In Lake Saiko, Kunimasu Oncorhynchus kawamurae was discovered and has been caught by recreational angling indistinguishable from a related species Himemasu Oncorhynchus nerka. From 2012 to 2014, we conducted a creel survey for two days just after the start of the angling season, aging by scale, and species identification by DNA analysis. The total length composition was transformed into the age composition using age-length keys. Under the assumption of an equilibrium condition, total mortality Z was roughly estimated from the age composition. Fishing mortality F was estimated from Z by subtracting natural mortality empirically estimated from longevity (four to seven years). The populations of Kunimasu and Himemasu were estimated by the number of total catch C, F, and Z. Multiplying N and the ratio of the number of Kunimasu in the recreational catch, the population of Kunimasu was roughly estimated to be 4,300-11,000 individuals.

 The growth characteristics of the hair crab Erimacrus isenbeckii in the coastal waters of Hokkaido, Japan were clarified using an identification method based on external morphology, frequency analysis of carapace length (CL), a mark-recapture sampling protocol, sampling of molted individuals, and rearing experiment. Specifically, the relationship between instar stages and CL was clarified using the former two methods, and to estimate actual growth, a growth formula was derived by applying the initial and post-exuvial CLs of molted individuals that were obtained using the latter three methods to Hiatt's method. There were not large differences between estimations of the CL of each instar by the CL frequency analysis and by the growth formula, so it was considered that both results were reasonable. Since no significant differences were observed between the growth formula results obtained for males after the ninth instar at three sites off Hokkaido, we propose that the growth formula for mature male hair crabs off Hokkaido can be represented as L_n+1=1.035L_n+10.575.

 To assess the influence of factors expected to affect the collection rate of natural bivalve spats, we conducted an on-site spat collection experiment using mesh bags and artificial turf as spat collectors in Banzu tidal flat in Tokyo Bay. In a series of 28 fortnightly spat collections from October 2012 to December 2013, the collection rate [log(collected clam density/ambient clam density+1)/2 weeks] of Ruditapes philippinarum changed seasonally ranging from 0.06±0.01 to 1.67±0.08 (mean±SE) for mesh bag and from 0.24±0.04 to 1.78±0.08 for artificial turf; similar figures were 0-0.28±0.18 and 0-0.94±0.09 for Mactra chinensis, and 0-0.24±0.09 and 0-0.79±0.13 for M. veneriformis. The effects of three assumed explanatory variables (shell length, occurrence of shear stress >0.3 N m⁻² during a single collection, and water temperature) on the collection rate were examined using multiple regression analysis. The shell length showed the largest standard partial correlation coefficient for both types of collector, and the collection rate was higher for smaller spats in the three bivalve species. The occurrence of high shear stress (i.e. physical erosion index for bivalves) and high water temperature, which affects the burrowing velocity of bivalves, showed minor effects on the collection rate.

 Mixed ratios of native and released ayu Plecoglossus altivelis altivelis caught in the Takatsu River were estimated for years 2009 to 2012. The stock of ayu in this river consists of three types: native amphidromous and two released hatchery-reared forms bred at different hatcheries. Mixed ratios were estimated by fitting a multiple normal distribution to the frequency distribution of the scale number above the lateral line using the maximum likelihood method. The data of scale numbers above the lateral line of ayu caught in the upper and lower waters of the dam in this river were resampled by the bootstrap method at 2,000 repetitions, and then each bootstrap distribution was fitted to the multiple normal distribution with two peaks. The native ayu ratios were estimated at 26.3-58.8% and 82.6-100%, respectively. These estimates reflected upstreaming conditions of native ayu, and were considered to be proper values.

 Recently, discoloration of cultured nori Porphyra yezoensis and wakame Undaria pinnatifida has often occurred due to insufficient dissolved inorganic nitrogen (DIN). Therefore, to supply the insufficient DIN in seaweed aquaculture areas, we attempted to develop a new nitrate fertilizer. This fertilizer was made of ammonium nitrate solution congealed with gelatin. Two types of fertilizer without a case (naked) and two types with a case were examined; the naked-type fertilizers had different amounts of gelatin, and the covered-type fertilizers were filled with ammonium nitrate solution and the cases had holes. The fertilizer cases had different diameters and numbers of holes. An experimental tank test was performed using an ultraviolet nitrate analyzer to measure the concentration of nitrate eluted into the water. 70% elution of ammonium nitrate from the naked-type fertilizer took approximately 5 days, but 60% elution of the covered-type fertilizer required about 20 days. The elution rate of the covered-type fertilizer with larger opening area of holes was more rapid. These results suggest that the elution rate of ammonium nitrate from the gelatin fertilizer could be regulated by changing the functional structure of the case.

 To develop an extruded pellet (EP) appropriate for the cultured bluefin tuna Thunnus orientalis, we compared the feeding effects of EP and raw fish feed on their growth and digestion. EP-fed tuna had slower growth compared to raw fish-fed tuna after a decrease in pepsin activity, and growth was restored to the same level as that of the control when protease activity increased resulting from a bloated pyloric caeca. EP feeding delayed the intestinal arrival time of food compared to raw fish feeding, because food is retained for longer in the stomach. This corresponded with the time of expression of digestive-related factors in both groups, resulting in delayed expression of digestive enhancer (cck) and digestive suppressor (pyy) in the EP group compared with the control group. The expression levels of Ghrelin, a growth related factor expressed when nutrients are absorbed into the intestine, were also significantly higher in the EP group than in the control group. Moreover, expression levels and times of growth hormone (gh) were not significantly different between EP and raw fish feed. Therefore, bluefin tuna has the capacity to adapt their digestive physiology in response to EP in order to grow efficiently.

 Red tides of the fish-killing dinoflagellate Karenia mikimotoi were recorded in Hakodate Bay for the first time in northern Japan during mid-October to late-November 2015. Mortalities of chum salmon Oncorhynchus keta, Japanese common squid Todarodes pacificus and Pacific abalone Haliotis discus hannai were observed during the mass occurrences of K. mikimotoi. The red tides of K. mikimotoi (3,200-11,500 cells/mL) were found at a temperature range between 9.9 and 15.7℃ and a salinity range between 31.8 and 32.2. The northward expansion of occurrence area of K. mikimotoi is thought to be related to natural dispersal by the Tsushima and/or Tsugaru warm currents or by anthropogenic transportation via ships' ballast water.

 Ascophyllum nodosum, a kind of brown algae, has long been used as one of the most cost-effective materials in alginates production. Natural flocculants that are prepared from A. nodosum by acidic and alkaline treatments with different conditions were analyzed for the flocculation properties and the chemical components, such as ash content, total sugar content, uronic acid content and neutral monosaccharides composition. The optimum flocculation activity was obtained by the treatment with 0.06 M HCl and subsequent immersion in sodium carbonate solution for 24 h, while the second best activity was obtained from the treatment with 0.6 M HCl and immersion in sodium carbonate solution for 1 h. According to the results of component analysis, it was suggested that the natural flocculant obtained from the former condition had a larger amount of fucoidan which induces high flocculation ability in the flocculant. In the case of the latter condition, the ash content of the flocculant was significantly decreased, therefore, it was concluded that the enhanced flocculation ability of this flocculant was driven by the increased elution of alginic acid by the substantial elimination of minerals.