# Methodological Pitfalls of Monitoring: Water Conditions Affect the Efficiency of Bottle Traps and Capture Success

## TL;DR

This study shows that water levels and habitat changes affect how well bottle traps catch smooth newts in a Hungarian wetland.

## Contribution

The study reveals how water replenishment and habitat changes impact the effectiveness of passive trapping methods for monitoring aquatic vertebrates.

## Key findings

- Smooth newts were caught at 10% relative frequency, while other species were caught at less than 1%.
- Bottle trap efficiency and capture success declined after increased water volume.
- Male smooth newts were disproportionately caught compared to females after water replenishment.

## Abstract

The Szaporca Old-Drava oxbow, one of Hungary’s earliest wetlands designated under Ramsar Convention, has faced significant threats due to drying. This necessitated the implementation of water replenishment measures. Our study aimed to assess the assemblage of aquatic vertebrates detectable by using bottle traps and to examine how water replenishment influences the traps’ efficiency and capture success. We found that the relative frequency of smooth newts caught with bottle traps was 10%, while that of other amphibian and fish species did not even reach 1%. Following the increase in water volume in the oxbow, both the efficiency of bottle traps (measured as the number of traps that captured smooth newts) and their capture success (number of caught newt individuals) declined significantly. The proportion of male smooth newts caught in traps increased significantly relative to females. Water replenishment also altered flow conditions and increased the extent of inundated areas, which in turn changed the substrate beneath the bottle traps. These environmental changes likely contributed to variations in trap performance. Our finding suggests that both water volume and associated habitat modifications can significantly influence the outcome of monitoring efforts using passive trap methods.

Wetland diversity and associated wildlife is declining globally. The Szaporca Old-Drava oxbow, one of Hungary’s first wetlands designated under the Ramsar Convention, has been threatened by desiccation, prompting the implementation of water replenishment interventions. This study aimed to determine which aquatic vertebrate species can be detected using bottle traps in the oxbow and whether the traps’ efficiency and capture success change following hydrological restoration. Our results showed that the relative frequency of smooth newts caught with bottle traps was 10%, while that of other amphibians (Danube crested newt, common spadefoot toad, edible frog) and fish (European weather loach, European mudminnow, Danube whitefin gudgeon) species did not even reach 1%. Based solely on the relative frequency data of the smooth newt, we found that both the capture efficiency (10.4% vs. 3%) and capture success (17% vs. 7.4%) of bottle traps significantly declined following an increase in water volume in the oxbow. Sex ratio patterns also shifted markedly: in the year of water scarcity, the male-to-female ratio of smooth newt was 2.7:1, whereas during water-abundant conditions it increased to 7:1. Water replenishment raised the water level by nearly one meter, resulting in the partial inundation of terrestrial vegetation and consequent habitat alterations. While water supplementation clearly supports the persistence of rare, protected, strictly protected and endemic aquatic species, our findings highlight the importance of considering hydrological conditions when interpreting the results of long-term monitoring in wetland ecosystems.