Partial nocturnal behaviour of giant mottled eels, Anguilla marmorata , in a small river, Amami‐Oshima Island, Japan, from acoustic telemetry

Anguillid eels typically exhibit nocturnal behaviour, but giant mottled eels A. marmorata are often observed during daytime in rivers on Amami-Oshima Island, Japan. We investigated their diel activity patterns and spatial use with stationary-receiver acoustic telemetry. Three tagged eels demonstrated partially nocturnal behaviour with some daytime activity. They displayed strong site fidelity to specific receiver locations, with occasional longer excursions to distant receiver locations. Because A. marmorata is a unique large-bodied apex predator in tropical rivers and likely faces little predation risk, it might not need to restrict activity to night to avoid predators.