As part of the Japan/East Sea (JES) initiative supported by the U.S. Office of Naval Research, we conducted an observational experiment to understand the physics of the mesoscale circulation in the Ulleung Basin, located in the southwestern corner of the JES. The current passing through the Korea Strait divides upon entering the JES, with portions of the current flowing along the Korean and Japanese coasts. The variability of these currents is especially energetic in our study region. Our objectives were to measure the time-varying currents in the upper and deep levels of the JES. We relate the population density of vertically migrating fish or squid to the time-varying locations of fronts.