Preliminary Evaluation of Barrens Topminnow (Fundulus julisia) Behavioral Response Along a Density Gradient of Western Mosquitofish (Gambusia affinis)

Jacob T. Westhoff^1,2, Hayden T. Mattingly^3,*, and William G. Wells⁴

¹Department of Biology, Box 5063, Tennessee Tech University, Cookeville, TN 38505. ²Current address - School of Natural Resources, University of Missouri, Columbia, MO 65211. ³School of Environmental Studies, Box 5152, Tennessee Tech University, Cookeville, TN 38505. ⁴Department of Biology, Sewanee: The University of the South, Sewanee, TN 37383. *Corresponding author.

Southeastern Naturalist, Volume 24, Special Issue 13 (2025): 27–40

First published early online: 21 March 2026

Abstract
Invasive Gambusia affinis (Western Mosquitofish [WMF]) are spreading across the Barrens Plateau region of Tennessee and contributing to the imperilment of the native, federally listed Fundulus julisia (Barrens Topminnow [BTM]). Aggression by WMF may induce an avoidance behavior such that BTM leave preferred habitats for less favorable locations, thereby compromising BTM population viability. We conducted 2 laboratory experiments to determine whether BTM would co-inhabit prime habitat with WMF or avoid WMF and occupy non-prime habitat. Conditions for both experiments consisted of a tank simulating prime habitat connected by a channel to an equal-sized tank with non-prime habitat. The first experiment (250-min duration) examined BTM habitat selection when exposed to accruing WMF densities. Habitat use of adult BTM was monitored as WMF were added gradually until they comprised 90% of the test community. BTM spent significantly more time in prime habitat until WMF represented >89% of the community, at which point topminnows rapidly left prime habitat. The second experiment (7-d duration) examined BTM habitat selection when exposed to static WMF densities in an array of tanks with different WMF:BTM ratios. Here, BTM use of prime habitat in daylight hours was negatively and linearly related to WMF density; however, there was no significant habitat selection during dark hours. Both experiments revealed either a threshold-type or linear-type avoidance behavior of adult BTM to increasing WMF densities. Taken with findings of other studies, our results suggest that BTM conservation efforts would be enhanced by regular monitoring of WMF densities in the Barrens region, along with deterrence or prevention of WMF access to sites inhabited by BTMn.

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