In order to obtain high-quality cultured Pacific bluefin tuna Thunnus orientalis, we examined the influence of body constitution on flesh quality, in comparison with wild Pacific bluefin tuna. Although there was no significant difference in the thickness coefficient between wild and cultured specimens, histological fat content in dorsal ordinary muscle of cultured specimens was higher than that of wild specimens. Significant relationship between histological fat content and fork length with growing in dorsal ordinary muscle were found in cultured (r=+0.377, n=29, P<0.05) and wild specimens (r=-0.515, n=18, P<0.01), respectively. The patterns of color L^＊ value and fork length were consistent with the results of histological fat content. While color a^＊ value was increased with growth of fork length in wild specimens (r=+0.837, n=18, P<0.01), but not cultured specimens. Furthermore, total contents of ATP related compounds in dorsal ordinary muscle of cultured specimens were significantly lower than those of wild specimens. These results suggest that histological fat content in muscle may influence on the flesh quality of cultured Pacific bluefin tuna.

View full abstract

We investigated the possibility of utilization of a closed recirculation system (CRS) for egg incubation of tiger puffer Takifugu rubripes. We put 140-200 g of eggs in 6 l - hatching jars, and the jars were integrated into two experimental systems, a CRS and a flow-through system. Hatching rate of eggs incubated in the CRS and the flow-through system were 50.2-95.4% and 42.1-84.1% in experiment 1, and 17.7-64.5% and 10.6-69.9% in experiment 2, respectively. In experiment 1, hatching rate in the CRS was significantly higher than in the flow-through system, but not significantly different in experiment 2. During egg incubation, it was necessary to pay attention to the rapid increase of ammonia caused by the hatching. Success of egg incubation in the present study enable the completely controlled aquaculture of tiger puffer by CRS.

View full abstract

Characteristics of larval and juvenile fish communities and diversity of semi-natural and artificial beaches and a river mouth in the Hiuchi-Nada, Seto Inland Sea were surveyed. A total of 40,112 fishes belonging to 72 or more species in 34 families were collected at four stations with a 2-m beam trawl, 1-8 times a month, from May 2002 to April 2005. The 10 most dominant fish taxa were Favonigobius gymnauchen (70.1% of total fish number), Acanthopagrus schlegelii (4.7%), Kareius bicoloratus (2.7%), Pholis nebulosa (2.7%), Repomucenus spp. (2.2%), Gymnogobius heptacanthus (1.7%), Acanthogobius flavimanus (1.7%), Sillago japonica (1.7%), Paralichthys olivaceus (1.5%) and Rudarius ercodes (1.4%). Density (individuals per 100 m²) and species number were higher in the semi-natural beaches than the artificial beach and the river mouth. And on the semi-natural beaches, various fish species appeared although the density of F. gymnauchen was extremely high. On the artificial beach, a unique distribution characteristic was shown as occurrence in the highest density of A. schlegelii juveniles among stations.

View full abstract

We surveyed the population genetic structure of a devil stinger (Inimicus japonicus) in northwestern Pacific coastal region using six microsatellite loci; our aim was to investigate or examine the genetic effects of stock enhancement. We collected wild specimens of the species from eight localities in Japan and one in South Korea. We also compared four wild cohorts with three cultured stocks tagged with otolith ALCs; all cohorts were from Okayama Prefecture. Allelic richness detected similar levels of diversity (7.5~9.5) between wild populations and cultured stocks. Similar levels of heterozygosity was detected (0.723~0.767) between wild populations and cultured stocks. AMOVA did not identify significant genetic differences between wild populations and cultured stocks. Genetic distances among wild populations in Japan were low (0.026~0.058). Genetic distance between wild and cultured stocks in Okayama Pref. were also low (0.029~0.076). Thus, stock enhancement of the devil stinger should maintain the number of effective parents at levels that at least exceed those in the current populations.

