Rainbow trout were fed diets including 0‑30% microalgae meal for 56 days. We observed accumulation of nicotinamide, pantothenic acid, and thiamin in the fish muscle and intestine with less amounts of the same vitamins in the intestinal content. Interestingly, there was a little increase of nicotinic acid but nicotinamide in fish muscle and intestine even though the diet contained nicotinic acid and nicotinamide, suggesting that nicotinamide is a main storage form of niacin. We also observed increasing trends of pantothenic acid and nicotinamide levels in fish muscle when fish were fed with diets including higher levels of those compounds in diet. These results suggest that microalgae can be used as one of the dietary vitamin B sources for fish.

To develop a management method for the Neptune whelk Neptunea arthritica, we examined its reproductive cycle, maturation size, relationship between age and growth, and population structure in Suttsu Fishing Port, southwestern Hokkaido, Japan. Histological observations of the gonads indicated that N. arthritica has one spawning season per year, encompassing May-July for females and April-June for males, with maturation sizes of 70 and 50 mm, respectively. Seasonal changes in the appearance of rings on the statoliths suggested that the annual ring primarily forms during December through January, with the first ring forming at 0 years of age. Analysis of growth models showed that the logistic curve fit the data better than did the von Bertalanffy and Gompertz curves, predicting the theoretical maximum shell height to be 77.4 mm for females and 67.8 mm for males. The 2017-2019 year classes comprised the highest percentage of the population, with no individuals aged nine years or older being detected. In conclusion, for the effective management and propagation of N. arthritica, fishing individuals under 70 mm should be prohibited, and a closed season from April to June should be established to protect immature individuals and the reproductive season.

Short-chain fatty acids (SCFA) produced by gut microbiota influence intestinal morphology. Previously, we reported differences in intestinal morphology between laboratory-raised (Lab) and wild-captured (Wild) fish; however, the mechanisms underlying these differences remain unclear. This study investigated variations in intestinal morphology, SCFA production, bacterial abundance, and community structure between Lab and Wild Kurosoi rockfish Sebastes schlegelii. Wild fish had shorter, heavier intestines and thicker circular and longitudinal muscle layers in the tunica muscularis compared to Lab fish. Additionally, Wild fish showed higher bacterial abundance and the present of predominant genera including Clostridium subcluster XIVb, Vibrio, Photobacterium, and/or Cetobacterium. Malonic and acetic acids, which are essential for SCFA production and act as metabolic intermediates, were found in higher concentrations in the intestinal contents of Wild fish than in Lab fish. This study used Lab and Wild fish to investigate feed-induced changes in intestinal morphology. Given the role of SCFA in intestinal morphological development, we hypothesize that alterations in the intestinal muscular layer are influenced by SCFA production mediated by resident intestinal microbiota derived from feeding.

To ensure the availability of sufficient adult chum salmon Oncorhynchus keta for hatchery stocking, voluntary fisheries restrictions have been implemented in some coastal regions of Hokkaido, Japan. In this study, we aggregated records of fisheries restrictions in 14 coastal regions of Hokkaido from 1997-2022. By focusing on years without restrictions, we constructed two capture-process models from generalized linear models to estimate the expected catch in the absence of restrictions. One model incorporated sea surface temperature as a factor and the other did not. The results show that sea surface temperature positively affected river catches in seven regions. Focusing on the four regions with high interpretability, we evaluated the effects of fisheries restrictions by comparing actual catches during years with restrictions against the catches estimated by our models (i.e., assuming no restrictions). In the Nemuro-North region, actual catches during many of the restricted years (10/14) exceeded estimated catches; this was especially pronounced during the final decade of our analysis. However, no clear positive effects or patterns were observed in the remaining regions. These results suggest that future fisheries restrictions should resemble those implemented in recent years in the Nemuro-North region to maximize the potential for positive effects on river catches.

Information on the low-salinity tolerance of D-shaped larvae, full grown larvae, and new settlers is limited. In this study, a newly developed small-scale method was used for low-salinity exposure experiments on young stages Manila clams. A generalized linear model (GLM) was used to analyze the results and estimate mortality under low-salinity conditions. The change in mortality, which was estimated from a GLM over 24 h of low-salinity exposure, indicated that low mortality occurred above a salinity of 15.0 at all growth stages. On the contrary, mortality increased rapidly below a salinity of 10.0 in D-shaped larvae, full grown larvae, and new settlers, which showed 50% lethal salinity at 6.0, 5.3, and 8.4, respectively.

In this study, spawning characteristics of blacktip grouper Epinephelus fasciatus were investigated using broodstock derived from artificial seed production and long-term-reared wild-captured individuals. Results revealed that the spawning period spanned from June to October and that the onset and cessation of spawning were dependent on water temperature. Water temperature thresholds regulating spawning were determined to be 22-23°C. Moreover, spawning of this species may temporarily halt during periods of high water temperatures. Spawning quantity, buoyant egg rate, and hatching rate were high from the start of the spawning period through July but declined after the high-temperature period. The survival activity index was generally lower than that of the other grouper species, with a survival rate of <50% at mouth opening. These results indicate that maintaining an appropriate water temperature facilitates prolonged spawning in this species. However, challenges remain in improving egg quality.

Focusing on the handling of broodstock in chum salmon, Oncorhynchus keta, I investigated the effects of time from broodstock to gamete collection and from gamete collection to artificial fertilization on hatchability. Hatchability increased from 47% using semen collected from males immediately after initial spermiation to >90% for that collected 8 or 11 days later. During the 11-day period, plasma 17α, 20β-dihydroxy-4-pregnen-3-one levels in males increased from 16 to 52 ng/ml and the spermatocrit increased from 26% to 35%. With fertilization using eggs collected in the 8 days after ovulation, hatchability was maintained at >90%. Use of semen within 30 min after male collection gave hatchabilities >90%, but at 240 min after male collection this declined substantially to about 36% after fish storage at room temperature. Hatchability when eggs were used from females 240 min after collection from the pond decreased slightly, although it remained above 90% at 120 min. These results indicate that semen collection from males immediately after initial spermiation or 60 min or more after collection from the pond can reduce hatchability. Hatchability was less affected by the equivalent factors in females.