Spawning-condition Japanese eels Anguilla japonica, fertilized eggs, and newly-hatched preleptocephali have been captured, and studies for observing spawning eels with underwater camera systems have begun. This study describes a new, less invasive, free-drifting underwater camera observation system that was deployed from the research vessel (R/V) Natsushima in June 2013. Three drifting buoy camera systems (Una-Cam) with lights-on/lights-off programmed sequencing during daytime and nighttime hours were deployed over a period of seven days at 20 locations south of a salinity front along the southern West Mariana Ridge. Live artificially matured A. japonica eels held in transparent chambers were used as an attractant source through the release of reproductive pheromones and other odors. Each system was suspended from a buoy array at a depth of 174–200 m, with four cameras and three lights pointed downward at different angles towards the eel chamber. The Una-Cam systems were stable and were effective at recording images of fish, crustaceans, and gelatinous zooplankton. Olfactory cues may have attracted male and female Derichthys serpentinus eels, which showed what seemed to be reproductive behavior and attraction to the Japanese eels in the chamber. Una-Cam systems are capable of recording images of anguillid eels, if they approach, and may be useful for observing spawning eels in their offshore spawning areas.
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