Tsushima Warm Current stock of chub mackerel is a commercially important fishery resource that is caught mainly by purse seine fisheries of Japan, South Korea, and China. However, there are still many questions about the biology of the stock. Stock status has been at a low level since the 1990s and more effective resource management is required. In this paper, we made a review of resource management, such as fisheries and stock assessment, as well as the biology of this stock, such as distribution, migration, stock identification, and maturity. We also discussed topics that should be further studied in the future. In order to manage this stock more efficiently, it is necessary for Japanese scientists to conduct research together with scientists from related countries to understand the fisheries as well as the mechanisms of recruitment variability and migration patterns.
Although there is a huge outbreak of juvenile Manila clams Ruditapes philippinarum in Rokujo tidal flat in Mikawa Bay every year, the Manila clam population declines from autumn to winter. However, the cause is not yet clear. To understand the mortality cause, we conducted field observation from spring to winter for three years (2014–2016), and monitored the physiological state, e.g., growth, condition factor, and maturity. In addition, we conducted a field experiment by using a stainless cage in 2015. Results of the field observation indicated that the number of Manila clams decreased by less than 10% from October to December each year. Results of the field experiment in 2015 indicated that Manila clams in the stainless cage died and the population equally decreased compared to the control group that was not protected by a cage. Consequently, it was not considered that windswell in winter and predation pressure of dabbling ducks and large carnivorous gastropods were the main causes of mortality. On the other hand, the population maturity rate had increased throughout the months of October until December in 2016. It was considered that sexual maturation and spawning were related to the mortality in autumn. Because the growth of the Manila clam population had slowed down from July or August every year, it was considered that individual Manila clams had not been in healthy condition from summer to autumn due to the influence of density effect and oligotrophication in estuaries. Therefore, these results suggest that large energy loss through sexual maturation and spawning in autumn under an oligotrophic condition causes mass mortality of the Manila clam population.
Seasonal changes and the influence of the Kuroshio path on catch and body size of mackerel scad Decapterus macarellus in waters around Hachijyo-jima Island in central Japan were studied using generalized linear models (GLM) and parameter estimates of a mixture of normal distributions. The results of the selected GLM using Akaike’s information criterion indicated that year, month, and changes in the Kuroshio path influenced catch and body size. The number of fishing boats per day was also found to decrease fishing efficiency (catches per fishing boat per day) probably due to restrictions on catches imposed by the Hachijyo-jima Island Fishermen’s Cooperative Association (FCA). The FCA restriction takes into account shipping capacity to prevent unit price drops. The coefficients of the year of the selected GLM indicate that stock abundance trends and body sizes decreased or increased after 2015, respectively. The selected GLM also indicated seasonal changes, in which catches increased from August through November and decreased from November through December. The results of parameter estimates of the mixture of normal distributions indicate that seasonal changes in catch and body size were probably due to growth and changes in age composition (age 1–3), which were targets for the fishery. Moreover, in the case of the C-type of the Kuroshio path, catches in the same month increased 1.1–1.2 times more than in the non-C-type of the Kuroshio path. Changes in catch by Kuroshio path were considered probable due to recruitment of high-growth fish associated with improvements in feeding environments, including increased nutrient levels, in addition to increases in body weight due to aging.