Diel variability in catches of major bottom-dwelling fish species in the southwestern part of the Sea of Japan was analyzed based on survey trawls and fisheries data. Ten pairs of day and night catches were produced by survey trawls at ten sampling points. The catch weight, catch number, occurrence, and standard length were recorded for each species caught by survey trawls. Further, the catch rate and frequency of occurrence in each species were compared between day and night using fisheries data of 487 cases. When the daytime catch rate was over two times higher than the nighttime catch rate, the fish was defined as a daytime species, and vice versa. Based on the results of both analyses, Doederleinia berycoides, Glossanodon semifasciatus, Heterololigo bleekeri, Pagrus major, Takifugu porphyreus, Todarodes pacificus, Uroteuthis edulis, Zenopsis nebulosa, and Zeus faber were classified as daytime species. Conger myriaster, Dexistes rikuzenius, Hippoglossoides pinetorum, and Tanakius kitaharae were classified as nighttime species. Most of the fish species analyzed differed in catch rate between day and night. Therefore, it is necessary to introduce a resource management method and standardization of stock index values based on information about diel variability in catches of each fish species.

The bigeye scad, Selar crumenophthalmus, is an important coastal fishery resource in Okinawa Prefecture. However, much is still unknown about its life history traits. Age, growth, sexual maturation, reproductive cycle, and food habits of the bigeye scad were demonstrated based on 418 specimens collected in coastal waters near Okinawa Island. Annual formation of otolith opaque zones from May to October was validated by monthly changes in otolith edge types. Maximum ages observed in the specimens were 6.7 years and 6.1 years for females and males, respectively. No significant difference was observed in growth between sexes. Younger individuals [8–9 cm fork length (FL)] were recruited in July and grew up in shallow coastal areas, reaching 19–24 cm FL one year after hatching. At this time, piscivory became more evident, and 80% of females reached maturity. The spawning season was estimated to last from April to October, peaking in July through September. Mean fecundity was 1.09 × 10⁵ oocytes. In the coastal waters near Okinawa Island, this species was caught mainly using set nets, and small-sized immature individuals generally accounted for more than 30% of the examined individuals between August and December. For a sustainable fishery of S. crumenophthalmus in coastal waters of Okinawa Island, immature individuals must be protected to avoid growth overfishing.

To evaluate the effect of marine protected area (MPA) of spangled emperor Lethrinus nebulosus, tag-and-release method was conducted between 2013 and 2019. In recaptured fishes, the distance travelled of 44 individuals were revealed, and 70% of those individuals were recaptured within one year. In addition, 77% of the individuals that were recaptured after more than one year were recaptured within 10 km from the release point (median 3.3 km). Those results suggested that the home range of L. nebulosus was relatively small. On the other hand, three of 20 mature individuals moved more than 10 km during or just before the spawning season, and two of them were recaptured in a set net off Yomitan Village 43–46 km away. Since those individuals were caught with a large number of L. nebulosus that is presumed to aggregate for spawning, those individuals may travel long distances for spawning. This study carried out the tag-and-release method on wild L. nebulosus and clarified migration and growth of this species. This study shows that the MPA is effective for protecting sedentary-young L. nebulosus and has spillover effect to the sea areas that are relatively distant from the MPA.