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Changes to the trophic structure of a desert fish community following river regulation and species turnover: implications for an endangered top predator
  • Eliza I. Gilbert,
  • Nathan Franssen
Eliza I. Gilbert
US Fish and Wildlife Service Southwest Region

Corresponding Author:[email protected]

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Nathan Franssen
US Fish and Wildlife Service Southwest Region
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A poor understanding of factors leading to species decline can result in inefficient or ineffective species restoration. Endangered Colorado Pikeminnow ( Ptychocheilus lucius) was nearly extirpated from the San Juan River, NM, USA and recent efforts to reestablish the species via hatchery augmentation of juveniles has yet to reach the targeted number of adults in the system. To assess how changes to the river’s food web could be limiting reestablishment of this top predator, we used stable isotope (δ 13C and δ 15N) signatures of the fish community pre- (museum specimens) and post river regulation with coincident extirpations and invasions. Following river regulation, four of five community-wide trophic structure metrics used to quantify resource use diversity were reduced and species turnover likely had little effect per se. For species sampled in both time periods, shared trophic resources generally increased (mean niche overlap = +35.7%). However, Colorado Pikeminnow experienced a large decrease in niche breadth (-72%) and diet mixing models suggested a shift from piscivory towards insectivory in contemporary collections. Our results suggested an overall reduction in basal resource availability after river regulation. We suggest increased reliance on similar resources may be limiting fish prey for Colorado Pikeminnow, ultimately contributing to the slow and limited reestablishment in the San Juan River. This study adds to the growing body of knowledge provided by museum specimens and stable isotope analyses to identify food-web dynamics that are a departure from historical conditions which can provide novel information critical to imperiled species management in modified systems.
20 Sep 2022Submitted to River Research and Applications
20 Sep 2022Submission Checks Completed
20 Sep 2022Assigned to Editor
23 Sep 2022Review(s) Completed, Editorial Evaluation Pending
10 Oct 2022Reviewer(s) Assigned
11 Nov 2022Editorial Decision: Revise Major
13 Jan 20231st Revision Received
19 Jan 2023Submission Checks Completed
19 Jan 2023Assigned to Editor
19 Jan 2023Review(s) Completed, Editorial Evaluation Pending
01 Feb 2023Reviewer(s) Assigned
17 Mar 2023Editorial Decision: Accept