Distribution, Abundance, and Detection Frequency of Lepomis peltastes Cope (Northern Sunfish) in the Illinois Waterway, Illinois

Andrya L. Whitten Harris^1,*, Brandon S. Harris¹, Michael J. Spear¹, Brian A. Metzke², Christopher A. Taylor³, and James T. Lamer¹

¹Illinois River Biological Station, Illinois Natural History Survey, University of Illinois Urbana-Champaign, 704 N. Schrader Avenue, Havana, IL 62644. ²Illinois Department of Natural Resources, Division of Natural Heritage, 1 Natural Resources Way, Springfield, IL 626702. ³Illinois Natural History Survey, Prairie Research Institute, University of Illinois, 1816 S. Oak Street, Champaign, IL 61820. *Corresponding author.

Northeastern Naturalist, Volume 33, Issue 2 (2026): 263–281

First published early online: 31 May 2026

Abstract
Lepomis peltastes (Northern Sunfish) is a small non-game fish of the Centrarchidae family that was recognized as a separate species from L. megalotis (Longear Sunfish) by the American Fisheries Society in 2013 (genetically supported in 2022). Post species designation, the Northern Sunfish was documented in 14 basins in eastern Illinois based on historical collections, yet its distribution and population status has not been documented in the Illinois Waterway (IWW). We used data from the standardized Multi-Agency Monitoring program, which spans 8 navigation pools and >500 rkm of the IWW, to report spatial distribution, length–frequency distribution, relative abundance, and detection frequency of the Northern Sunfish from 2019 to 2024. In this study, the Northern Sunfish was collected in 5 of 8 IWW navigation pools from Brandon Road Pool downstream through Peoria Pool, expanding its known range within the IWW downstream of Ottawa, IL. Estimates of relative abundance from boat electrofishing and mini-fyke net sets were the highest in Dresden Island through Starved Rock pools and generally increased from 2019 to 2024 in all pools (as much as 14-fold in boat electrofishing catch per unit effort). Pool-specific detection frequency increased or remained the same over time. Similar increases in Northern Sunfish relative abundance and detection frequency have not been documented in other known monitoring datasets of Illinois rivers. We were unable to explain the increases in relative abundance and detection frequency of Northern Sunfish through time after looking for similar increases in the relative abundance of other sunfish species or changes in environmental covariates such as the presence of aquatic vegetation. Continued species-identification education and monitoring of Northern Sunfish is needed to track changes in the IWW population and connected tributaries, inform their population status, apprise fish biologists of their current ranges, and aid in classification where Northern Sunfish and Longear Sunfish ranges could overlap in Illinois.

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