The role of zooplanktivorous mesopelagic fish in the ocean carbon cycle is attracting increasing attention. However, little information is available regarding the carbon budget of marine zooplanktivorous fish, let alone that of mesopelagic fish. Here, we propose a carbon release model that divides fish-released carbon into two parts (i.e., food carbon release and body carbon release, based on the source (ingested food and the fish body, respectively)) and three forms (dissolved organic carbon (DOC), CO2, and particulate carbon (PC)). By feeding a model marine zooplanktivorous fish, marine medaka (Oryzias melastigma), a radiocarbon-labeled living rotifer, Brachionus plicatilis, we quantified a detailed carbon budget for the fish that includes the allocation of released food and body carbon to DOC, CO2, and PC. The results indicate that 53%–75% of the ingested food carbon was not assimilated but was released mainly as DOC (48%–59%), followed by CO2 (30%–40%) and PC (11%–13%). The release (/efflux) rates of fish body carbon changed from 0.12 to 0.053 d-1 when daily food rations shifted from 2.2% to 4.3% of the fish biomass. DOC, CO2, and PC accounted for 39%–42%, 40%–45%, and 16%–18% of the carbon released from the fish body, respectively. By using the carbon release model and the parameters derived from the model fish and from the literature, we estimate that mesopelagic fish in the global open ocean produce 1.34–15.2, 0.95–10.8, and 0.35–3.97 Pg C/y of DOC, CO2, and PC, respectively. Our results show that marine zooplanktivorous fish can transform substantial fractions of their daily ingested food and released body carbon into DOC and that mesopelagic fish may be important sources of DOC and fast-sinking PC in the ocean.