Sampling bias of invasive gibel carp and threatened crucian carp: Implications for conservation

Abstract

Invasive alien species cause significant loss of biodiversity. The presence of invasive alien species is often left unnoticed until they become highly abundant, and early detection by citizens can serve as an early indicator of invasion. Here, we aimed to investigate the detection probability of invasive gibel carp (Carassius gibelio) and native critically endangered species, crucian carp (Carassius carassius), using typical methods employed by conservation agencies (fyke nets) and citizens (angling). Three sites with native crucian carp, nine sites with species syntopy and five sites with invasive gibel carp were investigated. The proportion of species caught by each method was compared using catch per unit effort (CPUE) as a measure of fish density and calculated as the number of fish of each species caught per hour. Species detectability by trapping and angling and the effects of waterbody type on the species catch success were analysed using a Beta regression model. Although the species are closely related and occupy similar positions in the ecosystem, the results showed a high affinity of invasive gibel carp for detection by angling (citizen method), while native crucian carp showed higher catch success in trapping (common professional monitoring method), to an extent that one species can be easily missed when sampled by only one of the methods. These results bring two important findings, namely the need to include citizen knowledge as an indicator of the progress of invasive gibel carp, and the potential danger of underestimating the invasion by conservationists and scientists. The results also suggest that the presence of native crucian carp in syntopy with invasive gibel carp may be overlooked by citizens. This study provides an example of detection bias that can hinder conservation efforts if neglected.