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The export of carbon mediated by mesopelagic fishes in the northeast Pacific Ocean

Abstract

Mesopelagic fishes are under-studied in relation to their importance in pelagic ecosystems. Traditional sampling methods (trawls and acoustic surveys) are biased, yet synergistic, and it has become clear in recent decades with the increased use of acoustic methods that the biomass of mesopelagic fishes is larger than thought from net sampling. For this research, I investigated the importance of mesopelagic fishes to the biogeochemical flux of carbon in the northeast Pacific Ocean. Calculation of the fluxes mediated by mesopelagic fishes requires an estimate of their abundance, and to this end I developed a new technique using combined trawl and acoustic data. The interpretation of acoustic data requires knowledge regarding the acoustic properties of fishes, and thus I made basic measurements on over 70 species of mesopelagic fishes. My results indicated that two of the major determinants of the acoustic reflectivity of a fish, body density and swimbladder inflation, vary both between and within species. Mesopelagic fishes were less dense than epipelagic fishes, and those that did not have inflated swimbladders as adults had decreased body density with increased length. Acoustic models of captured mesopelagic fishes were used with inverse modeling of multi-frequency acoustic data to estimate the capture efficiency of the Matsuda-Oozeki-Hu trawl (MOHT). Capture efficiency of the MOHT was estimated to be 14% for mesopelagic fishes. Trawl data, corrected for efficiency of capture, were used to estimate mesopelagic fish biomass at 77 stations in the northeast Pacific Ocean. The export of carbon out of the epipelagic ocean mediated by those fishes ("fish export") was estimated using individual-based metabolic modeling and extrapolated over the study area. Fish export was estimated to be 17% of total carbon export, estimated from satellite data, in the study area. Fish export varied spatially in both magnitude and relative importance. Although overall fish export increased with total export, its fraction of the total export decreased. Fish export exceeded 40% of the total carbon export in the oligotrophic North Pacific Subtropical Gyre. Because subtropical gyres comprise over half of the surface area of the global ocean, mesopelagic fishes are of global biogeochemical importance

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