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

Mean and standard error of <i>Pseudodiaptomus forbesi</i> and native copepod prey consumed by four different predator types in single-prey experiments.

<p>Black triangles represent the invasive copepod, <i>Pseudodiaptomus forbesi</i>, and gray circles represent native copepods, <i>Cyclopidae</i> spp.</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

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

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

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

Mean and standard error of <i>Pseudodiaptomus forbesi</i> and native prey consumed by four different predator types in two-prey experiments.

<p>(A) Black triangles represent the invasive copepod, <i>Pseudodiaptomus forbesi</i>, and gray squares represent native cladocerans, <i>Daphnia retrocurva</i>. (B) Black triangles represent the invasive copepod, <i>Pseudodiaptomus forbesi</i>, and gray circles represent native copepods, <i>Cyclopidae</i> spp.</p