Balanus rostratus is the largest species of barnacle in the coastal area surrounding Japan. It has not only been known as an edible barnacle in northern Honshu, but is also recognized as a fouling animal for the commercially important scallop Mizuhopecten yessoensis. This barnacle is a lesser-known marine resource in itself in Ofunato Bay, in which oyster and seaweed aquaculture is actively undertaken. The Bay has high primary productivity as the result of nutrient inputs from coastal waters, rivers and sewages. Under such environmental conditions, it is expected that this barnacle would grow faster and would be a new candidate for marine products grown locally. We aimed at elucidating its growth, feeding, and the seasonal appearance of its larvae for future cultivation. Individual barnacles grew 38–43 mm in basal diameter during the first 18 months after settlement and reached 47–50 mm after an additional 9 months. This growth surpassed those in Mutsu Bay and Okirai Bay with which we made a comparison. The results of stable isotopic ratio analysis indicated that these barnacles did not prey on zooplankton, but they might consume phytoplankton and particulate organic matter. They released nauplii in November to March and cyprid density reached about 120 ind./m³ in May. Clearly, this barnacle has a great capacity to be able to adapt to the nutrient-rich environment in Ofunato Bay.