The first thing we need to know to help save whales from extinction is how many of each type of whale there are, and where they are located. You might think it would be easy to count very large whales, but they can be hard to find in the deep offshore waters. Scientists also want to know where whales might get together to be social, feed, and breed, as these actions are very important for whales' survival. Scientists also want to know other things, like whether the whales have enough food, and whether people are changing the oceans in ways that make it harder for whales to live.
Sometimes scientists cannot look for the whales in person—it might be hard, dangerous, or just too far. Instead, we use underwater microphones, called hydrophones to listen for them. We will describe how hydrophones were used to listen for whale calls in the Canadian Pacific Ocean, off the coast of Vancouver Island. These hydrophones were located both on the ocean floor and on ocean gliders, which are small submarines.

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Go to source: https://kids.frontiersin.org/article/10.3389/frym.2020.00055