To investigate the effects of topographical differences on biological communities and production of the planktonic food web in surf zones, we conducted monthly surveys from November 2022 to October 2023 at 17 sandy beaches on Obama and Takahama Bays. Water temperature, salinity, nutrient (dissolved inorganic nitrogen: DIN and phosphorus: DIP), and chlorophyll-a concentrations, copepod density, and larval and juvenile fish communities were measured. In Obama Bay, compared to Takahama Bay, water temperatures tended to be lower in winter and higher in summer, and salinity was generally lower throughout the year. A generalized linear model analysis was conducted using four explanatory variables: temperature, salinity, distance from the bay mouth, and bay (Obama or Takahama). In the best models, three or four of the explanatory variables were included. Salinity had a negative effect on DIN and DIP concentrations, and a positive effect on densities of copepod nauplii and copepods (including copepodids and adults). Distance from the bay mouth had a negative effect on fish density and number of fish species. These results suggest that the main factors determining densities and production processes differ among biological communities belonging to different trophic levels, and that the interactions between the lower and higher trophic levels are not clear.

To provide fundamental knowledge for the sustainable use of fishery resources and the conservation of marine ecosystems, we investigated the distribution and abundance of fish larvae and juveniles in spring along the northern Hokkaido coast from the western Sea of Japan to the southern Sea of Okhotsk, Japan. A total of 38,736 individuals belonging to 32 taxa were collected using a frame trawl net (mean towing depth: 28.6 m) in April from 2010 to 2023, excluding 2013 and 2017. Gadus chalcogrammus consistently dominated the catches in most years, followed by Gymnocanthus spp. and Myoxocephalus spp. In 2014, exceptionally cold, low-salinity water prevailed, and Liparis agassizii was found in abundance, resulting in a distinctive taxonomic composition. Interannual fluctuations in the densities of several taxa were observed, including a notable decline in Lumpenus sagitta and an increased occurrence of Lepidopsetta mochigarei and Platichthys bicoloratus after 2015. Partial canonical correspondence analysis indicated that taxonomic composition was influenced by the Tsushima Warm Current, distance from major river mouths, and towing depth. In contrast, the effects of local temperature and salinity were not clearly discernible, possibly due to limitations in detecting their influence at small spatial and temporal scales.

Vertical movement of skipjack tuna Katsuwonus pelamis provides fundamental information for considering fishing effort and eliminating the effect of fishing location on catch per unit effort. Vertical movement of skipjack tuna associated with the surface-type fish aggregating devices (FADs) on the coast of Wakayama, Japan, was examined using the archival tag records. Based on nine fish that were recaptured around the surface-type FADs where they had been released on 24 June 2020, the elapsed period between release and recapture was from 3 days 19 h to 10 days 20 h. Skipjack tuna remained shallower than 20 m in depth for 91.2–99.5% of the time during both the nighttime and daytime. The differences in mean swimming depth between nighttime and daytime were as small as 0.3–3.7 m. Overall, the results of the present study suggested that there was no clear pattern of vertical movement between nighttime and daytime in skipjack tuna associated with the surface-type FADs on the coast of Wakayama. The peritoneal cavity temperatures were generally above 22°C regardless of the ambient temperature, suggesting that skipjack tuna maintained its body temperature to maintain its metabolism.