Can a sonar used to graph the ocean floors detect through polluted and highly populated waters?
Using sonar to map the ocean floor is not usually affected by pollution or by the occasional fish, if it's a normal herring or cod sized fish. Plastic pollution usually floats or sinks so it isn't a problem. A fish the size of a basking shark would show up on the sonar, but it wouldn't be a problem. Large shoals of fish also show up, but it would have to be a very large shoal indeed to interfere with mapping the ocean floor. There are seasonal shoals large and continuous enough to do this, but only at certain times of the year in a very few locations where they are on migration in their billions. I have never heard of one causing problems for a vessel whose mission was to map the ocean floor or search for a sunken ship. Sonar is sometimes used by trawlers to search for shoals of fish.