A hypothesis to explain the cause of the decline in Japanese eel Anguilla japonica is proposed based on the analysis of catch trends and information on environments downstream of the Tone River. It was evident the Tone River used to contribute more than 80% of the total seed catch. Spawning migration was inhibited by a water gate constructed in 1963, and by the complete closure in 1975. Based on this, a hypothesis was proposed, stating that the Japanese eel decline was caused by the inhibition of spawning and upstream migration of eel into lakes, which were used as habitats, due to the water resource development projects. Support for this hypothesis was obtained by examining the seed-eel relationship and the influences of the project on catches in areas other than the gates.
We investigated the relationship between the nutrients (nitrogen and phosphorus) in the medium and the total amino acid of in Pyropia tenupedalis thallus. The aim of this study is finding a new medium that will increase the total amino acid content in P. tenuipedalis thallus. First, we investigated the relationship between the total amino acid content and Nitrogen and Phosphorus content in P. tenuipedalis thallus. As a result, a positive correlation was found between total amino acid content and N content. Next, we examined the relationship between the presence and absence of nitrogen and phosphorus in the medium and the N and P contents in P. tenuipedalis thallus. In this study, modified 1/2 SWM-III medium was used as the reference medium. As the result, in the medium supplemented with no phosphorus, although the P content decreased, the N content was the highest. Finally, based on the above results, we newly prepared mediums with 50% and 10% phosphorus to the reference medium. We measured the growth, the effective quantum yields (ΦII) and dry weight of cultured with them, analyzed N and P contents. In the medium with the 10% phosphorus to the reference medium, the N content in P. tenuipedalis increased and the P content in was retained. In addition, the growth of the thallus cultured in the medium was also maintained. From the above results, it is considered that a medium in which the amount of phosphorus contained in modified 1/2 SWM-III medium is reduced to 10% is suitable for land cultivation of P. tenuipedalis.
The effects of gibberellin (GA3) and stevioside, plant growth regulators, on the maturation and growth of gametophytes of Eisenia arborea were investigated. Zoospores collected from sporophytes were cultured in 500 µM gibberellin and stevioside. The cell number of female gametophytes in gibberellin was lower and the cell size was larger than those in the control medium. The maturation rate of female gametophytes was 77.0% in the control medium, while it reached 99.0% in stevioside after 17 days culture. The oogonia number per gametophytes was 0.39/ind. in control, while it was 1.71/ind. in gametophytes cultured in gibberellin. No oogonium formation was observed on the gametophytes cultured in stevioside. These results indicate that gibberellin acts as a maturation stimulator and stevioside acts as an inhibitor of germling stage of E. arborea gametophytes. Both GA3 and stevioside will be useful for the efficient control and maintenance of Eisenia arborea gametophytes.
Eucheuma serra (J. Agardh) J. Agardh is an edible red alga distributed from warm-temperate to tropical waters in the Indo-Pacific region. In northern Taiwan, the wild thalli are collected by local residents and eaten as a seaweed dressing. The present study examined the phenology of E. serra in Shen-Ao Bay, Taiwan from May 2017 to July 2018. The length, miscellaneous epiphytic algae (MEA) and bite marks of selected thalli in 21 tagged E. serra stands were monitored along with the thallus weight, generation of 50 randomly selected thalli in the vicinity. The thallus length and weight were highest in June or July and minimum in November. Female gametophytes and tetrasporophytes exceeded 50% of collected thalli in May and July to September, respectively, but only immature thalli from December to March. The number of thalli with MEA and bite marks exceeded 50% between May and September. Furthermore, feeding test with an aquarium using eleven species of herbivores (n = 3), showed that the sea urchin Tripneustes gratilla and two herbivorous fishes Acanthurus dussumieri and Prionurus scalprum heavily grazed the thalli. To manage or enhance the wild stock of E. serra, establishment of rules for harvesting and protection from grazing are needed.
We verified changes in the burrowing behavior of yellow-phase Japanese eels depending upon substrate type by observing burrowing by individuals in the following laboratory experiments: 1) flat sand (approximately 1 mm) substrate; 2) a small rock (approximately 60 mm in long diameter and 20 mm in short diameter) on flat sand substrate; and 3) five small rocks on flat sand substrate. Burrowing behaviors with rapid body undulations were observed least in sediment type 3 (five small rocks on a flat sand substrate) than in the others. Burrowing behavior tended to successful on sand-small rock substrate (type 2 and type 3) compared to sand alone (type 1). We found that eels often excavated the sand-small rock substrate when burrowing by the leveraging their bodies against the edges of small rocks. This behavior enabled eels to hide within the substrate without having to use the rapid body undulations to bury themselves. These results suggest that higher densities of yellow eels found distributed on gravel and rock substrates in rivers are probably because yellow eels can save time when burrowing and avoid the use of rapid body undulations when doing so.
