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12 pages, 4 tables, 4 figures.-- This article is licensed under a Creative Commons Attribution 4.0 International License, Long-term monitoring datasets are fundamental to understand physical and ecological responses to environmental changes, supporting management and conservation. The data should be reliable, with the sources of bias identified and quantified. CETUS Project is a cetacean monitoring programme in the Eastern North Atlantic, based on visual methods of data collection. This study aims to assess data quality and bias in the CETUS dataset, by 1) applying validation methods, through photographic confirmation of species identification; 2) creating data quality criteria to evaluate the observer’s experience; and 3) assessing bias to the number of sightings collected and to the success in species identification. Through photographic validation, the species identification of 10 sightings was corrected and a new species was added to the CETUS dataset. The number of sightings collected was biased by external factors, mostly by sampling effort but also by weather conditions. Ultimately, results highlight the importance of identifying and quantifying data bias, while also yielding guidelines for data collection and processing, relevant for species monitoring programmes based on visual methods, CETUS programme was partially conducted within OBSERVA.PT (ref: MAR-01.04.02-FEAMP-0002), supported by the EU FEDER/FEMP and Fundação para a Ciência e Tecnologia (FCT, Portugal) under the Portugal2020 (Lisboa2020, Algarve2020, and MAR2020) Programme; and by the ATLANTIDA project (ref. NORTE-01-0145-FEDER-000040), supported by the Norte Portugal Regional Operational Programme (NORTE 2020), under the PORTUGAL 2020 Partnership Agreement and through the European Regional Development Fund (ERDF). Three Ph.D. fellowships for authors Ana M Correia (SFRH/BD/100606/2014), Ágatha Gil (PD/BD/150603/2020), and Raul Valente (SFRH/BD/144786/2019) were granted by FCT under the auspices of Programa Operacional Regional Norte (PORN), supported by the European Social Fund (ESF) and Portuguese funds (MECTES)