Field measurements of sex and size in Chinese mitten crabs (Eriocheir sinensis) from the region of Flanders (Belgium)

Introduction The Chinese mitten crab (Eriocheir sinensis H. Milne Edwards, 1853) is a species of true crab (Crustacea: Decapoda) that is native to eastern Asia, but has been introduced in Western Europe in the early 20th century. The mitten crab is listed as an invasive alien species of Union concern under Regulation (EU) No 1143/2014. This legal regime implies that European Member States must gain a good understanding of the occurrence and population status of the species on their territory. In the Region of Flanders (Belgium), the mitten crab has been the subject of various studies by the Research Institute for Nature and Forest (INBO). These studies often serve requests from local authorities with respect to the species' local status and options for management. The output of these studies can be consulted via the INBO portal (e.g. D'hondt et al. 2022, 2023). The coordination of management is led by Flanders Environment Agency (VMM). Generally, the sex and size of individual crabs become recorded in each of these studies as baseline biometric data. Although subsets of these data have been analyzed for specific reasons, the raw data have not yet been disclosed. Given the number and consistency of these measurements, and the potential interest for scientific or educational purposes, we consider it useful to bring these data together. Objective This dataset collects measurements of sex and size of Chinese mitten crab (Eriocheir sinensis) at the level of individuals. Method Note that the Chinese mitten crab is a catadromous species, living in freshwater environments but spawning in brackish environments. Every spring, a cohort of juvenile crabs migrates upstream; while every fall, a cohort of sexually mature crabs migrates downstream. The spring migration might become targeted by managers for the containment of the species. Campaigns The dataset includes data from the following campaigns ('campaign', coded as location_year-of-sampling): Melsenbeek_2022 : measurements of crabs caught in a lowland stream (Melsenbeek), near its confluence with the river Scheldt, at the time of spring migration. Zwalm_2024 : measurements of crabs caught in a lowland river (Zwalmbeek) and its accompanying fish ladder, at the time of spring migration (2024). Zwalm_2025 : idem (2025). Captures All crabs were obtained by means of capturing devices, either by fykes or so-called 'postbox traps' (see references above). This implies that every crab has been caught somewhere in the time span of the device's deployment. The start and end dates of these deployments are included as 'date_trapping_start' and 'date_trapping_stop'. Measurements The sex of crabs is determined by the shape of the abdomen ('sex'). The size refers to the width of the carapace ('size_mm'). More details on the methods are provided in the Appendix (.pdf). Occurrences The coordinates ('latitude', 'longitude') of the trapping locations are included for reference. The occurrences linked to these observations have been incorporated into datasets that are being published on GBIF, with additional location data included. These GBIF occurrences refer to higher number of crabs (since only a subset of crabs become measured), and thus exclude the biometric data that are the focus of the current dataset. Use of data As of July 2025, the dataset contains 1933 records. Those familiar to R, can easily read the dataset directly from Zenodo.