The oblong rockfish Sebastes oblongus is important in commercial and recreational fisheries in the coastal waters of Japan. The stock enhancement project for this species was began in 2003 in Kagawa Prefecture. This study examined the age, growth, and reproductive seasonality of 533 fish samples collected from the Seto Inland Sea from April 2011 to December 2014. The opaque zones (annuli) in the sagittal otoliths were formed annually between April and June. The maximum age observed was 6 years for males and 7 years for females. The von Bertalanffy growth equations were estimated as TL_t = 274{1 − exp[−0.46(t + 0.22)]} for males and TL_t = 466{1 − exp[−0.20(t + 0.44)]} for females, where TL_t is the total length (mm) at age t (year). After 3 years of age, females grew larger than males. The peak gonadosomatic index of males and females declined in November and between December and January, respectively. This indicates the occurrence of mating and parturition seasons in males and females, respectively. Our results provide, for the first time, life history traits such as age and growth, for stock assessment, management and enhancement of S. oblongus.

To improve the techniques for breeding aquarium fish including freshwater pufferfish, we induced artificial maturity in ocellated pufferfish Leiodon cutcutia from India through a hormone injection and observed its spawning behavior, morphological development, and growth. The male first marked his territory on a flat surface, and then invited the female by vibrating his body. The female entered the territory and laid eggs on the spawning bed, whereafter the male followed. After the departure of the female, the male protected the eggs until hatching. The eggs were spherical, translucent, demersal, and adhesive, with some small yellow oil globules, with a diameter of 1.42 ± 0.07 mm (n = 6). The hatched larvae, measuring 2.38-3.12 mm NL (n = 9), emerged with their mouths open. They reached juvenile stage 13 days after hatching (4.77 mm SL). The ocellated pufferfish can be bred in captivity even through the year, and a hormone injection would possibly result in smaller fish breeding.

Breeding habits and early ontogeny of Siphonogobius nue were observed in aquarium. Three males and five females spawned nine times from April 2 through July 27 in 2011, and a single pair spawned eight times from March 30 through July 12 in 2012, under the water temperature 12.0-23.5°C. Clutch size per spawning ranged from 1603-2461. Transparent fusiform eggs hung in a layer from the ceiling of the spawning nest of the guarding male. Egg size was 3.80±0.07 mm (n=20) in the major axis and 0.93±0.33mm (n=20) in the minor axis. Hatching began about 500 hours after spawning under the water temperature 12.2°C. Total length (TL), and number of myomeres (n=10) were 4.90±0.23 mm TL, and 13+19-20=32-33, respectively. Mouth and anus had already opened but the yolk still remained. After hatching, 20-50 individuals/ml of Brachionus plicatilis were fed. Artemia salina was given 8 days after hatching. 55 days after hatching, they reached the juvenile stage at 36.40 mm TL. Specific characteristics of S. nue larvae are as follows: number of myomeres 32-33, and three single large dendritic melanophores on the ventral side of the trunk above the anus, dorsal, and ventral sides of the caudal peduncle.

We investigated the effects of different nutritional condition in early life stage and selective breeding with a low fish meal diet on growth performance of rainbow trout Oncorhynchus mykiss juveniles. Individuals from a non-selected line (Cont) and an F2 generation obtained by selective breeding with a low fish meal content diet (LFMD) (Select) were exposed to different diets (either a fish meal diet (FMD) or LFMD), during first feeding for three weeks. After reaching a body weight of approximately 9 g, the juveniles were subjected to a feeding trial with LFMD for ten weeks. The Select groups showed higher growth rates in the LFMD-feeding trial than the Cont group. There were no significant differences in growth performance of individuals from the Select line, regardless of early nutritional history. In contrast, the juveniles from the Cont line fed LFMD during first feeding showed a significantly lower growth rate than those fed FMD. These results suggest that the growth delay observed in juvenile rainbow trout fed LFMD is caused by the experience of LFMD-feeding in early life stages while the utilization of LFMD is improved by the selective breeding.

Apostichopus japonicus has been an important export of Japan since the seventeenth century. However, with increase in demand, overfishing of this sea cucumber has become a concern. Consequently, we designed a nursery facility for the juvenile sea cucumber A. japonicus. However, growth stagnation occurred depending on population density. Therefore, we investigated the optimal density of juveniles based on the food supply. Sediment traps were installed and sediments were collected monthly from two facilities in a fishing port, Hokkaido, Japan, for a year. Settling particle flux and sediment mass were measured. It was found that the food supply was the lowest in June, and calculations were based on these values. The optimal density for juveniles with a length of 20 mm was calculated to be 141-151 individuals in a facility with a lower surface area of 5.8 m². In addition, 150 juveniles with a mean length of 23.3 mm were housed in one facility and 1,500 in the other facility in October 2019. After 1 year, the length and survival rate were 46.3 mm and 87.3% of the former and 34.5 mm and 33.2% of the latter, demonstrating the validity of the optimal density calculated.

