Walleye Trophic Position Before and After a Gizzard Shad Extirpation

Walleye (Sander vitreus) are an ecologically and recreationally important sport fish species. Reduced growth and condition in walleye can occur when prey availability is limited. In two Nebraska reservoirs, walleye consumed gizzard shad (Dorosoma cepedianum) as their primary prey until a winterkill extirpated the gizzard shad in 2001. Because of the winterkill, walleye in the two reservoirs had to change to alternative prey items. Our objective was to determine if stable isotope analysis on archived walleye scales can be used to detect a known food web shift in two reservoir food webs. We quantified the changes in walleye trophic position following the loss of gizzard shad using stable isotope analysis of carbon (δ13C) and nitrogen (δ15N) from archived scales. Walleye δ15N decreased and δ13C increased in both reservoirs after the extirpation of gizzard shad, indicating walleye likely fed at a lower trophic level on more benthic or littoral prey resources post winterkill. A replacement of gizzard shad by white perch (Morone americana) in Pawnee Reservoir may have ameliorated the loss of gizzard shad; in the other system, walleye appeared to feed on a wider variety of prey items as indexed by increased δ13C variability. Our results indicated that walleye were robust to gizzard shad extirpation

Walleye Trophic Position Before and After a Gizzard Shad Extirpation

Walleye (Sander vitreus) are an ecologically and recreationally important sport fish species. Reduced growth and condition in walleye can occur when prey availability is limited. In two Nebraska reservoirs, walleye consumed gizzard shad (Dorosoma cepedianum) as their primary prey until a winterkill extirpated the gizzard shad in 2001. Because of the winterkill, walleye in the two reservoirs had to change to alternative prey items. Our objective was to determine if stable isotope analysis on archived walleye scales can be used to detect a known food web shift in two reservoir food webs. We quantified the changes in walleye trophic position following the loss of gizzard shad using stable isotope analysis of carbon (δ13C) and nitrogen (δ15N) from archived scales. Walleye δ15N decreased and δ13C increased in both reservoirs after the extirpation of gizzard shad, indicating walleye likely fed at a lower trophic level on more benthic or littoral prey resources post winterkill. A replacement of gizzard shad by white perch (Morone americana) in Pawnee Reservoir may have ameliorated the loss of gizzard shad; in the other system, walleye appeared to feed on a wider variety of prey items as indexed by increased δ13C variability. Our results indicated that walleye were robust to gizzard shad extirpation

Disentangling the effects of a century of eutrophication and climate warming on freshwater lake fish assemblages

<div><p>Eutrophication and climate warming are profoundly affecting fish in many freshwater lakes. Understanding the specific effects of these stressors is critical for development of effective adaptation and remediation strategies for conserving fish populations in a changing environment. Ecological niche models that incorporated the individual effects of nutrient concentration and climate were developed for 25 species of fish sampled in standard gillnet surveys from 1,577 Minnesota lakes. Lake phosphorus concentrations and climates were hindcasted to a pre-disturbance period of 1896–1925 using existing land use models and historical temperature data. Then historical fish assemblages were reconstructed using the ecological niche models. Substantial changes were noted when reconstructed fish assemblages were compared to those from the contemporary period (1981–2010). Disentangling the sometimes opposing, sometimes compounding, effects of eutrophication and climate warming was critical for understanding changes in fish assemblages. Reconstructed abundances of eutrophication-tolerant, warmwater taxa increased in prairie lakes that experienced significant eutrophication and climate warming. Eutrophication-intolerant, warmwater taxa abundance increased in forest lakes where primarily climate warming was the stressor. Coolwater fish declined in abundance in both ecoregions. Large changes in modeled abundance occurred when the effects of both climate and eutrophication operated in the same direction for some species. Conversely, the effects of climate warming and eutrophication operated in opposing directions for other species and dampened net changes in abundance. Quantifying the specific effects of climate and eutrophication will allow water resource managers to better understand how lakes have changed and provide expectations for sustainable fish assemblages in the future.</p></div

Stressor-specific changes in abundance.

<p>Stressor-specific changes in model-estimated standardized relative abundance (z-score of log_e(CPE+0.1)) presented in units of standard deviations for 130 Minnesota lakes in the prairie, 508 lakes in the transition, and 598 lakes in the forest ecoregions from 1896 to 2010. TW = eutrophication-tolerant warmwater, TC = eutrophication-tolerant coolwater, IW = eutrophication-intolerant warmwater, and IC = eutrophication-intolerant coolwater.</p

Changes in abundance by tolerance and guild.

<p>Changes in abundance by tolerance and guild.</p

Climate and lake productivity changes.

<p>Changes in Minnesota lakes from (a) climate warming (MAT—mean annual air temperature °C) and (b) eutrophication (TP—mean summer epilimnetic total phosphorus concentrations μg/l) from pre-disturbance (1896–1925) to contemporary periods (1981–2010) for 1,236 lakes in Minnesota. Figure insets are box plots of interquartile ranges. The background in (a) represents Level 1 ecoregions [<a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0182667#pone.0182667.ref045" target="_blank">45</a>] and (b) simplified land cover classes derived from the 2001 National Land Cover Database [<a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0182667#pone.0182667.ref044" target="_blank">44</a>].</p

Species list.

<p>Species list.</p

Niche space changes.

<p>Trajectories of estimated 2-dimensional niche space changes in climate (mean annual air temperature °C) and productivity (mean summer epilimnetic total phosphorus concentration μg/l) in 1,236 Minnesota lakes from 1896 to 2010.</p

Improved understanding and prediction of freshwater fish communities through the use of joint species distribution models

Two primary goals in fisheries research are to (i) understand how habitat and environmental conditions influence the distribution of fishes across the landscape and (ii) make predictions about how fish communities will respond to environmental and anthropogenic change. In inland, freshwater ecosystems, quantitative approaches traditionally used to accomplish these goals largely ignore the effects of species interactions (competition, predation, mutualism) on shaping community structure, potentially leading to erroneous conclusions regarding habitat associations and unrealistic predictions about species distributions. Using two contrasting case studies, we highlight how joint species distribution models (JSDMs) can address the aforementioned deficiencies by simultaneously quantifying the effects of abiotic habitat variables and species dependencies. In particular, we show that conditional predictions of species occurrence from JSDMs can better predict species presence or absence compared with predictions that ignore species dependencies. JSDMs also allow for the estimation of site-specific probabilities of species co-occurrence, which can be informative for generating hypotheses about species interactions. JSDMs provide a flexible framework that can be used to address a variety of questions in fisheries science and management.The accepted manuscript in pdf format is listed with the files at the bottom of this page. The presentation of the authors' names and (or) special characters in the title of the manuscript may differ slightly between what is listed on this page and what is listed in the pdf file of the accepted manuscript; that in the pdf file of the accepted manuscript is what was submitted by the author