The ocean is divided into what are known as “pelagic zones” which are basically just layers divided up by how much sunlight the water receives, water pressure, and temperature. The surface layer is known as the epipelagic (sunlit) zone, as it receives the most light and is warmest. It then progresses downward, through the mesopelagic, bathypelagic, abyssopelagic, and the hadopelagic, which is the water more than 6,000 meters below the surface. 

A recent international collaboration of marine biologists led by Carlos Duarte from the University of Western Australia have discovered that around 95% of all of the fish biomass in the ocean live in the mesopelagic zone; about 10-30 times higher than previously believed. The results were published as open access in the journal Nature Communications.

The mesopelagic is known as the twilight zone, due to the diminished levels of light. The water in this region ranges from 200-1,000 meters below the surface, which can be seen on echo-locators, but does not have enough penetrating light to drive photosynthesis. The fish that live here feed on feces or dead organisms that have floated down from the epipelagic zone, herbivores that eat epipelagic plants at night but hide from birds during the day, and carnivores that feed on the scavengers and herbivores. 

Calculating the amount of biomass in this region has been attempted in the past, but it turns out that the fish are very adept at avoiding the nets used to capture and measure them. When Duarte and his team set sail to circumnavigate the globe for seven months, they relied much more on sonar and echolocation to view the fish, as opposed to physically collecting them. They concluded that there are approximately 1 billion tons of fish living in this zone; which dwarfs the amount of biomass in any other zone.

These results have some large implications about how marine ecosystems are affected by ocean gyres; the currents which affect the top 2,000 meters of the water. It was found that mesopelagic fish respire about 10% of all primary production in the water, which is more efficient than previously believed. 

Though many species of fish in the epipelagic are feeling the pressure from overfishing, pollution, and habitat destruction, that isn’t really the case for those in the mesopelagic. As the researchers discovered, the fish are great at avoiding nets. That means that these populations are essentially untouched by fishermen and are described as “pristine.” However, since the data was gathered acoustically and physical samples weren’t taken, it becomes hard to determine how these fish might be impacted by pollution.