Listening for echoes underwater lets scientists find the zooplankton that fish feed on. Having a conversation in a noisy party can be frustrating. It's even more difficult if the other person's whispering. We're out here to listen to whispers in the ocean. The animals we're studying produce very weak echoes when we bounce sonar off them, but these aren't the only sounds we pick up.
Oceans are very noisy.
Snapping shrimp, for example, make a loud click when they snap their claws to defend territory and stun their prey. Noises from ships, other sonars, and a wide range of machinery, all make it hard to hear zooplankton. Even just the electric current that we use to power our computers, oscilloscopes and sonars can produce noise. When we put the ROV in the water for the first test before heading to sea, noise overwhelmed our sonars. We wouldn't be able to distinguish the faint echoes of zooplankton.