Lifetime Trophic Ecology of Fishes and Invasive Species Management in Dryland Streams

Abstract

Invasive species are one of the greatest threats to freshwater fish biodiversity, with numerous documented impacts on freshwater food webs. This threat is especially acute in the American southwest, a region that is home to many endemic freshwater fishes. Invasive fish impacts have been studied most often at discrete points in time for pairs of species, but knowledge gaps remain regarding impacts across ontogeny in multi-species communities. Stable isotope analysis is commonly used to infer trophic ecology, including species interactions through competition and predation. We used a novel method of fish eye lens stable isotope analysis coupled with otolith growth measurements to construct lifetime trophic trajectories for native and nonnative fishes in Burro Creek, Arizona. Our results provide support for asymmetric competition wherein native species exhibited displaced trophic trajectories with respect to δ13C and δ15N, which may lead to reduced fish growth and fitness, with implications for population persistence.Competition for shared resources is a powerful mechanism in community ecology, contributing to the success of many invasive species. The balance between intra- and inter-specific competition is predicted to influence species coexistence, with theoretical and empirical studies historically focusing on single life stages. However, many fish species undergo ontogenetic dietary shifts, resulting in dynamic competitive interactions. Using the lifetime trophic trajectories of fishes from Burro Creek, Arizona, we measured isotopic niche distance as a proxy for competitive interaction strength and found shifts in competition through ontogeny within and between native and nonnative species pairs. Invasive species are increasingly targeted for suppression or eradication to reduce impacts on native communities. We leveraged two invasive green sunfish removal programs in intermittent streams of the Bill Williams River, Arizona, to quantify removal success and explore alternative effort allocation in time and space. Bayesian hierarchical modeling of removal data was used to estimate demographic parameters, finding that both removal programs resulted in at least a 0.39 probability of eradication. Simulated alternative management scenarios revealed that population suppression could be achieved with reduced effort, but eradication success was dependent on the high frequency of removal events, even after zero fish were captured. Given the accelerating spread of invasive species and limited resources to control invasive populations, quantitative removal models are valuable tools for improving and evaluating invasive species removal efforts. Though native to the Columbia River Basin, northern pikeminnow have been the subject of long-term population control. This began in response to their increased predation of anadromous juvenile salmonids as a result of habitat modification due to hydropower dam construction. The Northern Pikeminnow Management Program (NPMP) is a targeted harvest program with a sport reward fishery that has been operating in the Columbia and Lower Snake Rivers since 1991. Northern pikeminnow are tagged annually to model their exploitation rate, and the model used assumes closed populations separated by dams. We studied the movement of northern pikeminnow using passively collected passive integrated transponder (PIT) tag data from 2003-2022. Movement records reveal that individuals traveled at least as far as 968 km. Nearly 13% of northern pikeminnow tagged through the NPMP were detected in different river sections, separated by dams, from where they were initially tagged. Our increased understanding of northern pikeminnow movement informs the calculation of the exploitation rate, one of the key measures of NPMP success, and may contribute to future management strategies.