Goldfish, gibel carp, or crucian carp? Testing public recognition of Carassius species

Crucian carp ( Carassius carassius ) has declined across much of Europe and is listed as critically endangered in the Czech Republic. A major threat is the spread of non-native congeners, especially gibel carp ( Carassius gibelio ) and goldfish ( Carassius auratus ), which overlap in habitat use and can hybridise with crucian carp. Because these taxa are morphologically largely similar, public misidentification may bias citizen science monitoring and lead to counterproductive freshwater management by hobbyists or stakeholders. We quantified photo-based species recognition during the recreational angling fair “For Fishing 2023” in Prague (Czechia) using a three-alternative forced-choice task with printed photographs representing three individuals per species. After quality control, responses from 325 participants were retained for analysis, yielding 2925 trials. Overall accuracy across answered trials was 64.3%. Descriptively, accuracy was highest for goldfish, intermediate for crucian carp, and lowest for gibel carp; however, mixed-model analyses indicated that performance was dominated by image-level heterogeneity, with some photographs consistently easy or difficult regardless of respondent background. Misidentifications were directional: small gibel carp were commonly labelled as crucian carp, and a naturalised goldfish phenotype was also frequently classified as crucian carp. These results showed that photo-based Carassius records can contain systematic, phenotype-driven errors, supporting the need for uncertainty recording, validation workflows, and public training materials that explicitly include ambiguous phenotypes rather than only prototypical phenotypes. Although live individuals may be easier to identify, misidentification may still represent one pathway contributing to the spread of invasive Carassius species in Czechia.