In recent years, spawning of sandfish was observed in gulfweeds adhering to a breakwater in a sandy beach area of the Tomakomai East Port. This suggested that the breakwater served as a reef for sandfish to spawn on.This study focused on light and flows around a spawning ground of sandfish, for the purpose of adding a function as fish spawning grounds to breakwaters and other coastal structures. The study was based on the results offield surveys conducted in the Tomakomai East Port, where spawning on a breakwater had been observed, and in the Nemuro sea area, which is known as a natural spawning ground.As a result, it was found that gulfweeds were distributed mainly at depths where light attenuationfactors were over the approximate range of 0.05 to 0.09, although distribution depths differed between the Tomakomai East Port and the Nemuro sea area. Also, in spawning grounds of sandfish, the flow velocity at the time of adhesion of gulfweed embryos was considered to be of higher significance than that during spawning.