Basking sharks (Cetorhinus maximus) in the Northeast Atlantic show high occurrence of parasitism by sea lamprey (Petromyzon marinus)

Abstract Sea lamprey ( Petromyzon marinus ) are anadromous fish that spend one to two years as haematophagous ectoparasites in marine environments. This study provides the first quantitative insight into their feeding phase, which remains poorly understood. We investigate the occurrence of P. marinus feeding on basking sharks ( Cetorhinus maximus ) in the Northeast Atlantic. Between 2011 and 2020, 775 records of basking sharks were collected via video, photographs and written observations. After quality control, 434 records were analysed. Of these, 41.8% showed lampreys attached. The number of lampreys per shark ranged from one to eight, with an average of 1.9 (± 1.4 SD). Lampreys were significantly more likely to attach to the ventral surface, although attachment site was not linked to shark sex. We discuss mechanisms of attachment, as well as the role of shark migratory behaviour and olfactory cues from coastal aggregations that may increase opportunities for P. marinus to attach. This study provides the first evidence of a significant ecological association between the two species, suggesting C. maximus may be a preferred or more accessible host. These findings offer new insight into lamprey ecology and may inform understanding of how this species could respond to future environmental change.