Measuring seawater velocity using drone imaging of jellyfish swarms

Tracking individual jellyfish from a fixed drone provides information about their propagation velocity, the sum of the seawater and jellyfish swimming velocities. Using the drone to measure the jellyfish swimming velocity requires, therefore, measurements of the water velocity at the time and vicinity of the jellyfish. Obtaining such measurements is challenging. Instruments such as ADVs, ADCPs, and drifters are difficult to install, release, and operate, especially when the water velocity must be measured near the jellyfish individuals. To overcome this, we propose a method, by which each jellyfish pair provides a non-intrusive measurement of the horizontal water velocity near the jellyfish. The method is based on measuring the horizontal body orientation and the propagation velocity of two jellyfish relative to the drone frame of reference. A vector summation results in a formulation of the horizontal water velocity components from which the jellyfish swimming, relative to the water frame of reference, is calculated. We have tested the accuracy of the method by computer simulations and applied it in-situ by tracking >1100 jellyfish during three campaigns near the Haifa Bay. We found that the horizontal swimming velocity of Rhopilema nomadica is 10 +/- 1 cm/s. We will present the new method, the simulation results, and the impact of the swimming velocities on the where about of jellyfish swarms. The potential implementation of the method using other swimming and flying animals will be discussed.