Carbon export mediated by mesopelagic fishes in the northeast Pacific Ocean

•The biomass of mesopelagic fishes increases with NPP off the USA west coast.•Active transport of carbon by mesopelagic fishes is 15–17% of total carbon export.•Active transport of carbon by fishes varies spatially in magnitude and importance.•Active transport of carbon by fishes is equal to passive flux at a depth of 400 m.•Active transport fills most of the gap between sediment trap data and models.

The role of fishes in the global carbon cycle is poorly known and often neglected. We show that the biomass of mesopelagic fishes off the continental USA west to longitude 141°W is positively related to annual net primary productivity, and averages 17 g m−2. We estimate the export of carbon out of the epipelagic ocean mediated by mesopelagic fishes (“fish-mediated export”; FME) with individual-based metabolic modeling using the catch from 77 mesopelagic trawls distributed over the study area. FME was 15–17% (22–24 mg C m−2 d−1) of the total carbon exported in the study area (144 mg C m−2 d−1), as estimated from satellite data. FME varies spatially in both magnitude and relative importance. Although the magnitude of FME increases with increasing total export, the ratio of FME to total export decreases. FME exceeds 40% of the total carbon export in the oligotrophic North Pacific Subtropical Gyre, but forms <10% of the total export in the most productive waters of the California Current. Because the daytime residence depth of these fishes is below the depths where most remineralization of sinking particles occurs, FME is approximately equal to the passive transport at a depth of 400 m. The active transport of carbon by mesopelagic fishes and zooplankton is similar in magnitude to the gap between estimates of carbon export obtained with sediment traps and by other methods. FME should be considered in models of the global carbon cycle.