View full abstract

This research examined the effect of rotifer density on growth of the juvenile seahorse Hippocampus kuda. Body weight and height were measured under various rotifer densities. Furthermore, the effects of juvenile swimming speed in relation to its growth were investigated. Under high rotifer density conditions (518 ind./ml), the maximum rate of pivot feeding was 32 suctions per hour, which equates to a maximum of 6.4 rotifers per suction. Syngnathids are pivot feeders; sucking rotifers through their snout with seawater. This enables them to ingest multiple food organisms simultaneously. Hence, increasing the rotifer density promoted a higher growth rate at no　additional energy expenditure; this is because there was a higher concentration of rotifer available for consumption. The swimming speed of juvenile in seahorses at the birth was 10.6±4.0 mm/s. After 15 days postpartum, the difference in swimming speed was nominal, decreasing slightly to 10.3±3.4 mm/s. Generally, other fish develop faster swimming speeds after the hatching. However, the seahorse's swimming speed remains very slow, meaning it is unable to swim towards food efficiently in low rotifer density conditions.

View full abstract

The sexual maturation and age of the black-spot tuskfish Choerodon schoenleinii captured from the Yaeyama Islands area, Okinawa, Japan, was investigated. In total, 422 fish landed from April 2009 to March 2011 were measured for total length (TL), and 45 fish were purchased for the analysis of the gonad and otolith. All fish < 60 cm in TL were female, and the mature females were over 35.4–48.1 cm in TL (age: 1-6 years old). Ovarian development started in November, and well-developed oocytes were found in March and April. All fish ≥ 60 cm in TL (age: 5–14 years old) were males.

View full abstract

This study aimed to present diagnostic DNA markers for the identification of 34 Japanese parrotfish species (family Scaridae) and to reveal the species composition of parrotfish juveniles in the seagrass beds of the Ryukyu Islands, southern Japan. Using eight restriction enzymes, we determined that the polymerase chain reaction-restriction fragment length polymorphism technique can be used to identify parrotfish species based on the digestion pattern of recognition sites. Applying this technique, we were able to identify seven species (Chlorurus sordidus, Scarus psittacus, S. ghobban, S. rivulatus, S. schlegeli, S. globiceps, and S. quoyi) from 224 parrotfish juveniles collected from seagrass beds in Ishigaki, Okinawa, and Yagaji islands in August and September during 2011-2013. Of these, the first three species were common to all islands, whereas the last two species were found only in Okinawa or Yagaji islands.

View full abstract

The purpose of this study is to research factors inducing the initiation of spawning season for Acheilognathus melanogaster. A. melanogaster was reared in different combinations of water temperatures and daylight lengths from the autumnal equinox and the winter solstice. There were no spawning activities under the condition of 12L during the experiments on October 10, 2011 and September 23, 2013. On the contrary, the spawning was observed in the case of long day treatment of 14 and 15L. There were no spawning activities under the condition of 9L during the experiments on December 22, 2011 and December 22, 2013. But, there was spawning in the long day treatment of 12 and 15L. Moreover, the spawning was not observed in the cryogenic water temperature of 14.0℃. However, when the water temperature rose higher than 16.0℃, A. melanogaster spawned under the long day treatment. These results indicate that A. melanogaster requires long day treatment in the case of artificial spawning. But, the time required for long day treatment depends on the previous daylight lengths.

View full abstract

We evaluated the feasibility of using otoliths from heat-cooked ayu for stock identification based on Sr/Ca ratio analysis. We exposed hatchery-reared, amphidromous, and landlocked ayu to one of three heating treatments then measured the otolith Sr/Ca ratio using a wavelength dispersive X-ray electron probe microanalyzer. The control group was untreated. Identification among hatchery-reared, amphidromous, and landlocked populations was possible by comparing with the Sr/Ca ratio profile in the otolith during the early life stages. The Sr/Ca profile was not affected by heat-treatments in ayu from any of the three stocks. These results suggest the otolith Sr/Ca ratio patterns determined by electron probe microanalyzer can be used for stock origin identification, even when ayu have been heat-cooked.

