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Simple Summary: Chub mackerel is an economic fishery species that is key in transferring energy from zooplankton to large-bodied carnivores through the local food webs. Understanding the ecological conversion efficiency of this species is important for ecosystem-based fishery. However, the ecological conversion efficiency cannot be accurately determined in the laboratory. In this study, an in situ enrichment method was designed to simulate the wild conditions and used for the determination of ecological conversion efficiency of chub mackerel. Additionally, the method described in this study provided insights into the determination of other wild fishes that are difficult to sample in situ and domesticate in the laboratory. Understanding the ecological conversion efficiency of a fish species can be used to estimate the potential impact of the marine food web and accordingly provides scientific advice to ecosystem-based fishery management. However, only laboratory experiments may limit the accuracy of determining this index. In this study, food ingestion and ecological conversion efficiency of wild chub mackerel (Somber japonicus), a typical marine pelagic fish, were determined with gastric evacuation method in laboratory and in situ enriched simulation conditions. Additionally, the effect of temperature and body weight on ecological conversion efficiency was further estimated based on the 2D interpolation method. The results showed that, at 25.1 °C, the ecological conversion efficiency determined in-lab (35.31%) was significantly higher than in situ (23.85%). Moreover, the interpolation model estimated that with an increase in temperature (10–27 °C), the ecological conversion efficiency initially decreased, followed by an increase when the temperature reached 18 °C, but the ecological conversion efficiency generally decreased against the body weight at each temperature. The findings of this study enhanced the understanding of the energy budget of chub mackerel and also provided an efficient method for the determination of wild fishes that are difficult to sample in situ and domesticate in the laboratory. [ABSTRACT FROM AUTHOR]