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Research to understand the distribution and density of jellyfish is actively being conducted using training ships, but this is hindered by the high cost of manpower and the limitations of the irradiation area. Unmanned aerial vehicles (UAVs or drones), however, provide cost-effective means for assessing marine animal populations. Therefore, we tested the application of UAVs in estimating jellyfish density and probed the altitude-dependent suitability of these devices. We analyzed images obtained by a drone as well as by manual counting and used ImageJ to measure the density of  Nemopilema nomurai  off Sang-Chuja Island, Jeju, South Korea. Analysis of the image obtained at altitudes of 5–120 m allowed for the identification of 2–173 individuals, while 1.49–9.09 individuals were identified per 100 m 2 . Jellyfish density data measured by manual count and by ImageJ did not show any difference below 90 m; however, a difference was presented at altitudes of 100 m (98%) and 120 m (95%). These results demonstrate the potential of drones for jellyfish monitoring and recommend an optimal altitude for observation.

Density Estimation of Nemopilema nomurai (Scyphozoa, Rhizostomeae) Using a Drone