The relationship between the quality of dried nori (Pyropia yezoensis), harvested in the Bisan Strait in 2017-2018, and its unit price was examined. The dried nori used in this study was classified as 1st, 2nd, and 3rd grades. Instrumental analysis and sensory evaluations were used to evaluate nori quality objectively, and to investigate the consistency of these techniques. The color, the soluble attribute in the mouth, and the taste of the nori were measured by instrumental analysis. The results had correlation with the unit price of them. Multiple linear regression analysis of the data showed that the umami of dried nori was significant in establishing its price. The results from the Tensipresser® and taste-sensing system corresponded reasonably well with those from the sensory evaluations. This suggested that instrumental analysis can evaluate the quality of dried nori objectively so it can then be used to determine an appropriate price.
The purpose of this research was to test the effectiveness of using toasted laver chips (a discard product of the laver processing industry) as culture feed for Stephanolepis cirrhifer (thread-sail file-fish, Family Monacanthidae). Fishmeal was replaced with different proportions of laver (0, 11, 34, 56 or 67% w/w), and best growth was obtained with mixtures of 11 or 34% laver, despite the consequential decline in the total amount of protein through partial replacement of fishmeal. In a second experiment, each laver-substituted mixture (0, 11, 34, 56 or 67% w/w laver) received equal amounts of wheat flour and cod liver oil supplements to provide similar amounts of protein and lipid content, respectively. Best growth was obtained with 67% laver. From these two experiments it is concluded that toasted laver chips are acceptable as an alternative feed material to lower the proportion of fishmeal required for efficient culture of S. cirrhifer.
In currently procedures, Japanese eel Anguilla japonica larvae are fed at the bottom of rearing tanks. Therefore, the swimming ability of larvae should be considered for designing procedures for rearing them. This study analyzed the swimming ability and the amount of intestine fullness of Japanese eel larvae in 1 kl-scale tanks. Our results indicate that larvae can swim downward with faster than 6 mm/s at 7 days after hatching (DAH). Moreover, food in the intestines of larvae saturated at around 7 DAH. Consequently, a swimming speed of about 6 mm/s might be necessary to feed them efficiently in a 1 kl-scale tank.
To evaluate the effects of four Umami substances (inosine-monophosphate: IMP, monosodium glutamate: MSG, guanosine monophosphate: GMP, succinic acid: SA) on feed intake and neuropeptide Y (NPY) expression in yellowtail. IMP and GMP increased both feed intake and npy expression, while MSG did not increase feed intake or npy expression. SA did not increase feed intake, but significantly increased npy expression. npy expression levels reflected feed intake, except for the SA treatment, suggesting that some Umami substances might increase feed intake by increasing hypothalamus npy expression.
Rearing experiments were conducted using newly settled rabbitfish collected in subtropical seagrass meadows. Mean standard length on day 92 in the treatment fed on krill and seagrass (Thalassia hemprichii) was significantly larger than those of rabbitfish fed on only krill. The mixing models demonstrated that contribution rates of seagrass in the treatment fed on krill and seagrass was 0.7-15% (95% confidence interval). Contribution rates of seagrass for wild rabbitfish were equivalent to those of reared fish. These results suggest that the contribution of seagrass as an energy source is relatively small for juvenile rabbitfish in subtropical seagrass meadows.
The effects of green light from LEDs on the quality of spotted halibut were examined using commercial-size fish (approximate initial total length [TL] and body weight [BW] 37 cm and 730 g, respectively). The fish exposed to green light for 60 days grew faster than those reared under ambient light. Statistically significantly higher values were observed in measured TL and specific growth rates of TL and BW. Thus, green light can enhance somatic growth in adult fish, as indicated by the present findings, as well as in juvenile fish, per our previous study. In contrast, no significant difference was observed in muscle characteristics between the fish reared under green light and the control fish with regard to the proximate composition, concentration of free amino acids, and taste intensity characteristics such as acidity, bitterness, umami, saltiness, and richness. Moreover, no difference was observed in the response of panel members who tasted sliced muscle. This, therefore, indicated that spotted halibut exposed to green light were of a similar meat quality to those reared conventionally.