Understanding the overall fishery and fish stock trends from commercial fisheries data throughout Japan is important for discussing the concrete management policy of the Japanese fishery. In this study, fishery information from logbook data of offshore trawl fisheries throughout Japan were compiled into a database with spatial and temporal criterion (logbook database). The temporal transitions of the catch, effort (number of tows) and standardized catch per unit effort (CPUE, kg/tow), and the spatial changes of the fishing grounds were revealed using this database. Additionally, the impacts of various factors (transitions of domestic and international social situations, developments of fishing vessels and gears, fish stock fluctuation, and so on) on these changes were discussed, and the problems for the quantitative evaluation of the overall fisheries and stock conditions were indicated. After 1972, the catch and number of tows in most areas have decreased, and the fishing grounds were reduced to the coast of Japan. The catch and number of tows in some areas were attuned to the fish stock fluctuation due to environmental changes. On the other hand, the CPUEs remained stable on a long-term basis. These CPUE trends were probably affected by the changes in the fishing strategies, the vessel sizes, and so on. In the future, it will be possible to evaluate the overall fishery and stock conditions from the logbook database by standardizing fishing effort and investigating the catch data.
We compared stock assessment for the Tsushima Current stock of chub mackerel based on the fishing year with that based on the calendar year to evaluate how the definition of “year” influenced the estimation of resource fluctuation. The total catches of chub mackerel in the East China Sea, the Sea of Japan, and Korean waters did not have a large difference between the two definitions of year. While most catches consisted of Age-0 and Age-1 in the calendar year basis, over 80% of catches was regarded as Age-0 in the fishing year basis. The fishing year basis estimated less stock biomass and less spawning stock biomass by virtual population analysis tuned to resource abundance indices than the calendar year basis, but no marked difference was found in the trends of the abundance estimates. In contrast, the estimated number of recruitments was about the same between the two bases of year. Retrospective patterns demonstrated that stock biomass was continuously overestimated in both types of year. The degree of overestimation for the fishing year basis was less than that of the calendar year basis in most years, however, it happened to be larger in a few years. Allowable biological catch was also overestimated in such years. Lastly, we listed the characteristics of the stock assessment results based on the fishing year and discussed their future challenges.
Dynamics of dissolved inorganic nitrogen (DIN) in relation to the color of fresh nori (Pyropia) were examined around nori farms in the coastal waters of the northeastern part of Harima Nada (off the coast of Akashi City and Harima Town in Hyogo Prefecture). DIN concentration in surface water was higher inside the ports due to industrial effluent and rivers, and around the outfall of treated sewage effluent. Horizontal distribution of high DIN concentration in surface water extended in the east-west direction along the coastline. On the other hand, offshore concentrations were lower. The dynamics of salinity and NH4–N concentration suggest that during the ebb tide (eastward flow in this area), discharge from ports and treated sewage effluent in the western area can influence the coastal nori farms east of this study area. Also, the numerical simulation method results showed that DIN supplied from the treated sewage effluent flowed eastward during the ebb tide, and reached the coastal nori farms. The quality of fresh nori produced in the eastward coastal nori farms from this study area was better than that from the offshore area, presumably influenced by the nutrients discharged from the ports and the treated sewage effluent.