The annual reproductive cycle and shell length (SL) frequency distribution of the clam Mactra chinensis was examined based on specimens collected from the dredge fishing ground in Mikawa Bay from February 2017 to February 2018. Mature specimens were collected mainly from April to July. The gonads began to degenerate from August and most specimens collected from November to February were immature. These results indicate that the major spawning season of M. chinensis in Mikawa Bay is from April to July. Based on cohort analysis using SL data, most specimens collected from February to May were commercial-sized (>40 mm SL), and new cohorts (<30 mm SL) recruited in September 2017. The fraction of the newly recruited cohorts increased from September to November 2017. They showed rapid growth, attaining the commercial size by February 2018. These results indicate that the early juveniles of M. chinensis settle down from spring to early summer, and grow up to the commercial size within one year.
We examined stock abundance trends and the influence of the Kuroshio path on catches of Japanese butterfish (Hyperoglyphe japonica) in waters around Hachijyo-jima Island in central Japan. The results of a selected generalized linear model (GLM) using Akaike’s information criterion indicated that year, month, and changes in the Kuroshio path influenced catches of Japanese butterfish. The number of fishing boats per half month and the Kuroshio path were also found to influence fishing efficiency (catch per fishing boats per half month). Assuming that the coefficients of the year of the selected GLM indicated the stock abundance trend, no decrease in the stock abundance of Japanese butterfish was detected. Moreover, fishing efficiency improved with an increase in the number of fishing boats per half month, which can be attributed to the improvement in effective searching for fishing grounds. In addition, the effect of improvement in fishing efficiency with an increase in the number of fishing boats was generally higher in the C-type than in the except C-type of the Kuroshio path. In the case of the number of fishing boats per half month exceeding 13.3, it was predicted that the catches were higher in the C-type of the Kuroshio path than in the except C-type. In the case of the C-type, the depth of suitable water temperature for Japanese butterfish was shallower than that of the except C-type because of cooling of deep waters. Therefore, in the case of the C-type, fishing grounds may form in shallower depths. Therefore, fishing efficiency may improve because of a decreased influence of current and increased operation frequency per day due to a reduction in operation time.
In order to elucidate factors affecting seasonal and interannual variability in crude fat content (Fc) and condition factor (Cf) of the Pacific stock of chub mackerel Scomber japonicus, we examined Fc and Cf in relation to fork length (Fl), capture date (Cd, number of days elapsed from January 1st in each year) and year. We measured Fc, Fl and body weight of 2,085 individuals of chub mackerel (21–42 cm Fl) captured by purse seine fleets from waters off northeastern Japan during 2012–2017. Fc and Cf were high from September to January, and slightly decreased in February. During March–August, when sample size was small, Fc was lower than in February, and Cf level was close to that in February. Non-linear relationships were detected between Cf and Fc, between Fl and Fc, and between Fl and Cf. The best GAM model in terms of AIC, with Fc or Cf as a response variable, and year (categorical variable), Cd and Fl (spline functions regardless of their independent or simultaneous usages) as explanatory variables, effects of year were statistically significant. Potential causes of strong positive effects of 2016 and 2017 on Fc were discussed.
Total organic carbon (TOC), total nitrogen (TN), δ13C and δ15N in sediments collected from 135 stations in the central and western parts of the Seto Inland Sea in July 2016 by Ministry of the Environment, Japan (MOF) were analyzed to evaluate the bottom environment. The principal component analysis based on these parameters plus C : N ratio and six parameters provided by MOF shows that the area is divided into four zones: zone A characterized by fine and organically enriched sediments, zone B characterized by coarse sediments showing low contents of organic matter, zone C1 characterized by low δ13C and δ15N values and the nearshore location and zone C2 characterized by low δ13C and δ15N and the offshore location. The terrestrial organic matter ratio in the sediment of zone C1 was high (37%), whereas the ratio in zone C2 must be low, considering low values of the C : N ratio. The comparison in the sediment parameters between the 2016 and previous surveys shows no clear trend in the sediment parameters during the 30 years except for TOC and TN in sediments collected from zone A in Hiroshima Bay.