The yellowfin tuna (Thunnus albacares) is potentially an important species for Taiwan’s cage culture industry in the future. Feed is a major cost component during the breeding process, and with the aim of accurately estimating feed quantity and reducing feed wastage, the Fisheries Research Institute (FRI, Council of Agriculture, Executive Yuan) was able to successfully develop an underwater residual feed monitoring system that can reduce feed costs by an estimated 5-10% and is already being used by two companies after technology transfers were carried out. Every year, yellowfin tuna migrate to the southwestern waters of Taiwan. However, they consist mostly of juvenile fish that do not taste good and cannot be sold at good prices, and the overfishing of small and immature fish is detrimental to resource sustainability. For these reasons, FRI has been working with aquaculture operators to conduct yellowfin tuna marine cage culture experiments and cultivate brood fish, in the hope of quickly establishing artificial propagation technologies that will allow for the mass production of seedling fish that industry operators can cultivate or release, leading to an increase in resources. The FRI Coastal and Offshore Resources Research Center and a partnering aquaculture operator carried out yellowfin tuna aquaculture experiments along the coastal waters of Hengchun. The experiments involved the placement of a net cage (approximate diameter: 20 m) at a water depth of about 27 m. The net cage was designed to be submersible and can be lowered to a water depth of 10 m or lower in the event of a typhoon. Preliminary test results have indicated that the infection rates and parasite risks for yellowfin tuna are considerably low, and that feed is the primary cost component during the breeding process. The 300 juvenile yellowfin tuna that are currently being raised in the net cage will require about three years to reach a harvestable weight of 36 kg. During this time, feed expenses can reach up to NT$8 million and account for half or more of aquaculture costs. FRI’s underwater residual feed monitoring system enables its user to remotely release feed to the fishes and monitor feeding conditions from his or her boat or land-based office. Through the system, land-based staff can monitor and adjust feed quantity, such that the annual feed costs for a single net cage can be reduced by an estimated NT$200,000, and observe the fish’s activity and health conditions. At present, aquaculture operators are already utilizing the system to monitor their marine net cages, so as to be able to promptly identify any cage damage and prevent their fish from escaping, which will help improve overall aquaculture efficiency. The system has won praise from the industry and been acquired by two operators via a technology transfer. In late September, this system will be showcased at the 2019 Fisheries & Seafood Show held at the Kaohsiung Exhibition Center.