Figure 1 . Schematic diagram of the carbon release model. Ingested food carbon is absorbed after digestion. Unabsorbed food carbon is released as feces through defecation. The absorbed carbon is first allocated to the metabolic carbon pool (e.g., blood and liver). Part of the absorbed carbon in the metabolic carbon pool is released from the fish through respiration and excretion. The remaining absorbed carbon is further assimilated into the structural carbon pool (e.g., white muscle). Meanwhile, part of the structural carbon is replaced, transformed through catabolism and reproduction, and released from the fish. The solid and dotted lines show the carbon flows starting from the ingested food and from the structural carbon in the fish body, respectively. The released carbon originating from ingested food and the fish body are called the food carbon release (red solid arrow lines) and the body carbon release (red dotted arrow lines), respectively. All the released carbon is in the form of dissolved organic carbon (DOC), CO2, and particulate carbon (PC).
To develop the conceptual model, we examined the food carbon release and body carbon release parameters of a model marine zooplankivorous fish, marine medaka (O. melastigma ), through short-term (36 h) depuration (the process of releasing 14C from the fish) experiments performed by labeling fish food with14C, and through long-term (8 days) depuration experiments performed by labeling the bodies of the fish with14C. The lab experiments were also designed to be extrapolated to mesopelagic fish. By using carbon release parameters derived from the model fish and parameters from the literature, we composed the model scenarios for estimating the carbon release of mesopelagic fish in the global open ocean (Figure 2).
Figure 2 . A flow chart showing the relationships among the conceptual model, the experimental work parameterizing model fish carbon release, and the model scenarios for estimating the carbon release of mesopelagic fish. DOC, CO2, and PC indicate dissolved organic carbon, carbon dioxide, and particulate carbon, respectively.