The deepsea smelt Glossanodon semifasciatus is one of the most important commercial fish for the offshore demersal trawl fisheries around Japan. Although growth is an influential factor for survival during the early life stage and recruitment to the fishery resource, little is known about growth and survival in deepsea smelt larvae and juveniles. Larvae and juveniles of deepsea smelt were collected with a small otter-trawl net in Tosa Bay, southwestern Japan, monthly from April 2008 to March 2010. From the monthly variations of the standard length (SL) histograms, small specimens (SL<40 mm) were recruited from February to July and were continuously sampled up to 100 mm SL. In each cohort, increments in the number of otolith rings between sampling campaigns corresponded approximately with the number of days of the sampling interval, indicating the daily formation of otolith rings. The date of first ring formation (DFRF) was distributed continuously from February to July, which is considered the main spawning season of deepsea smelt in Tosa Bay. This finding does not correspond with previous studies that found two spawning seasons (spring and autumn) in the deepsea smelt sub-population of the Sea of Japan. Monthly distributions of DFRF indicated two groups of cohorts in spring and summer in 2008, whereas in 2009 one group of cohorts was found only in spring. To characterize growth patterns, mean increment widths at 5-day intervals from the 1st to the 56th otolith daily rings were calculated in each group in 2008 and 2009. Narrower increment widths were found in the spring group of 2008 and 2009, within which those sampled later (i.e., long-term survivors) tended to have wider increment widths than those sampled earlier (i.e., short-term survivors). However, wider increment widths were found in the summer group of 2008, within which those sampled earlier had wider increment widths than those sampled later. The results suggest that growth-selective survival is perceptible in deepsea smelt larvae only under unfavorable growth conditions in Tosa Bay.

The catch characteristics of large bigeye tuna fish caught by near-shore longline boats, NL, ‘Kajiki-nawa’ along the Boso coast were analyzed. NL were operated around the Kuroshio channel of the Boso coast from November to June of previous year. The fishing line operated a shallow line with a reach of about 70 m deep during the daytime, and mainly caught fish with a fork length of 110 cm or more. Monthly mean surface water temperature for the bigeye tuna was 18.5–23.2°C, which is different from records for bigeye tuna so far, with fish caught in the high-water temperature area at shallow water depth during the day.

Sexual maturation and spawning season of jack mackerel Trachurus japonicus caught in the sea areas around Sagami Bay and Sagami-nada were examined based on gonad histological observation and gonad somatic index (GSI) of males and females. The males and females of high value GSI appeared from March to September, and GSI average of males and females was highest in May, and June, respectively. Based on observation of gonad tissue section, the males with matured testes appeared from March to July, and the females with ovaries which contained hydrated oocytes or migratory nucleus appeared from April to July, respectively. The minimum age at first maturity was 0 years old for males, and 1 year old for males in this area. On the other hand, the oldest matured male individual was 11 years old, and the oldest female was 23 years old. Estimated 50% maturity size was 188.2 mm in males and 230.6 mm in females, in fork length. This suggests that males mature at a smaller size and younger age than females. From the results of comparing age and GSI, the tendency that the GSI value decreases was recognized in the old fish, but jack mackerels in this area continue contributing to reproduction for over ten years from the time they mature and reproduce for the first time.

In this study, we detected pair-trawling ships using a synthetic aperture radar that can observe fisheries without a fishing lamp. Automatic detection of pair-trawling ships is possible by extracting all ships from the image, examining the distance to the surrounding ships from each ship, and using the combination that minimizes the distance between ships as a pair. For the cases where the ship pair is incorrect, correction methods are presented. In addition, in order to understand the activities of the pair-trawling ships in the target area, the density of pair-trawling ships and distance between ship pairs is calculated. In this study, if sufficient data can be obtained during the fishing season, there is a possibility to estimate fishing effort.

Age, growth, and maturation of the coastal rock fish Sebastes zonatus were estimated on the basis of 1,485 specimens (78–359 mm in standard length: SL) collected in the Pacific coastal area off southern Iwate, northeastern Honshu, Japan by bottom long-line surveys from 2008 to 2018. The length (SL)–weight (BW) relationship was fitted by different allometry equations between both sexes. Transverse otolith sections indicated that an opaque zone was formed once a year, during May–October. The ages identified ranged from 1 to 9 years old, and a von Bertalanffy growth equation was calculated due to no significant difference between both sexes. Seasonal variations of the gonad somatic index indicated that matured females were occurred during March–May at age four or older. The features in age, growth and maturation of S. zonatus were equivalent to previous studies for the Sebastes vulpes species complex (S. zonatus and S. vulpes) along the Pacific coast off Iwate and off Aomori. The time-series for CPUE (inds./100 hooks) in each year class indicated that the relatively strong year classes were occurred in 2005, 2007, 2008, and 2015 from the present waters in the survey period.