Juvenile salmon (<i>Oncorhynchus</i> spp.) use of reservoir food webs is understudied. We examined the feeding behavior of subyearling Chinook salmon (<i>O</i>. <i>tshawytscha</i>) and its relation to growth by estimating the functional response of juvenile salmon to changes in the density of <i>Daphnia</i>, an important component of reservoir food webs. We then estimated salmon growth across a broad range of water temperatures and daily rations of two primary prey, <i>Daphnia</i> and juvenile American shad (<i>Alosa sapidissima</i>) using a bioenergetics model. Laboratory feeding experiments yielded a Type-II functional response curve: <i>C</i> = 29.858 <i>P</i> *(4.271 + <i>P</i>)^-1 indicating that salmon consumption (<i>C</i>) of <i>Daphnia</i> was not affected until <i>Daphnia</i> densities (<i>P</i>) were < 30 · L^-1. Past field studies documented <i>Daphnia</i> densities in lower Columbia River reservoirs of < 3 · L^-1 in July but as high as 40 · L^-1 in August. Bioenergetics modeling indicated that subyearlings could not achieve positive growth above 22°C regardless of prey type or consumption rate. When feeding on <i>Daphnia</i>, subyearlings could not achieve positive growth above 20°C (water temperatures they commonly encounter in the lower Columbia River during summer). At 16–18°C, subyearlings had to consume about 27,000 <i>Daphnia</i> · day^-1 to achieve positive growth. However, when feeding on juvenile American shad, subyearlings had to consume 20 shad · day^-1 at 16–18°C, or at least 25 shad · day^-1 at 20°C to achieve positive growth. Using empirical consumption rates and water temperatures from summer 2013, subyearlings exhibited negative growth during July (-0.23 to -0.29 g · d^-1) and August (-0.05 to -0.07 g · d^-1). By switching prey from <i>Daphnia</i> to juvenile shad which have a higher energy density, subyearlings can partially compensate for the effects of higher water temperatures they experience in the lower Columbia River during summer. However, achieving positive growth as piscivores requires subyearlings to feed at higher consumption rates than they exhibited empirically. While our results indicate compromised growth in reservoir habitats, the long-term repercussions to salmon populations in the Columbia River Basin are unknown.</p></div

Parameter estimates for linear and nonlinear functional response models describing the relation between <i>Daphnia</i> density and subyearling Chinook salmon consumption.

<p>The two data sets are from all the data and an ecologically relevant subset (<i>Daphnia</i> densities < 40 · L^-1).</p

The Wisconsin bioenergetics model [5] with species specific parameters developed by Stewart and Ibarra [37] and modified consumption parameters from Plumb and Moffitt [26] used to model juvenile Chinook salmon growth.

<p>The Wisconsin bioenergetics model [<a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0185933#pone.0185933.ref005" target="_blank">5</a>] with species specific parameters developed by Stewart and Ibarra [<a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0185933#pone.0185933.ref037" target="_blank">37</a>] and modified consumption parameters from Plumb and Moffitt [<a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0185933#pone.0185933.ref026" target="_blank">26</a>] used to model juvenile Chinook salmon growth.</p

Maximum July and August <i>Daphnia</i> densities from various literature sources, predicted subyearling consumption rates (# · hr^-1) from a laboratory derived functional response curve in this study, and estimated time to satiation (h) from McNary and John Day reservoirs, lower Columbia River, 1982–2010, based on previously derived evacuation rates from Haskell et al. [14].

<p>Mean <i>Daphnia</i> lengths used for 1982 were 1.4 mm in July and 1.5 mm in August. All others used a <i>Daphnia</i> length of 1.2 mm.</p

Type-II (solid line) functional response curve of subyearling Chinook salmon fit to a range of <i>Daphnia pulex</i> densities from laboratory trials and the 95% confidence interval about the mean (region between dashed lines).

<p>Type-II (solid line) functional response curve of subyearling Chinook salmon fit to a range of <i>Daphnia pulex</i> densities from laboratory trials and the 95% confidence interval about the mean (region between dashed lines).</p

Map of the Columbia River basin depicting mainstem hydroelectric dams (white rectangles) and the Hanford Reach (shaded oval), an important spawning area for fall Chinook salmon.

<p>Map of the Columbia River basin depicting mainstem hydroelectric dams (white rectangles) and the Hanford Reach (shaded oval), an important spawning area for fall Chinook salmon.</p

Bioenergetics modeled specific growth (g · g · d^-1) of subyearling Chinook salmon at varying consumption rates ranging from 2,000 to 32,000 <i>Daphnia</i> · day^-1 (A) and juvenile American shad ranging from 10 to 40 shad. day^-1 (B) at water temperatures ranging from 16 to 24°C.

<p>Bioenergetics modeled specific growth (g · g · d^-1) of subyearling Chinook salmon at varying consumption rates ranging from 2,000 to 32,000 <i>Daphnia</i> · day^-1 (A) and juvenile American shad ranging from 10 to 40 shad. day^-1 (B) at water temperatures ranging from 16 to 24°C.</p

Energy densities and proportions of prey items collected from juvenile fall Chinook salmon in John Day Reservoir, Columbia River from 21 July and 11 August 2013 [14].

<p>Twelve juvenile Chinook stomachs were examined from each date.</p

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

<p>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 <i>Oncorhynchus tshawytscha</i>, we evaluated the monthly production of daphnia <i>Daphnia</i> 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 <i>Daphnia</i> 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 <i>Daphnia</i> 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 <i>Daphnia</i> 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 <i>Oncorhynchus</i> spp. in lentic systems.</p> <p>Received April 24, 2016; accepted July 28, 2016 Published online October 11, 2016 </p

Appendix D. Lake trout growth models, lake trout catch rates, and predation risk for kokanee.

Lake trout growth models, lake trout catch rates, and predation risk for kokanee