This study predicted the catch of spawning spring chub mackerel that migrates to spawn in the Hyuga-nada Sea, which substantially impacts the business of the medium-sized purse-seine fishery in Miyazaki Prefecture. After selecting the explanatory variables that are important for the conditions of migration to the Hyuga-nada Sea using the Boruta feature selection algorithm, a catch forecasting model (R²=0.79) was constructed using random forest regression. The explanatory variables selected for the model were the sea surface temperature (SST) of the Kumano-nada Sea and the distance of the Kuroshio current from Cape Ashizuri and Cape Toi. The influence of these explanatory variables on the model is examined in conjunction with partial dependent plots, regional SSTs, and previous findings. The results indicate that the optimum conditions for spring catches of chub mackerel in Miyazaki Prefecture’s medium-sized purse-seine fishery during high abundance period of the Pacific stock of chub mackerel are the typical Kuroshio large meander when warm water spills over into the Kumano-nada Sea and the Kuroshio comes in contact with the waters off Cape Ashizuri and Cape Toi.

Rabbitfish Siganus fuscescens, an herbivorous fish, is considered a cause of seaweed bed extinction “isoyake” due to its heavy foraging activity, and it is culled using gillnet for conservation of seaweed beds around Jogashima, central Japan. To improve the fishing efficiency of rabbitfish culling, the oceanic environmental factors were analyzed in relation to catch-per-unit-effort (CPUE; individuals caught by a fisherman in a day). As a result, water temperature, moon age, chlorophyll fluorescence, and salinity were found as influential factors of rabbitfish CPUE. CPUE rose with water temperature, possibly because rabbitfish became more vulnerable to gillnets due to increased activity at higher water temperatures. However, CPUE decreased when water temperature exceeded 20°C during the spawning season in June–July. CPUE was highest at the moon age of 14.4 days during the spawning season, suggesting a change in behavior for spawning. The CPUE decreased when chlorophyll fluorescence became both higher and lower, as the sight of rabbitfish might have been affected by phytoplankton density: higher visibility of gillnets at lower chlorophyll conditions and lower activity of rabbitfish at higher chlorophyll conditions. CPUE also became lower as salinity decreased, possibly because rabbitfish became less active. To catch rabbitfish efficiently, we suggest that rabbitfish culling should be operated when the preferred environmental condition of rabbitfish exists during the spawning season with a full moon, based on oceanic environmental monitoring such as water temperature, salinity, and chlorophyll fluorescence.