This dataset contains validated presence/absence data of invasive crayfish species in Flanders, Belgium. The data were collected through the citizen science project Craywatch and submitted as observations to Waarnemingen.be. Here they are aggregated to indicate the presence/absence of 7 target species per sampling session. Data processing Observations were quality-controlled and corrected where necessary (e.g. merging split entries, removing duplicates, correcting location identifiers, adding missing location IDs based on volunteer confirmation). The aggregation and validation was done using the following criteria: Sampling session: observations from the same location_id were grouped into a single sampling session (session_id) if they occurred within a contiguous period with gaps no larger than 7 days. The sampling session starts with start_date and ends with end_date. The total number of crayfish caught during a sampling session is indicated in count. Effort calculation: trap_days were calculated as the sum of the maximum number of active traps on each day of the session. Missing daily trap counts were imputed from adjacent days within the session. Validation status (validation_status): valid: assigned if a species was present (presence = 1) OR if the species was absent (presence = 0) with sufficient sampling effort (i.e. at least 12 trapdays). invalid: assigned if the species was absent, (presence = 0) but the sampling effort was insufficient (<12 trapdays) to reliably confirm absence. Source data (observation_ids): the identifiers of the raw observations that constituted to a record are retained. It is possible to look up this records through Waarnemingen.be or GBIF (e.g. for ID 318178915: https://waarnemingen.be/observation/318178915/ or https://www.gbif.org/occurrence/search?occurrence_id=Natuurpunt:Waarnemingen:318178915&advanced=1). Target species Procambarus clarkii Procambarus virginalis Procambarus acutus Faxonius limosus Faxonius virilis Pacifastacus leniusculus Pontastacus leptodactylus Column definitions session_id: Unique identifier for the aggregated sampling session (location_id + sequence number). location_id: Unique identifier for the sampling location. latitude: Latitude in decimal degrees (WGS84). longitude: Longitude in decimal degrees (WGS84). start_date: Date when the sampling session started (YYYY-MM-DD). end_date: Date when the sampling session ended (YYYY-MM-DD). trap_days: Total sampling effort (sum of active traps per day), used to determine validation status. validation_status: Reliability of the record: valid (proven presence or validated absence) or invalid (absence with insufficient effort). scientific_name: Scientific name of the target species. presence: Binary indicator: 1 = Present, 0 = Absent. count: Total number of individuals caught during the session. observation_ids: Semicolon-separated list of original observation IDs from Waarnemingen.be (for traceability). Only populated for presences.