Ok so the most amazing reblog I've seen of this says:

There's a globe-spanning layer of mesopelagic fish that is so dense it distorts SONAR. For decades we had no idea what created the Deep Scattering Layer or why it moved. We still know almost nothing about it.

It contains 65% of all fish biomass.

Marine Ecology has Dark Fish.

And:

FYSA: 65% is a low-end estimate. Dark Fish may comprise 95% of all fish biomass, but we just don't know: https://phys.org/news/2014-03-ninety-five-cent-world-fish-mesopelagic.html

Apparently 40% to 50% of that hidden biomass is mostly likely these guys, bristlemouths, and they are thought to be the most abundant vertebrates on Earth, numbering in the quadrillions. The mesopelagic zone makes up 20% of the oceans so while the estimates are based on limited sampling, so far bristlemouths have been there wherever scientists go looking.