Photos and measurements of otoliths from fish caught in the southern part of the North Sea is a photo dataset published by the Research Institute for Nature and Forest (INBO). It contains photos of fish otoliths – taken by a binocular microscope on a black background and edited to include a ruler – and measurement/reference data for these otoliths and fish. The dataset can be used as reference data to identify fish species in the southern part of the North Sea based on their otoliths, e.g. dietary/food ecology research of seabirds. Files reference_data.csv: Fish information (scientific name, sampling information, length) and otolith measurements (width/height). Each row corresponds to a fish, which fish_id can be found in the otolith photo name. Scientific name.zip: Photos of otoliths, grouped by species. Photo names are of the form fish_id_suffix.jpg, where suffix indicates if it was a left (Lx) or right (Rx) sagittal otolith or a smaller lapillus/asteriscus (Bx) otolith.