https://phys.org/news/2018-11-oceanographers-first-ever-images-entire-cod.html

But the ocean is a murky place, and fish are highly mobile by nature, making them difficult to map and count. Now a team of oceanographers at MIT has journeyed to Norway—one of the last remaining regions of the world where cod still thrive—and used a synoptic acoustic system to, for the first time, illuminate entire shoals of cod almost instantaneously, during the height of the spawning season.

The team, led by Nicholas Makris, professor of mechanical engineering and director of the Center for Ocean Engineering, and Olav Rune Godø of the Norwegian Institute of Marine Research, was able to image multiple cod shoals, the largest spanning 50 kilometers, or about 30 miles. From the images they produced, the researchers estimate that the average cod shoal consists of about 10 million individual fish.