Why so Blue? Assessing the Accuracy of Blue and Nile Tilapia (Oreochromis spp.) Field Identifications in South Florida

Janelle A. Goeke^1,2,3,*, Lauren Ballou^1,3, Heather Bracken-Grissom¹, and Nathan J. Dorn¹

¹Florida International University, Institute of Environment, Department of Biological Sciences, North Miami, FL 33199.²University of Southern Maine, Casco Bay Estuary Partnership, Portland, ME 04101.³Berea College, Biology Department, Berea, KY 40403. *Corresponding author.

Southeastern Naturalist, Volume 25, Issue 2 (2026): 237–253

First published early online: 31 May 2026

Abstract
Oreochromis aureus (Blue Tilapia) and Oreochromis niloticus (Nile Tilapia) are both established in South Florida freshwater systems, but are rarely distinguished in the field due to their similar morphology, making it impossible to disentangle the ecological impacts of these 2 invasive fishes. The typical morphological characteristic used to differentiate these 2 species is the presence of clear barring on the caudal fin of Nile Tilapia. We assessed the accuracy of field identifications based on this characteristic by sampling tilapia from freshwater systems in South Florida, identifying them in the field, then subsequently examining them via molecular analyses. As far as we are aware, this study is the first to directly compare caudal-fin barring patterns with molecular identifications in these species. We constructed a 3-gene phylogeny to compare morphological and molecular identities. Morphological and molecular identifications matched only 72% of the time, with most mismatches occurring for Blue Tilapia individuals misidentified in the field as Nile Tilapia. Of the misidentified fish, a majority had ambiguous caudal fin barring and discordant placement between nuclear and mitochondrial single-gene trees; suggesting hybridization may play a significant role in inaccurate taxonomic placement. Better understanding of hybridization and the ecology of hybrid crosses is needed to understand population distributions and ecological impacts of these species in South Florida.

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