A turtle releasing device (TRD) for a submerged bag net of set nets consists of a 1.0-meter-square escape vent on a 2.0-meter-square base net and a flap door covering the vent. In this study, the closing force of the flap door in the TRD was evaluated to investigate the optimum design of the TRD such that the flap door closes automatically to prevent fishes from getting out. The closing force in static or flowing water (10-40 cm/s) was measured with a tension meter. The closing force became larger with a wider width of the overlapping (fixed) part of the flap door and a larger weight of the flap door. Closing moments relating to the overlap increased with a larger opening angle of the flap door, although that relating to the weight of the flap door decreased. The closing force in the flow from the opening side of the flap door was smaller than that in static water because of fluid resistance. A flap door with a frame diameter of 6.0 mm and overlap width of 0.2 m closed automatically in the 0.8-knot flow from the opening side, and the closing force of this flap door in the flow from the fulcrum side was small enough for turtles to open.

　To evaluate the population recovery of masu salmon Oncorhynchus masou by fish ways, we investigated the number and distribution of spawning redds, population density and body size of juveniles for a total of 10 years before and after the construction of fish ways in the Shirai River, Hokkaido, Japan. Before the fish ways were constructed, most juveniles were distributed in the lowermost reach of the river. The population density of juveniles was high and their growth rate was low. After the fish ways were constructed, the distribution of spawning redds expanded into the upper reaches of the river, while in the lower reaches the population density of juveniles reduced and their growth rate increased. After construction, the number or size of potential smolts increased. The total number of spawning redds of the third generation after construction was 2.8 times as many as that of the first generation. We supposed that expansion of the spawning area and habitat of juveniles by constructing fish ways might lead to an increase of the number or size of potential smolts and an increase of the number of ascending adults and spawning redds.

　In this study, we conducted three experiments to determine the performance of a multi-stereovision technique to enhance the accuracy of fish body measurement for aquaculture management. We compared the performance of the stereo cameras using two, three, and four stereo cameras in each experiment. First, we compared the performance of camera calibration. Next, we calculated the difference between the estimated three-dimensional positions of the interpolated or extrapolated control points and the true values. Further, we confirmed the relationship between the observational error of the stereo cameras and the distance from the centroid of the analytical space to measure the interpolated or extrapolated control points. Subsequently, we estimated the fork length of the fish in an aquaculture net-cage. We investigated the relationship between the observational error of the stereo cameras and the distance from the centroid of the analytical space to measure the fish positions. The result of each experiment suggests that the multi-stereovision using four cameras has higher performance than that using two or three cameras. The accuracy of stereovision using four cameras was improved more than the conventional stereo measurement using two cameras. As the error in the depth direction is reduced by the multi-stereovision using four cameras, the multi-stereo measurement may have a wider measurement space than the conventional one.

　Walleye pollock Gadus chalcogrammus were reared indoors from egg to adulthood in order to record their life history in a controlled rearing environment. Next, a laboratory experiment using the reared fish was conducted to examine the relationship between the adults' feeding conditions and the quantity and/or quality of spawned eggs. The hatchery-reared fish spawned spontaneously for the first time at age 2 years. The fish were fed at two ration levels (to satiation or 25-50% of the satiation amount) for periods of 8 months prior to and during the first-time spawning season. The relative egg production (number of spawned eggs per female somatic weight) was similar between the two groups. The diameters of eggs spawned by fish fed to satiation were larger than those spawned by fish fed on the low ration. Laboratory experiments using hatchery-reared walleye pollock are useful for examining the quantity and quality of spawned eggs in relation to the biological characteristics (e.g. age, standard length, nutritional status and recruit/repeat spawner) of adult fish.

　In Japanese landing markets, fish prices are usually determined in auctions. Previous theoretical research argued that under certain conditions, the price was the same regardless of the style of auction. However, no empirical studies have been conducted in Japan to verify this theoretical assertion. In Japanese local landing markets, fish prices may be influenced by auction style or other parameters such as the number of auction participants. To reveal the factors contributing to fish prices at local landing markets, actual transaction data of Japanese spiny lobster in landing markets in Mie prefecture were analyzed. The price was significantly influenced by the auction style. In particular, (1) the price is higher when the prices bid by other bidders are open to all auction participants, (2) the price is higher when a larger number of buyers participate in the auction, (3) the price is higher when the trading time per auction is longer, and (4) the price is higher when Japanese spiny lobster is traded collectively, by a single large unit per auction per day.