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Madenjian C.P.,U.S. Geological Survey | Rediske R.R.,Grand Valley State University | O'Keefe J.P.,Grand Valley State University | David S.R.,Daniel erther Center For Conservation And Research
Journal of visualized experiments : JoVE | Year: 2014

A technique for laboratory estimation of net trophic transfer efficiency (γ) of polychlorinated biphenyl (PCB) congeners to piscivorous fish from their prey is described herein. During a 135-day laboratory experiment, we fed bloater (Coregonus hoyi) that had been caught in Lake Michigan to lake trout (Salvelinus namaycush) kept in eight laboratory tanks. Bloater is a natural prey for lake trout. In four of the tanks, a relatively high flow rate was used to ensure relatively high activity by the lake trout, whereas a low flow rate was used in the other four tanks, allowing for low lake trout activity. On a tank-by-tank basis, the amount of food eaten by the lake trout on each day of the experiment was recorded. Each lake trout was weighed at the start and end of the experiment. Four to nine lake trout from each of the eight tanks were sacrificed at the start of the experiment, and all 10 lake trout remaining in each of the tanks were euthanized at the end of the experiment. We determined concentrations of 75 PCB congeners in the lake trout at the start of the experiment, in the lake trout at the end of the experiment, and in bloaters fed to the lake trout during the experiment. Based on these measurements, γ was calculated for each of 75 PCB congeners in each of the eight tanks. Mean γ was calculated for each of the 75 PCB congeners for both active and inactive lake trout. Because the experiment was replicated in eight tanks, the standard error about mean γ could be estimated. Results from this type of experiment are useful in risk assessment models to predict future risk to humans and wildlife eating contaminated fish under various scenarios of environmental contamination.

Glon M.G.,Central Michigan University | Larson E.R.,Daniel erther Center For Conservation And Research | Larson E.R.,University of Notre Dame | Larson E.R.,Urbana University | Pangle K.L.,Central Michigan University
Hydrobiologia | Year: 2016

Ecological applications of stable isotope analysis are dependent on knowing consumer-diet discrimination factors (Δ) and consumer metabolic turnover rates (m). We used an 80-day laboratory experiment to test for differences in the δ13C, δ15N and m of two species of crayfish (Orconectes rusticus and O. virilis) fed one of two diets (algae wafers and bloodworms). Over the course of the experiment, the δ13C and δ15N signatures of the crayfish approached equilibrium with those of their diets. We fit our data to a growth-based model and found δ13C, δ15N, and m to be largely indistinguishable between species, except in the case of δ15N and m of crayfish on the algae diet. We thus pooled parameters to calculate Δ13C (algae diet: 1.57‰ [95% confidence interval: 0.86–2.35]; bloodworm diet: 0.8‰ [0.14–1.55]) and Δ15N (bloodworm diet: 1.2‰ [0.32–2.11]), and used species-specific data to calculate Δ15N for the algae diet (O. rusticus: 2.54‰ [2.06–3.08]; O. virilis: 3.35‰ [2.53–4.51]). Our results provide values of stable isotope Δ and m for applications to crayfish, and offer a rare comparison of these values between two closely related species and to commonly used literature values. © 2015, Springer International Publishing Switzerland.

Kreps T.A.,University of Notre Dame | Kreps T.A.,Bridgewater College | Larson E.R.,Daniel erther Center For Conservation And Research | Larson E.R.,University of Notre Dame | And 2 more authors.
Freshwater Science | Year: 2016

