We developed a technique to transfer second-stage snow crab Chionoecetes opilio zoeae between rearing tanks during mass seed production using zoeal positive phototaxis. Zoeae swarmed efficiently to incident light in the rearing tank from a 300 W reflector lamp located 25 cm above the water surface and a reflector made of expanded polystyrene board (30 cm× 30 cm) placed in the rearing water of 25 cm depth, and they were successfully transferred to the other tank by siphoning. At a siphoning hose current velocity of 10 cm/s, zoeae were not injured during transfer and their survival rates evaluated by culturing them in 1 l beakers for 7 days after treatment were not decreased compared with control (not transfer) and 5 cm/s current velocity treatment. Survival rates to the megalopae stage in treatment of transfer were about 2-fold higher than treatment of no transfer in large scale tanks of 20 kl volume.
We observed the diel activity and burrowing behavior of juvenile hatchery-reared red tilefish, Branchiostegus japonicus (45-100 mm standard length [SL]) in a sand-covered aquarium. Fish that were ≤50 mm SL exhibited diurnal patterns of activity, which remained constant between dawn and dusk, but none of the fish exhibited burrowing behavior. Juveniles within the range of 51-75 mm SL were also diurnal; their activity peaked around dawn when the fish frequently exhibited burrowing behavior. Fish within the range of 76-100 mm SL exhibited similar behaviors to those in the 51-75 mm SL size class. However, the time of peak activity was earlier in the 76-100 mm SL fish than those in the 51-75 mm SL size class. All juvenile size classes were inactive at night but their activity began to increase at dawn. The time at which activity began to increase shifted to earlier in the dawn period, and the duration of the increase shortened as fish grew. Juvenile hatchery-reared red tilefish acquired a burrowing habit and exhibited changes in the timing of peak activity with body size. These behavioral developments may be needed for survival in the natural habitat following release.
Many commercially important fishes associate with drifting seaweeds in their juvenile stage, however, the ecological significance of drifting seaweeds for juvenile fishes is still unclear. We postulated that the following two hypotheses may be applicable for juvenile fishes associate with drifting seaweeds, the "concentration of food supply" hypothesis: juvenile fishes are attracted by phytal animals on the drifting seaweeds and the "indicator-log" hypothesis: fish use accumulations of drifting seaweed as an indicator of productive areas (e.g. frontal areas) for food. We investigated the frontal areas, zooplankton abundance around the drifting seaweed, and the food availability of fish juveniles associated with drifting seaweed accumulations in the East China Sea in 2012 and 2013. A total of 14 drifting seaweed mass and 22 species (n = 408) of fish juveniles were collected. We found that 49.7-99.7% of the individual fed on planktonic food and the feeding incidence on phytal animals was less than 50%. Although drifting seaweeds were aggregated around the frontal areas of surface currents, the zooplankton abundance was not significantly different between these frontal areas and other areas. Our findings indicate that ecological significance of drifting seaweeds as feeding habit is relatively low for juvenile fishes associated with drifting seaweeds.
This study examined a new method for mass production of Undaria pinnatifida seedlings using free-living gametophytes in a large indoor tank. To attach the gametophytes onto strings of seedling collectors and to promote fertilization and germination, the stirring and spraying methods with flat vessels were used before culturing in the large tank. After the indoor culture, many juvenile sporophytes grew on the strings with uniformity. The juvenile sporophytes were transferred to the sea for nursery cultivation. After the cultivation, many seedlings were produced on seedling collectors. Thus, this mass seedling production is considered to be a simple and practical method, and will be helpful for stable and high-quality production of U. pinnatifida.
We investigated the breeding period, otolith annulus formation and growth of the striped snakehead Channa striata population in Nasaythong District, Central Laos. The estimated minimum sizes at maturation for females and males were ca. 19 cm SL and ca. 17 cm SL, respectively, based on the gonad somatic index. The monthly change in the gonad somatic indices of sexually mature-sized specimens indicated that the species began breeding in April when the water temperature was rising and was less active thereafter. Based on the monthly changes in the marginal width of translucent zones in the sagittal otoliths, rings in the otoliths were considered to be formed by development of opaque zones at the rate of one per year between March and July during which the water temperature was seasonally elevated. This finding indicated that the otolith rings were the annuli. Growth patterns for females, males and both sexes were estimated using the age data and regressed by von Bertalanffy growth equations. The estimated sizes were ca. 10 cm SL at 1 year-old and reached ca. 30 cm in 6 year-old fish for both sexes; the growth did not differ significantly between sexes.
