Citation: Hotopp, I.; Walther, B.; Fuelling, O.; Reil, D.; Hesse, C.; Below, D.A.; Imholt, C.; Jacob, J. Habitat and Season Effects on Small Mammal Bycatch in Live Trapping. Biology 2022, 11, 1806. https:// doi.org/10.3390/biology11121806 Academic Editor: Carles Vilà Received: 18 November 2022 Accepted: 7 December 2022 Published: 13 December 2022 Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affil- iations. Copyright: © 2022 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). biology Article Habitat and Season Effects on Small Mammal Bycatch in Live Trapping Ines Hotopp 1 , Bernd Walther 2 , Olaf Fuelling 1 , Daniela Reil 2 , Christin Hesse 2 , Diana Alexandra Below 2 , Christian Imholt 2 and Jens Jacob 2, * 1 tier3 Solutions GmbH, Kolberger Str. 61–63, 51381 Leverkusen, Germany 2 Rodent Research, Institute for Epidemiology and Pathogen Diagnostics, Julius Kühn-Institute (JKI)–Federal Research Centre for Cultivated Plants, Toppheideweg 88, 48161 Münster, Germany * Correspondence: jens.jacob@julius-kuehn.de Simple Summary: Trapping particular small mammal species is frequently used for scientific pur- poses but unnecessary bycatch can occur. Live trapping conducted over the last decade in Ger- many using Ugglan multiple capture traps in grassland, forest and margin habitats revealed about 30% bycatch when target species were common voles (Microtus arvalis) in grassland and common voles and bank voles (Clethrionomys glareolus) in margins and forests. This was more pronounced in spring and along margins. Species mentioned on the early warning list according to the Red List Germany were higher in numbers and proportion in spring and in grassland. The results of the study will help to avoid periods with enhanced presence of bycatch including endangered species (if the purpose of the study allows) or to pay particular attention in certain seasons and habitats when the occurrence of bycatch is most likely. Abstract: Trapping small mammals is frequently used to study the dynamics, demography, behavior and presence of pathogens. When only particular small mammal species are in the focus of interest, all other species are unnecessary bycatch. We analyzed data from extensive live trapping campaigns conducted over the last decade in Germany, following a consistent standard trapping protocol that resulted in about 18,500 captures of small mammals. Animals were trapped with Ugglan multiple capture traps in grassland, forest and margin habitat. Trap success and the proportion of bycatch were about 30% when target species were common voles (Microtus arvalis) in grassland and common voles and bank voles (Clethrionomys glareolus) in margins and forests. This was more pronounced in spring and along margins. Species mentioned in the early warning list according to the Red List Germany were higher in numbers and proportion in spring and in grassland. The results will help to avoid periods with enhanced presence of bycatch, including endangered species (if the purpose of the study allows) or to pay particular attention in certain seasons and habitats when the occurrence of bycatch is most likely. Keywords: conservation; non-target species; endangered species; rodents; Ugglan traps; voles 1. Introduction Capturing wildlife is important to humans as wild animals are recognised as a source of nutrients and material for shelter, tools, medicine and cultural objects, etc. For millennia, wild animals have been vital in the process of domestication, for support in hunting and herding, lifting and transporting objects and other work, religious rites and as guards, as well as companions. The importance of wildlife in these aspects was later supple- mented by the relevance of wildlife in research. In many cases, the species required needs to be obtained. While capture for food does not necessarily require the animal to stay alive, their use for observational studies does. In some aspects of research such as the study of animal Biology 2022, 11, 1806. https://doi.org/10.3390/biology11121806 https://www.mdpi.com/journal/biology