The following four topics in aquaculture and fishery resource management were reviewed with focus on marine benthic species; 1) relationship between distribution pattern and establishment of fishing grounds, where those grounds are usually maintained in habitats with higher density of the target species, and thus basic knowledge on distribution factors of the species is required; 2) relationship between aquaculture phenomena in the field and basic sciences to understand biological processes occurred in extensive sea bed culture of Japanese scallop; 3) effects of selective harvest of large males on female reproductive potential due to sperm limitation, particularly in large crustacean fisheries, and 4) phenotypic plasticity and genetic changes in growth and maturity size caused by selective fisheries. Studies in ecology, genetics and physiology are needed to understand the above processes in detail, and both applied and basic sciences would be useful to develop solutions for aquaculture and fishery resource management.
In order to examine the dietary value of a marine microalga Rhodomonas sp. for the broodstock of Japanese pearl oyster Pinctada fucata martensii, a rearing experiment was conducted for 15 days. The reproductive performance of eggs from broodstock fed with Rhodomonas sp. was similar to that of those fed with microalgae conventionally used in hatcheries. Then, rearing experiments of larvae were carried out for 22 days. Larval growth and survival rate were not significantly different among broodstocks fed Rhodomonas sp. and the groups fed other microalgae. Moreover, as a result of biochemical analysis of the fatty acid of broodstock after rearing experiments, no significant difference was obtained on their nutritional status. These results suggest that Rhodomonas sp. is effective for broodstock as a live food and greatly contributes enhancement of productivity of the Japanese pearl oyster.
We examined the effects of desiccation on the ammonia oxidation ability of ceramic filter materials to maintain the nitrifier activity during washing procedure of filter. After the ammonia oxidation activity of filters were measured at 25℃, those removed from seawater and kept at 25℃ under different conditions (at 30%, 60% humidity in the air , and saturated humidity in plastic bag). After the filters were kept up to 30 days, the ammonia oxidation activity was estimated again. Compared to the initial activity, activity of those kept under the humidity of 30% and 60%, decreased to about 50% on sixth and 21st day, respectively. Activity of filters kept at 30% was lost on the seventh day, but under 60%, filters had 3.2% activity on the 30th day. In saturated humidity, filters had about 50% activity even after 30 days, and an ammonia-oxidizing bacteria and archaea of filters were similar to those at initial condition. When we returned the filters had kept in saturated humidity to the seawater and added an ammonia source, its activity recovered after three days. Therefore, it is necessary to prevent desiccation of the filters in the washing procedure to keep the ammonia oxidation ability.
Three test diets were formulated using non-enzyme treated fish meal (FM) and fish oil with protein and lipid levels (%) of 55 and 18 (D1), 51 and 22 (D2), and 47 and 26 (D3) to determine the suitable dietary level as well as the utility of non-enzyme treated FM at grow-out stage of Pacific bluefin tuna (PBT), Thunnus orientalis. Raw sand lance, Ammodytes personatus (D4) with 64% protein and 11% lipid was used as the control. Duplicate groups of 100 young PBT (mean weight 82.9 g) were stocked into 6×6×4.5 m3 net cages and reared for 35 days. Although there was no significant difference in mean body weight between D1 and D4, D4 showed significantly higher value than that of D2 and D3 (P<0.05). The survival was significantly higher in fish fed with D1 (59.0%) than that of D4 (47.5%) (P<0.05). Dietary protein/energy ratio and protein content showed strong positive linear correlation with final mean weight. Relative weights of viscera, pyloric caeca and intestine in fish fed with D1-D3 were significantly higher than those of D4 (P<0.05). The increasing level of dietary lipid in D1-D3 resulted in significantly lower retention efficiency than that of D4 (P<0.05). Results suggest that the non-enzyme treated FM can be utilized and dietary level of 55% protein and 18% lipid could improve the growth performance as well as the survival of juveniles PBT at grow-out stage.
The effect of water temperature on the time taken to reach each embryonic developmental stage and on hatching rates were studied in the three-lined tonguefish, Cynoglossus abbreviatus. Eggs hatched at temperatures ranging from 13 to 24°C. The time between each developmental stage of eggs decreased exponentially with increasing temperature. The time taken from spawning to first hatching decreased from 102 hours at 13°C, to 30 hours and 30 minutes at 24°C. An intersection between regression lines was observed at approximately 20°C, when the logarithm of time taken from spawning to hatching was plotted against water temperature. More than 50% of all eggs hatched normally at temperatures ranging from 13 to 20°C and eggs incubated at temperatures ranging from 13 to 24°C produced normal hatched larvae. The relationship between normal hatching rate and water temperature was expressed by a curve, with a maximum value of 15–17°C. We conclude that 15–17°C is the optimum water temperature for embryonic development and hatching of the three-lined tonguefish. This coincides with the water temperature observed during the peak natural spawning season in Okayama Prefecture, Japan, for this species.
