This brief summarizes the paper “Mesopelagic fish biomass in the southern California current system” by Davison et al., which was published in the journal Deep-Sea Research II: Topical Studies in Oceanography


 What is the mesopelagic zone?

The mesopelagic zone is an aquatic layer of the ocean that runs from approximately 200m to 1000m below the surface. It is bordered above by the photic epipelagiclayer and below by the aphotic bathypelagic layer. This means that the mesopelagic zone serves as a transition region between sections of the ocean illuminated by sunlight where plants can grow and sections that sunlight does not reach. Mesopelagic fishes are important marine life inhabiting this zone.

Why is the biomass of mesopelagic fishes relevant?

Because of this layer’s location between the explored and relatively unexplored regions of the ocean, mesopelagic organisms provide a link between deep sea marine life and the surface that is more familiar to humans. Many mesopelagic fishes take part in diel vertical migration, a feeding pattern in which organisms migrate vertically at night in order to feed on plankton in the productive epipelagic layer. These species play an essential role in the movement of organic matter between the surface and deeper parts of the ocean, significantly affecting open-ocean food web interactions. Mesopelagic fishes are the most abundant vertebrates on Earth, and diel vertical migration is the largest migration of animals on the planet.

In scientific literature, the influence of mesopelagic consumers of zooplankton and their role in carbon transport is often overlooked, plausibly due to a lack of knowledge about the biomass of mesopelagic organisms. Biomass is a measurement of the weight of organisms in a given area, measured in metric tons (one metric ton equals 1,000 kilograms).

How did we go about surveying mesopelagic biomass?

We surveyed mesopelagic fish populations in the Pacific Ocean off the coast of Southern California. Our data spans three years and was obtained through a combination of trawl net and sound (sonar) surveys so as to provide the most accurate result possible. In a trawl survey, large nets are used to capture organisms; the number of organisms caught is then used to make an estimate of biomass in an area. Trawl surveys tend to underestimate the biomass of mesopelagic fishes because many organisms are capable of avoiding the nets or simply pass through the mesh. We combined trawl surveys with acoustic survey methods (in which an estimation of abundance is made by using sound waves) to obtain a more accurate result.