Groupers in subfamily Epinephelinae, family Serranidae, are highly valuable marine resources, especially in Southeast Asia including Japan. The objective of this study was to understand the morphological characteristics and endocrine mechanisms underlying the sex differentiation and sex change in groupers. First, we analyzed the morphological characteristics and the role of sex steroid hormones on the gonadal sex differentiation using the large Malabar grouper, Epinephelus malabaricus. Next, the physiological mechanism of sex change was revealed by using the small Honeycomb grouper, E. merra, as a model fish. Finally, we succeeded in inducing an artificial sex reversal from female to male in the groupers by using aromatase inhibitor or gonadotropin; this is in addition to the alreaady-existing methods that use androgen and could be achieved by applying the basic information about the physiological mechanism of sex change in groupers.
In the stock assessment of the Tsushima Warm Current stock of chub mackerel Scomber japonicus using virtual population analysis (VPA), there was a remarkable retrospective pattern in which the estimate of stock size was revised downward each year. In this VPA, the catch at age and abundance indices were used as data, and assumptions such as a constant natural mortality coefficient across time and age and time-invariant catchability coefficients were used. To investigate the causes of the retrospective pattern, a retrospective analysis was performed by modifying the original data and assumptions used in the VPA and observing whether the retrospective pattern changed or not. The results showed that the retrospective pattern almost disappeared using an untuned VPA or by assuming the annual change of catchability coefficient. This suggests that the assumption of a time-invariant catchability coefficient may be the cause of the retrospective pattern.
To reveal the distribution patterns of mackerel in the Bungo Channel, western Japan, we investigated catch statistics and specimens of chub mackerel Scomber japonicus and spotted mackerel Scomber australasicus sampled at fish markets from 2007 to 2017. Chub mackerel were caught mainly in the northern part of the Bungo Channel while spotted mackerel were caught in the southern part of the channel. Spotted mackerel were observed at higher latitude in the eastern channel (Ehime Prefecture) compared to the western side (Oita Prefecture). This pattern suggested a correlation with the asymmetry in water temperature between the eastern and western sides of the channel. The distribution of 0-year-old mackerel was observed chiefly around nearshore areas from April to June, and thereafter it gradually shifted to offshore areas in July, indicating that 0-year-old fish expanded their habitat to offshore waters as they grew.
In this study, we proposed a simulation method for estimating the moving path-line of larval fish by the circular flow of aeration and discussed the relationship between the flow in an aquaculture tank and sinking death. First, computational fluid dynamics (CFD) analysis was conducted to visualize the flow in the aquaculture tank. Next, to simulate the moving path-line of larval fish of bluefin tuna Thunnus orientalis, the motion equation of the larval fish was solved. For the moving path-line simulation, the larval fish was assumed to be a non-swimming particle. The larval fish surfaced with the upstream flow by aeration, then sank to the bottom by downstream flow around the tank wall. To prevent this sinking, upstream flows of 2.1-2.4 mm/s and 8.0-9.0 mm/s were required for the larval fish of 3 and 9 days after hatching, respectively. Finally, we concluded that the proposed simulation method is useful for designing aquaculture tanks.
We examined how feeding treatment affects the growth of coho salmon Oncorhynchus kisutch in order to improve the conventional aquaculture method for this species in Miyagi Prefecture, northern Japan. We conducted feeding experiments during three culture periods: the seawater transfer period (average initial body weight, 151±20 g [mean±standard deviation]), low temperature period (351±105 g), and temperature increase period (1420±202 g). We compared the final average body weight and feed conversion ratio for three dietary treatments: conventional feeding (commercial feed pellets pre-soaked in water [“wet feed”] and supplied to satiation), restricted feed (wet feed at 80-90% of satiation levels) and dry feed (commercial dry pellets to satiation). Under both the wet feed and restricted feed, the average final body weight for each experimental period was generally lower than with wet and satiation treatment, but the differences were not significant (p>0.05). The feed intake was lower in the dry feed treatment, resulting in less growth in all three periods (p<0.05).
Heshiko packed in rice bran containing 3% NaCl with different weight ratios was stored at 1-2℃ for one month for desalting. Changes in water activity (aw), NaCl concentration and moisture content of the heshiko and rice bran were investigated during the desalting. During the desalting with little movement of moisture, aw and NaCl concentration were equilibrated between heshiko and rice bran packaged in a bag. It was possible to predict the equilibrium aw from the aws and moisture contents in heshiko and rice bran prior to desalting. On the other hand, when the aw decreasing coefficients, the magnitude of the aw-lowering effect of 1 g of two different dry solid matters (heshiko and rice bran in this work) working against 100 g of water inside these materials, became equivalent during desalting, the moisture contents of heshiko and rice bran also became equilibrated. These results may be useful technical information for the quality control of desalted heshiko.
This study examined the effect of ozone fine bubbles (OFB) processing (bubble size: 0.8-5.0 µm, number of bubbles: 1.6×1014 individuals) at 20℃ on the quality of the sea pineapple Halocynthia roretzi, as well as the effect of different storage methods (in cold seawater of 2℃ or in seawater ice of -1.5 to -2.0℃) on the quality. In the group stored in cold seawater after the OFB treatment for 24 hours, a significant decrease in the number of viable bacteria in the body and higher S and V values of the squirt fascia were found on day 7, comparing to those of the others. Also, the squirt fascia remained a fresh yellow color until 7 days after storage. On the other hand, the colors of the squirt fascia and the peeled juice of sea pineapple stored in seawater ice changed from yellowish or pale yellow to brown. Thus, the storage of sea pineapple in seawater ice around freezing temperature caused the degradation of quality. These results indicate that storage in cold seawater after OFB treatment is effective for maintaining the high quality of sea pineapple.