In order to examine mechanisms of inter-annual variability in age-aggregated biomass of Pacific saury Cololabis saira in the waters off northern Japan, which has been declining in recent years, we measured crude fat content in flesh (Fc), body (knob) length (B1) and body weight of 796 individuals of large-sized (age-1) Pacific saury captured from the waters off northern Japan from September to December during 2012–2017. Statistically significant negative and positive correlations were detected between capture date (Cd, number of days elapsed from January 1st in each year) and Fc, and between B1 and Fc, respectively. The best multiple regression model in terms of AIC, with Fc as a response variable and Cd, B1, and year as explanatory variables, indicated that effects of year-specific intercept, Cd, B1 and an interaction between Cd and year were statistically significant. Year-specific intercept, representing Fc level, indicated statistically significant positive correlations with Japanese annual catch and standardized CPUE of Pacific saury. The cause (s) of this phenomena were explored mainly by using survey-derived biomass in the high seas during June–July and age-1 fish ratio in estimated stock abundance, and oceanographic conditions off the southeastern coast of Hokkaido Island. As a result, we proposed a hypothesis that Fc level in August affects timing and location of the start of southward migration and subsequent migration routes of age-1 Pacific saury. We also discussed potential areas where fat is accumulated during the northward migration from May to August.
We are developing a system to estimate a migration period of juvenile yellowtail (Seriola quinqueradiata) in waters off Kagoshima Prefecture. As a part of this research, ages in days and growths of juvenile yellowtail associated with floating seaweeds were examined with an analysis of otolith daily increments. Sagittal otoliths were obtained from 343 juvenile yellowtails caught in waters off Kagoshima Prefecture in March and April from 2010 to 2012. The analysis suggested that ages in days of specimens were between 22 and 69. By grouping specimens into collection month groups of each year, birth months of those collected in March and April were estimated to be from mid-January to mid-February and early February to mid-March, respectively. Growth rates of the specimen groups in order starting with the fastest were in April of 2012, April of 2011 and 2012, and March of 2010 to 2012. The slowest growth rate was in March of 2010 to 2012 in this study, and greater than that reported in the previous study on growth rates of juvenile yellowtail collected in May 1992 around Goto Islands in the East China Sea. It is estimated that growth rates of juvenile yellowtail groups by months and areas are positively related to their water temperature environments, which they experience, depending on their hatching periods and growing areas.
This study investigated transport processes of red tilefish Branchiostegus japonicus from the East China Sea to the coastal area of Miyazaki Prefecture by particle-tracking experiments. Particles were released from areas along the shelf break of the East China Sea in autumn, which are the main spawning ground and season, respectively, and the movement patterns and paths were examined using reanalysis data of an ocean data assimilation system. After 45 days from the release, which is the average duration until settlement, 0.01–0.7% of particles released in the East China Sea reached the coastal area of Miyazaki Prefecture. Among them, 90% and 10% of particles passed through the Ohsumi and the Tokara Straits, respectively. The number of particles reaching Miyazaki Prefecture was positively correlated with those passing through the Ohsumi Strait, while mass arrival events were further related to the deceleration of the Ohsumi Branch Current caused by a small meander of the Kuroshio. Proportions of particles by destination at Day 45, including those not reaching Miyazaki Prefecture, were 10.8% to the Pacific, 1.5% to the Sea of Japan, and 88% remained within the East China Sea. Therefore, it is likely that red tilefish has a major reproduction cycle within the East China Sea, while larvae and juveniles were also supplied from there to Japanese coastal areas. Finally, it is suggested that the Ohsumi Strait is the main passage of larval/juvenile transport of red tilefish, from the East China Sea to the coastal areas of Miyazaki prefecture.