The daytime distribution of benthic juveniles of the genus Gymnogobius was described based on discrete depth sampling using a small trawl-net from 2 m to 40 m depth in Lake Biwa during July and August 1996. A total of 9,366 individuals were collected in this survey, most were G. isaza (93.0%) with a few G. urotaenia (3.9%) and morphologically unidentified specimens (3.1%). Juveniles of G. urotaenia were distributed in littoral areas shallower than 10 m depth, where the bottom is just at or above the thermocline. In contrast, juveniles of G. isaza were distributed in more off-shore areas, deeper than 10 m depth, and the density was highest depth of 10 m to 20 m i.e. just at or below the thermocline. The unidentified specimens were collected at depths 10 m to 20 m. In principal component analysis based on five morphometric characteristics (pre-anus length, caudal peduncle length, caudal peduncle depth, eye diameter and interorbital width), the unidentified specimens showed intermediate morphometric characteristics between G. isaza and G. urotaenia. This result suggests the possibility that they are hybrids between these two species.
We studied about supplemental effects of a dry powdered microalga Parachlorella kessleri (KNK-A001) for two shrimp species. As a result it was found that dietary supplementation of 0.005% KNK-A001 showed a tendency to improve growth performance of pacific white shrimp (Litopenaeus vannamei). Furthermore, kuruma shrimp (Marsupenaeus japonicus) fed diet containing 0.05% KNK-A001 showed significantly higher survival rate than control without KNK-A001 in WSSV challenge trial. Increased hemocyte cell density was observed in the hemolymph obtained from the shrimp fed 0.05% KNK-A001, and superoxide anion producing activity of the hemocytes was also increased as compared to the control. These results suggested that KNK-A001 had the beneficial effects on shrimp species.
We have developed a simple and compact method to determine sulfide tolerance of bivalves using glass syringe as an exposure container. The method could maintain sulfide concentration and pH precisely over 10 days of period in laboratory. In this method, lethal periods of sulfide exposure at 25℃ in 5 psu were determined 11, 6, and 4 days in the condition of anoxic, anoxic + 10 mgS/l, and anoxic + 30 mgS/l, respectively in both of pH 7 and 8. Increasing hydrogen sulfide and ammonium in mantle cavity due to continuous valve closure may have been one of the lethal factors.
The browsing ability of the grey sea chub, Kyphosus bigibbus, was examined under tank-rearing conditions. Ten fish (total length, 40.0-43.5 cm) were reared in each of two round 3 kl fish tanks. Brown seaweeds (Ecklonia kurome and Padina arborescens) were fed to the fish during different time periods (4:00-7:00, 11:00-14:00, 17:00-20:00 and 21:00-24:00). The average amount of seaweed browsed by the fish during the four periods ranged from 33.6 to 41.1 g/kg-fish/3-h for E. kurome and from 35.8 to 45.4 g/kg-fish/3-h for P. arborescens. The amounts of seaweed browsed were similar between the two species of seaweed and between the four time periods. These preliminary experiments suggest that K. bigibbus browses seaweed during any time period including night.
There is increasing concern about the sustainable supply of fish meal for the aquaculture industry. This has been elevating the priority of securing alternative feed ingredients. However, alternative ingredients often rank low in digestibility. We therefore evaluated the effects of enzyme pre-treatments to enhance digestibility of such ingredients. Ingredients tested were rapeseed meal (RM), soybean meal (SM), macrophyte meal (MM; made from Egeria densa), and algal meal (AM; by-product of agar production). Each was mixed with water containing citric acid and autoclaved, after which phytases (two types) or pepsin were added and incubated (35℃-5 days). The treatment effects were evaluated by measuring in vivo digestibility using common carp. Pepsin increased protein digestibility in SM and AM (P<0.05). Phytases increased protein digestibility in SM (P<0.05). Phytases and pepsin increased phosphorus digestibility in RM and SM (P<0.01). Fecal Ca, Mg, and Zn content tended to be lower in fish fed diets containing phytase-treated RM or SM, compared with fish fed the control diets. In summary, phytases and pepsin increase digestibility of RM and SM. But, these enzymes are less effective for MM and AM. Since MM and AM are potential feed ingredients, further research is warranted on these sources.
We evaluated the optimal season for transferring juvenile greater amberjack Seriola dumerili from indoor tanks to sea cages. Four batches of juveniles were reared in indoor tanks (total length: 52-118 mm; body weight: 1.9-19.9 g) beginning in June, August, November, December 2010 and March 2011. We transferred these 4 groups to sea cages in August 2010 (June batch), October 2010 (August batch), January 2011 [two groups: A (November batch) and B (December batch)], and April 2011 (March batch). The fish were then cultured for 1-2 years. The group transferred in August (summer) died from red sea bream iridoviral disease 1 month after transfer. The survival of fish in the October (autumn), January-A (winter), January-B (winter), and April (spring) transfer groups was 88%, 68%, 21%, and 41%, respectively, at the end of the culture period. The time to harvest was shortest (617 days) for the juveniles transferred in October. Our results suggest that the optimal season for transferring juveniles from indoor tanks to sea cages is October (autumn) during a period of lowering water temperature.
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