Basic rearing conditions for the production of settlement-stage larvae of Iwagaki oyster Crassostrea nippona were clarified using a large, semi-outdoor tank (100 kl). Four experimental treatments using different initial feeding and larval density conditions were established and the effects of these differences on the growth and survival of planktonic larvae were investigated. The present study showed that initial feeding with Isochrysis sp.（T.ISO）, mass-produced at low cost outdoors, followed by Chaetoceros neogracile were optimal for mass-production of settlement-stage larvae. In our cost and labor-saving procedure, D-larva density in the 100kl tanks was 1-2/ml, and water replacement at 5-7-day intervals was suitable for mass-production of settlement-stage larvae. Approximately 1.9 million spats with a mean shell size of 2.4 mm were produced by the single-seed method in the 100 kl tank using the settlement-stage larvae produced in the third trial.
The brackish water clam Corbicula japonica, on the east and west coasts of Lake Shinji, Japan, was sampled monthly for three years (April 2011 to March 2014) using a Smith-McIntyre grab sampler. Shell-length and the growth of new settlers was examined from continuous change of normal shell-size distributions by polymodal analysis. C. japonica individuals grew during May to September in each of the three years. New settlers in the former half of the spawning season, from mid-June to late September, grew to approximately 5 mm in mean shell-length by November but showed almost no growth in winter. However, new settlers in the latter half of the spawning season showed less growth as their mean size was less than 2 mm by November. In 2011 with salinity less than 3.5, no new settlers were found in the former half of the spawning season and existing clams showed little growth. Contrarily, under relatively higher salinity (5.6-7.4) in 2013, a large number of new settlers were found and showed faster growth rates. These findings suggested that the total amount of new settlers and growth of C. japonica in Lake Shinji were influenced by salinity of the lake water.
Two species of the genus Nematalosa, N. japonica and N. come, are the valuable target species for local fisheries in the Okinawan region of southwestern Japan. We examined the morphological development in larvae and juveniles of these two species collected in Nakagusuku Bay, Okinawa Island. Totally, 59 N. japonica, 89 N. come and 2 their hybrids were identified by morphological, ecological, and molecular methods. Their larvae and juveniles can be distinguished from those of other clupeid species by the number of myomers or dorsal and anal fin rays, location of dorsal fin base and anus, and pigment patterns. Two closely related species also appeared very similarly in general morphological characteristics such as body shape, dorsal and anal fin rays, and pigment pattern on body. Although morphological analyses showed differences in number of myomers between two species, their hybrids were difficult to identify. Accordingly, accurate identification of their larvae and juveniles could be achieved by a combination of distributional, morphological, ecological, and genetic tools.
The isotopic variations in the populations of the Japanese anchovy, Engraulis japonicus, in three sea areas of the eastern Seto Inland Sea were examined. The carbon and nitrogen isotope ratios for Japanese anchovy differed among the sea areas. The results showed much greater isotopic variations in the populations in Harima-Nada and Kii Channel than Osaka Bay. Moreover, clustering analysis showed that the populations in Kii Channel and Harima-Nada consisted of two sub-groups that are isotopically different, suggesting that the populations consist of groups of different feeding history. The spatial isotopic variations of the anchovies in our results may help us to understand the migration behavior of the Japanese anchovy in the eastern Seto Inland Sea.
We surveyed the distribution of Biwa salmon spawning redds in eleven inlet streams of Lake Biwa, from late October to early December 2015. We confirmed spawning redds in ten streams, with the exception of the Takashimakamo River, and analyzed their distribution in each streams. Spawning redds tend to be formed in the same section of stream every year, and are especially prevalent in lower sections of weir or dried up areas, which prevent upstream migration. However, when blockages or disturbances are cleared through stream discharge stabilization or the weir removal, mature fish spawn in the upper reaches of the stream. The results of this survey, along with information obtained from local resources, suggest that small inlet streams also serve as spawning site for Biwa salmon. Therefore, we propose that stream administrators avoid conducting river work projects during the peak of spawning season from October to May, when redds density within stream sections is the highest, due to large number of eggs or alevins that remain inside spawning redds during this time.
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