LAUSR

The invasive fish threaten ponds' ecological status and their ecosystem services. Therefore, obtaining a representative sample of fish community composition is fundamental to fishery management, research, and nature conservation. Estimates of size distribution, density and biomass of topmouth gudgeon (Pseudorasbora parva), model species of invasive fish in three ponds were compared among three sampling methods - electrofishing, fish-trapping, and throw-netting. The study illustrates that the invasive fish, P. parva, can be detected by all tested fishing methods, yet our results clearly showed that there are pronounced differences among methods in population characteristic estimates. Electrofishing and throw-netting gave biased information on size distribution of P. parva. Fish-trapping and throw-netting gave reasonable P. parva density and biomass estimates, while electrofishing clearly underestimated it. All tested methods showed a body size increment of P. parva between summer and autumn sampling sessions, yet neither throw-netting nor electrofishing recorded an increment of its density. Our study showed that fish-trapping is the most reliable and, at the same time, affordable method to estimate invasive P. parva population characteristics in ponds despite more time demanding sampling. The success depends on the mesh size of sampling gear, operator's skill, and habitat structure. Cost-effectiveness of selected methods and the importance of invasive fish monitoring in ponds is discussed. The sampling gear must be considered carefully according to the aim of the monitoring.