Linkages between habitats can strongly affect ecosystem function through exchange of energy and materials. In lake food webs, large piscivorous fishes integrate littoral and pelagic energy sources through the consumption of smaller fishes and macroinvertebrates. We tested the hypothesis that the introduction of the invasive rusty crayfish (Orconectes rusticus) decouples littoral and pelagic food webs by reducing the abundance of small fish and noncrayfish benthic macroinvertebrates, thereby shifting diets of large fish to increased dependence on littoral energy sources (i.e., crayfish). We used stable isotope ratios of C (?13C) and N (?15N) to examine differences in resource use between lakes with high abundance of O. rusticus (invaded) and lakes with native crayfish only or with low abundance of nonnative crayfish (uninvaded). The trophic position of juvenile crayfish was lower in invaded than in uninvaded lakes, results suggesting decreased availability of, and reliance on, macroinvertebrate prey and greater dependence on detritus and algae. In invaded lakes, diets of piscivorous fishes relied more strongly on littoral energy sources, and Walleye (Sander vitreus), an obligate piscivore, had a lower trophic position than in uninvaded lakes, results suggesting reduced availability of fish prey. In agreement with another recent stable-isotope study and observations from many smaller-scale but more mechanistic mesocosm and enclosure experiments, these results support the hypothesis that invasion by O. rusticus causes a reduction in the abundance of benthic macroinvertebrates and small fishes and consequently reduces trophic position and increases littoral dependence of large fishes. © 2016 by The Society for Freshwater Science.

Loh T.-L.,University of North Carolina at Wilmington | Loh T.-L.,Daniel erther Center For Conservation And Research | McMurray S.E.,University of North Carolina at Wilmington | Henkel T.P.,Valdosta State University | And 2 more authors.
PeerJ | Year: 2015

Consumer-mediated indirect effects at the community level are difficult to demonstrate empirically. Here, we show an explicit indirect effect of overfishing on competition between sponges and reef-building corals fromsurveys of 69 sites across the Caribbean. Leveraging the large-scale, long-term removal of sponge predators, we selected overfished sites where intensive methods, primarily fish-trapping, have been employed for decades or more, and compared themto sites in remote or marine protected areas (MPAs) with variable levels of enforcement. Sponge-eating fishes (angelfishes and parrotfishes) were counted at each site, and the benthos surveyed, with coral colonies scored for interaction with sponges. Overfished sites had >3 fold more overgrowth of corals by sponges, and mean coral contact with sponges was 25.6%, compared with 12.0% at less-fished sites. Greater contact with corals by sponges at overfished sites was mostly by sponge species palatable to sponge predators. Palatable species have faster rates of growth or reproduction than defended sponge species, which instead make metabolically expensive chemical defenses. These results validate the top-down conceptual model of sponge community ecology for Caribbean reefs, as well as provide an unambiguous justification for MPAs to protect threatened reef-building corals. An unanticipated outcome of the benthic survey component of this study was that overfished sites had lower mean macroalgal cover (23.1% vs. 38.1% for less-fished sites), a result that is contrary to prevailing assumptions about seaweed control by herbivorous fishes. Because we did not quantify herbivores for this study, we interpret this result with caution, but suggest that additional large-scale studies comparing intensively overfished and MPA sites are warranted to examine the relative impacts of herbivorous fishes and urchins on Caribbean reefs. © 2015 Loh et al.

Knapp C.R.,Daniel erther Center For Conservation And Research | Alvarez-Clare S.,North Central College
Journal of Tropical Ecology | Year: 2016

Herbivores are predicted to forage on a variety of plants in order to obtain a nutritionally sufficient diet. Most herbivores, however, forage non-randomly and may be influenced by morphological, chemical and physical traits in their food. We examined the influence of several leaf traits on food selection for the Exuma rock iguana (Cyclura cychlura figginsi). We expected the iguana to prefer leaves with higher nutrient concentration and lower physical defences, such as reflected by high N, P, Ca, K, Mg concentrations and low leaf density and per cent concentrations of cellulose, hemicellulose and lignin, respectively. We quantified selection by examining 30 faecal samples and analysing traits of leaves from the 10 most common plants on the island. Our results showed substantial variability in all measured traits among species but food preference only for less-dense leaves, a good indicator of low leaf toughness. Our results are the first to demonstrate that physical leaf traits can influence food selection in a true herbivorous lizard and offer a basis for future testing. Copyright © 2015 Cambridge University Press.

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