The effect of light quality on the maturation of conchosporangia formed on the free-living conchocelis or the shell-living conchocelis and the release of conchospores in Neopyropia yezoensis was investigated using blue, green, red, and green LEDs. In the experiment, conchocelis including immature conchosporangia which formed under the 26 ± 1°C, 15 ± 2 µmol/m²/s, 14 h light : 10 h dark cycle condition was transferred to 17 ± 1°C, 100 µmol/m²/s, 12 h light : 12 h dark cycle conditions, and then the number of released conchospore after exposure to each quality of light was counted. Several conchospores were released from the conchocelis cultured under the blue, green and white LED culture conditions. On the other hand, the red LED inhibited the maturation of conchosporangia and release of conchospores. When the conchocelis whose conchosporangia maturation was inhibited by red LED was transferred to the white LED fluorescent lamp condition, several conchospores were released after 5 and 10 days. These results suggest that this technique may contribute to the nori cultivation industry as an alternative to releasing conchospores by the temperature-controlling.

Ocean acidification has troubled many ecologists during recent decades. Elevated CO₂ levels have shown to have a significant impact on fish otolith formation; however, it is still unclear if the cause of these impacts is H⁺ or HCO₃^-, as this has not been studied yet. In this study, we used juvenile marbled flounder, Pseudopleuronectes yokohamae as a model organism, to examine the effects of acidification caused by H⁺, not elevated CO₂, on fish otolith formation. The fish were reared under a control (pH 7.8) and two HNO₃ treatments [pH 7.5 and pH 6.8]. The results of the principal component analysis showed a trend of larger otolith size (perimeters, areas, width, and length) under the pH 6.8 treatment than under the control and pH 7.5 treatments. The ANOVA results showed that there were significant shape differences (mainly in terms of solidity) between the HNO₃ treatment groups and the control group. Our results indicate that H⁺ could be the main factor causing the oversized and smoother otoliths under ocean acidification conditions.

Seasonal occurrence and food habits of larvae and juveniles of the threatened goby Gymnogobius castaneus were investigated using monthly seine-net samples in two salt marsh microhabitats (a small creek and marsh edge separated from the creek) in Lake Hinuma, eastern Japan from April 2011 to March 2012. A total of 1,425 G. castaneus individuals (5-55 mm in body length, BL), comprising mostly larvae and juveniles, were collected during the study period. The larvae and juveniles occurred mainly in the creek from April to June, thereafter growing gradually in the same habitat and attaining minimum adult size (42 mm BL) by December. In contrast, extremely few larvae and juveniles (9 individuals) and no adults occurred on the marsh edge during the study. The major food items of larval and juvenile G. castaneus in the creek changed progressively from calanoid and cyclopoid copepods to chironomid larvae and mysids. These results suggest that the goby remains in the salt marsh creek throughout early life stages, with a role as the dietary-shift area of the creek.

To investigate the settlement period, growth and nursery ground of juvenile marbled founder Pseudopleuronectes yokohamae near the southern edge of their distribution, we collected juveniles at two sites, Suo Nada and Beppu Bay, along the coast of the Kunisaki Peninsula, Seto Inland Sea, Japan in April, May and June 2018. Analysis of the otolith microstructure revealed that the settlement period at both sites ranged from January to March, and the peak was mid-February. Growth rates during 51-60 days after settlement in Suo Nada and Morie Bay were 0.42 ± 0.11 mm / day and 0.38 ± 0.11 mm / day, respectively. The relationship between temperature and growth showed that juveniles grow faster at ≥8.5°C during 1-60 day-age after settlement. Although the density of juveniles decreased from April to June with increasing water temperature, juveniles were still observed at both sites in June even when the temperature exceeded the threshold (20°C) to emigrate from the nursery ground. There was a strong correlation between biomass of benthos and density of juveniles at both sites in May and June, suggesting that some remained in the nursery ground for food intake.

The distribution and growth of juvenile chum salmon in early marine life are influenced by sea surface temperature, and the optimal water temperature for its growth is 8°C-13°C. However, little is known about the influence of water temperature on the feed intake (FI) and feeding efficiency of juvenile salmon. In this study, juvenile chum salmon (Oncorhynchus keta) originating from the Chitose and Shiriuchi rivers of Hokkaido were reared at 4°C-23°C and fed Artemia until satiation to investigate the effects of water temperature on feeding and growth. The FI, specific growth rate (SGR), and feeding efficiency (FE) of juveniles were significantly affected by water temperature. The threshold water temperatures that maximized of FI, SGR, and FE were estimated to be 18°C, 16°C, and 13°C, respectively. These parameters were not only low in cold water but also decreased in water temperatures above the threshold temperature. The triglyceride content in the body and glycogen content in the liver, which are indicators of metabolic fuel stores, also decreased at water temperatures of >18°C. The optimal temperature for FE (13°C) may coincide with the upper limit of juvenile distribution in coastal waters.

To understand effects of artificial barriers on the natural recruitment of chum salmon, spatial distribution of spawning redds was investigated for five years in a non-stocked river with a small dam. Spawning redds were observed from October to December every year, but not in January. Redd density in the section just below the dam was more than five times higher than that in downstream sections. Some redds in the high-density section were superimposed by later spawners. These results suggest that spawning ground expansion by artificial structure modification (e.g., installation of fish ways) is important for enhancing chum salmon reproductive success.

Tank experiments were conducted to develop a technique to remove fouling seaweeds from seedling production of Sargassum fusiforme. We set a substrate with S. fusiforme and fouling seaweeds (Ulva australis and U. intestinalis), and 12 juveniles Girella punctata in replicate tanks (n = 4). Coverage and length of the fouling seaweeds decreased as time proceeded, while those of S. fusiforme did not decrease. In addition, S. fusiforme were not observed in the stomach contents of G. punctata. This suggests that G. punctata can remove fouling seaweeds without apparent damage to S. fusiforme in the seedling production.