Trophic niches of native and nonnative fishes along a river-reservoir continuum

Casey A Pennock,Keith B Gido,Phaedra Budy,Mark C Mckinstry,Zachary T Ahrens

doi:10.1038/s41598-021-91730-1

Abstract

Instream barriers can constrain dispersal of nonnative fishes, creating opportunities to test their impact on native communities above and below these barriers. Deposition of sediments in a river inflow to Lake Powell, USA resulted in creation of a large waterfall prohibiting upstream movement of fishes from the reservoir allowing us to evaluate the trophic niche of fishes above and below this barrier. We expected niche overlap among native and nonnative species would increase in local assemblages downstream of the barrier where nonnative fish diversity and abundance were higher. Fishes upstream of the barrier had more distinct isotopic niches and species exhibited a wider range in δ15N relative to downstream. In the reservoir, species were more constrained in δ15N and differed more in δ13C, representing a shorter, wider food web. Differences in energetic pathways and resource availability among habitats likely contributed to differences in isotopic niches. Endangered Razorback Sucker (Xyrauchen texanus) aggregate at some reservoir inflows in the Colorado River basin, and this is where we found the highest niche overlap among species. Whether isotopic niche overlap among adult native and nonnative species has negative consequences is unclear, because data on resource availability and use are lacking; however, these observations do indicate the potential for competition. Still, the impacts of diet overlap among trophic generalists, such as Razorback Sucker, are likely low, particularly in habitats with diverse and abundant food bases such as river-reservoir inflows.

Highlights

Instream barriers can constrain dispersal of nonnative fishes, creating opportunities to test their impact on native communities above and below these barriers
Six nonnative species were only captured downstream of the waterfall in the river downstream and reservoir habitats, and most nonnative fish were in higher abundance downstream of the waterfall (Supplementary Fig. S1)
Variation in species overlap in isotopic space was apparent along the river-reservoir inflow gradient that coincided with changes in fish assemblage composition and likely variation in diversity of energetic pathways moving from lotic to lentic habitats

Summary

Introduction

Instream barriers can constrain dispersal of nonnative fishes, creating opportunities to test their impact on native communities above and below these barriers. Whether isotopic niche overlap among adult native and nonnative species has negative consequences is unclear, because data on resource availability and use are lacking; these observations do indicate the potential for competition. Of competition is often missing in research of freshwater systems, partially because of a lack of data on trophic resource use and availability Because of their rarity and conservation status, it is often difficult to directly assess the diet of imperiled s pecies[37,38,39], limiting our understanding of impacts of nonnative species as resource competitors. Δ15N can provide an estimate of trophic position[44,45], and several informative analytical metrics have been developed to quantify isotopic niche breadth (realized niche) and trophic overlap among species in an a ssemblage[46,47,48]

Methods

Results

Conclusion