An age-structured production model of stock dynamics was applied to data of the stock of roughscale sole Clidoderma asperrimum off the Pacific coast of northern Honshu, Japan, in order to estimate stock sizes from 1971 to 2020. The data sets used for estimations were series of catch over the years, that of standardized abundance index over 1972‒2020, average body weight over 2008‒2020 and several biological and operational parameters. Parameters of number of recruits at saturation are divided into those in three periods (1971‒1976, 1977‒1998, 1999‒2020) to incorporate the effect of regime shift into the models of stock dynamics. Unknown parameters were estimated minimizing the residual sum of squares of natural logarithms of the data of abundance index and mean body weight, and the parametric bootstrap method was used to estimate the variance of parameter estimates. The results of estimations showed that point estimates of spawning stock biomass in 1971, 2000 and 2020 were 72.3 (CL 56.9‒87.7), 2.8 (2.0‒3.6), and 9.3 (6.2‒12.4) thousand tons, and the fishing coefficient peaked in 1978 [0.46 (0.35‒0.57) per year] and gradually decreased to 0.03 (0.02‒0.04) in 2020. Interpretation of the results, modifications of the models, and the data to be collected in the near future are discussed.

　Stationary probability density function of number of recruits is applied to evaluate effect of the Harvest Control Rule (HCR) for the winter-spawning stock of Japanese flying squid Todarodes pacificus. The probability density function is calculated using the stock and recruitment model of Beverton and Holt with multiplicative process error and the mathematical condition for stationary state of Markov process. Maximum Sustainable Yield (MSY) under constant fishing coefficient is numerically searched using expectation of sustainable yield calculated with the probability density function. Allowable Biological Catch (ABC) depends on estimate of number of recruits that is random variable from the log-normal distribution. The results showed that the value of MSY was 178 thousand tons, the average of ABC from HCR was decreased with increase of variance of the estimate of number of recruits in log-scale, and the tentative estimate of variance of number of recruits calculated from available data of abundance surveys was within achievable range of the MSY. Merits of the theoretical models and issues to be resolved were discussed.

　Over the last few decades, the extent of marine kelp forests has decreased along the west coast of Kyushu in Japan. We monitored the distribution of seaweeds and seawater temperatures off Nomo, Nagasaki, western Kyushu from 2010 to 2019, to examine the relationship between seaweed distribution and abundance, and water temperature, with particular attention to Ecklonia radicosa. In 2010, Ecklonia cava ssp. kurome, E. radicosa and Sargassum macrocarpum dominated along the transect. However, E. cava ssp. kurome and S. macrocarpum disappeared by December 2013. Moreover, E. radicosa drastically decreased in 2016 and has not been observed since 2017. In contrast, small macroalgae such as Zonaria diesingiana increased in abundance after the larger macroalgal species disappeared. Water temperatures in the spring of 2016 were the highest recorded in the decade from 2010 to 2019 (18.2–19.7℃) and blades of E. radicosa were frequently lost in 2016, probably because of browsing by herbivorous fish. Previous studies showed that feeding rates of herbivorous fish increase, while relative growth rates of E. radicosa are stable, within the recorded temperature range. Changes in the balance between fish herbivory and seaweed productivity due to increasing water temperatures could be responsible for the decline of E. radicosa.

　The metabolic response of hatchery-reared juvenile masu salmon Oncorhynchus masou to prolonged starvation was examined in a rearing experiment. Moreover, the fish released into the Iburibashi River in autumn for stock enhancement were recaptured and their nutritional condition was biochemically evaluated. The hepatic glucose-6-phosphate dehydrogenase (G6PDH) activity and muscle triglyceride (TG) content of the starved fish were significantly lower than those of the fed fish, whereas the hepatic alanine transaminase (ALT) and muscle acid protease (AP) activities of the starved fish were significantly higher than those of the fed fish. These results indicate that hepatic lipogenesis was depressed and hepatic amino acid catabolism and muscle proteolysis were enhanced during starvation. In the release-recapture experiment, the recaptured fish had lower hepatic G6PDH activity and muscle TG content and higher hepatic ALT and muscle AP activities than the fish before the release. The nutritional condition of the recaptured fish was evaluated using a discriminant function constructed from the biochemical data of the rearing experiment. Sixty-nine percent of the recaptured fish were evaluated to be under the starvation condition. This result indicates that the nutritional condition of the released fish was considerably reduced in the river.

　This study estimated the size effect of recreational fishing prohibition on stock enhancement of resident stream salmonids. Data on environmental conditions and fish populations were collected in fishing-prohibited and -permitted areas in several tributaries with diverse environmental conditions. The average treatment effect of the fishing prohibition on the population density was estimated with the adjustment of environmental conditions by inverse probability weighting estimation using propensity scores. Fishing prohibition significantly increased the population densities of mature fish by an average of 2.18 times, but not those of immature fish.

　We examined changes in water quality parameters (water temperature, salinity, DO, and nutrients) in Sendai Bay before and after the Great East Japan Earthquake (GEJE). Principal component analysis (PCA) showed that, after the GEJE, DO and nutrients (DIN and DIP) decreased, and water temperature and salinity increased compared to before the GEJE. The highest decrease in nutrients was observed in spring at the bottom layer, followed by summer and autumn at the bottom layer. Intrusion of water mass with high temperature, salinity, and low nutrient concentrations increased in spring after the GEJE. In addition, regression analyses suggested that spring intrusion of the Oyashio current affected water quality and increased nutrient concentrations in Sendai Bay, and the mean southernmost latitude of the Oyashio in spring has tended to shift to the north in recent years. Therefore, the nutrient decrease in spring after the GEJE was likely due to effects of the increased intrusion of water mass with low nutrient concentrations from the Kuroshio Current, and the decreased intrusion of water mass from the Oyashio current. The nutrient decrease at the bottom layer observed in summer and autumn was likely due to decreases in nutrient supply from the bottom sediment caused by the change in bottom sediments after the GEJE.