Instrumented longline gears were deployed in collaboration with French commercial longline vessels to study underwater the megafauna capture/escapement processes. Underwater cameras affixed on the branchlines 1-2 metres above the hook deployed on hooks between enabled a precise description of interactions of several shearwater and one Atlantic puffin (Fratercula arctica) species with the baited hooks. Studying and decrypting the underwater behaviours of seabirds provides new insights into their interactions with the fishing gear. Videos showed that seabirds could interact with the fishing gear during the gear soak to a depth of ca. 20m, long after the gear setting ends and after the fishing vessel departed the fishing grounds. These observations revealed cryptic events of successful and non-successful attempt in grabbing and ingesting the bait at depths to ca. 20 m, suggesting that bycatch mitigation approaches are needed during multiple phases of fishing operations, and not just during setting and hauling as is the prevailing paradigm. No birds were observed captured during the underwater video monitoring.