View full abstract

The growth and leaf morphology of Sargassum miyabei were measured at four salinities (3, 7, 15 and 30) in outdoor tanks. The plant was collected in Aso Lagoon, Kyoto Prefecture. The length of plant and number of leaves were highest at 30. Most plants produced sinuate leaves at 3, serrate at 7, oblong at 15, and linear at 30. In our outdoor tanks, a strong correlation was observed between leaf complexity and leaf area at different salinity conditions.

View full abstract

We investigated the influence of the timings (days after hatching, DAH) of the prey switch from rotifers to yolk-sac larvae of spangled emperor on the growth and its variations of Pacific bluefin tuna larvae. The five timings of prey switch were conducted on 17, 20, 23, 26 DAH and no switched group. Mean larval body lengths (mm) in each group on 29 DAH were 27.27, 23.38, 17.23, 13.45 and 9.58, respectively, indicating that earlier prey switch induced larger larval body sizes. No relationship between the timing of prey switch and the growth variation in the coefficient of variation was found.

View full abstract

Dependency of pepsin activity on temperature and pH in the gastric juice (GJ) of farmed Pacific bluefin tuna Thunnus orientalis was determined in order to compare the enzyme properties of extracted and purified enzymes. Pepsin in fresh GJ had the highest activity at temperatures ranging from 25-30℃ and a pH value of 1.6; however, these temperature and pH values differed from those associated with both crude enzyme extracts and purified pepsin. The present study was the first to report the effects of pH and temperature on the activity of pepsin in the GJ of Pacific bluefin tuna.

View full abstract

The spawning of paired "honmoroko" (Gnathopogon caerulescens), a small cyprinid fish that is endemic to Lake Biwa, was investigated in relation to frequency of spawning, egg hatching rates, and occurrence of deformities in hatchlings through a spawning season. Four female/male pairs, which were reared in captivity in tanks supplied with lake water, spawned 6-9 times/pair between April and the end of June or July, with 44.4-100% (mean 84.6%) of the eggs in each brood hatching. The mean deformity rate of the hatchlings was low (1.5%). These results suggest that "honmoroko" is capable of normal spawning in pairs even though this species usually exhibits polyandrous spawning in nature, and that pair spawning can reveal some detailed reproductive traits of this fish.

View full abstract

Seven water quality parameters (pH, COD, DO, DO% saturation, TN, transparency and oil film) were monitored in cultivation sites of the Sanriku-Town area, southern Iwate Prefecture, Japan before and after the 2011 Great East Japan tsunami (summer 2008 to winter 2014). The pH, COD, DO and TN ranged within the highest level of water quality in the "Environmental Quality Standards for Conservation of the Living Environment" established by the Ministry of the Environment of Japan, except for the surface DO in the summer of post-tsunami period. The transparency after the tsunami fluctuated at a slightly high level in comparison with that before the tsunami. The DO% saturation retained close to saturation or supersaturation state, and no oil film was detected over the period of this study. In addition to these water quality studies, Undaria pinnatifida sporophytes from the same strain were cultured using the same method in pre- and post-tsunami periods. Their thallus density, length and wet weight did not differ between the two periods. These findings show that the high level of water quality in the cultivation sites of the Sanriku-Town area was maintained after the tsunami.

View full abstract

We investigated the growth and survival rate of the cultured cockle Fulvia mutica together with environmental factors, such as water temperature, salinity and chlorophyll a concentration (chl a) to estimate the possibility of the suspended cultivation at Igumi fishing port, Hyogo Prefecture, southwestern Sea of Japan. The growth was relatively rapid from March to July, but was stagnant from August to the following March. The survival rate showed a marked decrease just after the start of the culture experiment and from following August to October, and the final value was 45.8%. Water temperature exceeded 25℃ in July and August. Chl a concentration in summer showed frequently higher than 2μg l^-1. However, it decreased after autumn and was lower than 0.5μg l^-1 especially in winter. High water temperature in summer and low chl a after autumn are concluded to be factors causing negative effects such as growth stagnation and decrease of survival rate for the aquaculture in the port. The present study indicates that the cockle culture is not beneficial in Igumi fishing port and the other coastal water of Hyogo Prefecture in the Sea of Japan.

View full abstract