Data from: Impact of the invasive alien topmouth gudgeon (Pseudorasbora parva) and its associated parasite Sphaerothecum destruens on native fish species

Two datasets belonging to the paper "Impact of the invasive alien topmouth gudgeon (Pseudorasbora parva) and its associated parasite Sphaerothecum destruens on native fish species" published in Biological Invasions (https://doi.org/10.1007/s10530-019-02114-6), is provided here. The first dataset consists of individual measured and weighed fish per sampled water body. In case a large number (>50) of the same species and length were encountered, a representative number was weighed and measured, and the remaining individuals were only counted. The second dataset consists of parameters related to morphology and water quality, and number of specimens found per fish species, of each sampled water body. Below, methodological information is provided on the study site, the sampling process, and the water sample analysis. For references, see the published paper in Biological Invasions.
 
Study site
We selected 54 water bodies (oxbow lakes, shallow lakes and ponds) in river floodplains of the IJssel, Meuse, Nederrijn and Waal River. These water bodies were selected using the following criteria: a) Potential presence of P. parva according to the Dutch National Database Flora and Fauna, b) No permanent hydrological connection with the main stream or a side channel, c) Similarity in habitat characteristics (e.g., depth and surface area, for habitat characteristics per sampling site), d) Suitability for sampling with a seine net. These criteria were set to reduce variance in the fish species composition created by habitat variables, as our aim was to detect effects caused by P. parva. The areas of sampled water bodies ranged from 100 to 80,000 m2. In total 54 sites located in the floodplains were visited and sampled using a seine net (21 m long, 2.4 m high, mesh size 4x4 to 10x10 mm). Fifteen sites could not be sampled sufficiently with this gear type due to high vegetation cover and/or water depth. Hence, 39 sites were included in the analyses of effect on fish assemblages and body condition. The coordinate system used here concerns Amersfoort RD (EPSG: 28992). 
 
Sampling
Sampling of the fish populations was carried out from October to December 2015. The seine net was used while wading and provided adequate data on juvenile and small fishes in shallow habitats. The sampling area ranged from 0.04 to 82.35% of the surface area of water bodies and was used to calculate fish densities (number of fish m-2). All caught fishes were identified, weighed (accuracy 0.05 g) and their total lengths (TL, from tip of snout to longer caudal fin lobe, accuracy 1 mm) measured in the field. Young of the year (YOY) were distinguished, based on length. Each individual was assigned to being a YOY, based on known YOY thresholds in the Netherlands. Small fishes (<35 mm) were pooled for weighting. In case a large number (>50) of the same species and length were encountered, a representative number was weighed and measured, and the remaining individuals were only counted. Subsequently, the fishes were released. 
Habitat and soil parameters which included coverage percentage of aquatic vegetation, littoral vegetation, and substrate (mud, sand, gravel and rocks), and tree branches in the water were visually estimated. The water transparency (cm) was determined using a Secchi disk (measured vertically). Water temperature (°C), conductivity (μS cm-1) and salinity (PSU) were measured at the site with the use of a Model 30 meter (YSI incorporated). A water sample was taken and at the same day pH and alkalinity (eq l-1) were measured in the laboratory. Water samples in polyethylene bottles were stored in the freezer at a maximum storage time of 75 days until analysis. Metal ions were analysed using an ICP analyser (Thermo Electron corporation IRIS Intrepid ΙΙ XDL). Concentrations of nitrate (NO3-), ammonium (NH4+), phosphate (PO43-), chloride (Cl-) and potassium (K+) were determined using an Auto Analyzer 3 system (Bran and Luebbe, Norderstedt Germany). Physico-chemical data is missing for site 39 due to loss of the sample.
 
Abstract
The Asian cyprinid Pseudorasbora parva is considered to be a major threat to native fish communities and listed as an invasive alien species of European Union concern. Our study aims to gain evidence-based knowledge on the impact of both P. parva and it parasite Sphaerothecum destruens on native fish populations by analysing fish assemblages and body condition of individuals of native fish species in floodplain water bodies that were invaded and uninvaded by P. parva. We explored the use of environmental DNA (eDNA) techniques to detect S. destruens. Prevalence of S. destruens in native fish species was assessed. Fish samplings showed significantly negative correlations between the abundance of P. parva and the native Leucaspius delineatus, and Pungitius pungitius and three biodiversity indices of the fish assemblages (Simpson's diversity index, Shannon-Wiener index and evenness). Contrastingly, the abundances of the native Gasterosteus aculeatus and P. parva were positively related. In nearly all isolated water bodies with P. parva, this species is outnumbering native fish species. No effect of P. parva presence was found on body condition of native fish species. Sphaerothecum destruens was demonstrated to occur in both P. parva and G. aculeatus. Gasterosteus aculeatus is suggested to be an asymptomatic carrier that can aid the further spread of S. destruens,. Analysis of eDNA proved to be a promising method for early detection of S. destruens, here showing that S. destruens presence coincided with P. parva presence. The ongoing invasion of both P. parva and S. destruens is predicted to pose a significant risk to native fish communities.