Regional variability in catches of the Japanese flounder Paralichthys olivaceus in Japanese waters was investigated. Catches of Japanese flounder from coastal prefectures from 1978 to 2017 were analyzed using principal component analysis. The first to sixth principal components were extracted to examine variability in catch among regions. Although the values increased and decreased at intervals of approximately 10 years, the first principal component showed an increasing trend over time. The first principal component was positively correlated with catches from northern and central Honshu on the Pacific coast, and Hokkaido. However, there was a negative correlation with catches from the Japan Sea. The second principal component was positively correlated with catches from the southern part of Japan on the Pacific coast, the Seto Inland Sea and the East China Sea, where water temperature was high, but was negatively correlated with the northern region of the Japan Sea. Correlations between the first and second principal component scores and catch of Japanese flounder varied among regions. Based on such variability, modification of the current boundaries for Japanese flounder stocks is recommended.
The total allowable catch (TAC) is determined based on the allowable biological catch (ABC). The ABC of the next year is calculated by applying the harvest control rule to this year's stock assessment results. In consideration of the time delay, it was examined whether the ABC calculation method used so far can manage stocks appropriately. The behaviors of the autumn cohort of Japanese common squid Todarodes pacificus and the Tsushima Warm Current stock of Japanese sardine Sardinops melanostictus were examined when the catch was performed according to the ABC as calculated from the past time. The results show that if the fluctuations in the recruitment per spawning, the fishing mortality coefficient in the previous year of the ABC and the weight at age were large, the management was likely to fail. Therefore, it is important to eliminate the time delay and to conduct a simulation in advance based on the actual situation.
This study developed a method for automatically measuring fish-body sizes with a stereo-vision system calibrated using a three-dimensional frame for accuracy and precision. The three-dimensional frame was installed in a water tank and photographed using a stereo camera to obtain the parameters for calibration. An optical character recognition technique was used to detect the feature points on the frame. All the feature points could be detected, and the correct combinations of the points were matched automatically in the stereo images. To obtain the fish-body size, the snouts and tails of goldfish in the tank were detected in the stereo video sequences using the Faster R-CNN image recognition technique, and the fish-body lengths were calculated automatically. The accuracy and precision of the automatic calibration system were equivalent to the manual calibration, whereas those of the automatic fish-body size measurements were lower than the manual measurement. The automatic processes of the calibration and the fish body-size measurement were about 96% and 90% faster than those in the manual process. The issues of the accuracy and precision of fish-body size measurements can be resolved in the future by improving the image recognition accuracy.
Skipjack tuna Katsuwonus pelamis is known to aggregate around fish aggregating devices (FADs), and many ultrasonic biotelemetry investigations have been conducted around FADs. However, the investigation areas have been limited to the area around surface type FADs where receivers can be installed, and adequate tracking cannot be achieved. To clarify the behavioral ecology of skipjack in more detail, it is necessary to carry out extensive and long-term observations. We have developed a fishing boat-mounted receiving system that combines an ultrasonic biotelemetry receiving system and a data communication terminal. The developed system was installed on a fishing boat that operated around FADs in waters around Yonaguni Island, Okinawa, Japan, located in the upper Kuroshio Current. The effectiveness of behavioral investigation of skipjack around FADs was verified. As a result, we were able to expand the tracking area and received the locations of individuals by GPS and successfully tracked them for long periods. Furthermore, data were collected in an efficient manner by using an information and communication technology (ICT) network as a data acquisition method.
Age and growth of oonibe fish Argyrosomus japonicus, the fishery catch of which has increased in recent years off Miyazaki Prefecture, were examined by the otolith sectioning method. The spawning season peaked in February, and an opaque zone was formed from February to May. Opaque zone counts were the same as the actual age for released and recaptured individuals (n=142). Fish sampled from the fishery catch were 99-1110 mm in total length, and aged 0+ to 10+. The growth trajectory shows a rapid linear increase till 800 mm and 3+ and stagnant increment around 900-1100 mm after 5+ years old. VBGF was calculated as TLt=1089{1−exp[−0.364×(t+0.142)]}. Maturation length and age were estimated to be 800 mm and 4 years old, respectively.
Tridacnid giant clams are large bivalves found in coral reefs. Giant clams harbor symbiotic dinoflagellate, which is essential for their survival. Artificial seedling production is necessary because the natural stock of giant clams is declining, but this process usually results in low larval survival, likely caused by a failure to establish the symbiosis between clam larvae and symbiotic algae under artificial conditions. In the present study, experiments were conducted to determine how the origin of symbiont algae fed to larvae and the frequency of algal supply affect the survival rate of Tridacna squamosa larvae. Although the frequency of algal supply did not affect larval survival, enhanced larval growth was achieved in the bottle-scale experiment when algal cells were supplied once every 2 days. At the actual seedling production scale, a survival rate of 21.4 (20.9-21.7)% [average (minimum-maximum)%] was achieved in tanks supplied with algae extracted from several small T. squamosa individuals, which represented a two-fold increase as compared to the conventional method 9.4 (8.0-10.9)% of extracting algae from a single medium T. squamosa individual.
Growth of the brackish water bivalve Corbicula japonica in Lake Abashiri was assessed by analyzing the age and shell length composition of individuals captured in field sampling and reared through a field-caging experiment for the 2008 year class. The specific growth rate (SGR), growth model, and length frequency distribution at each age were calculated in relation to shell length. The growth rate of bivalves was high from June to September, when the water temperature was over 15℃. Because of the seasonal growth pattern, the von Bertalanffy growth model extended with a periodic function was determined as the best model when our data in the field-caging experiment were applied to several growth models. It was found that the growth of C. japonica in Lake Abashiri is markedly slower than that in other fishing grounds in Honshu during the initial stage to age 2, and is accelerated in summer when aged 2-3. In Lake Abashiri, fast-growing individuals reach commercial size (≥23.3 mm in shell length) at age 4, whereas most individuals take 5 or 6 years to grow to that size.
To elucidate the spatio-temporal dynamics of the harmful dinoflagellate Karenia mikimotoi and the relationship with environmental conditions in Uwajima Bay and its adjacent waters, we conducted field observations and analyzed the data in 2018 when a bloom occurred and in 2019 when it did not occur. In 2018, cell density of K. mikimotoi increased in Uwajima Bay, and then the bloom corresponding to low salinity water expanded to a broad area of the adjacent waters, suggesting that the spatio-temporal dynamics of the species were controlled by fresh water and nutrient discharges from rivers, and by passive transport due to a surface residual current. In 2019, although vegetative cells overwintered at low density, they did not form a bloom, possibly due to the environmental conditions for their growth, a large-scale intrusion of warm water (kyucho), and the existence of competing species.