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Sugahara K.,Shiga Prefectural Fisheries Experimental Station | Sugahara K.,Kinki University | Fujiwara-Nagata E.,Kinki University | Fukuda A.,Kinki University | Eguchi M.,Kinki University
Fish Pathology | Year: 2010

Bacterial cold-water disease (BCWD) caused by Flavobacterium psychrophilum leads to heavy mortality of ayu Plecoglossus altivelis in Japan. Previously, a 28°C warmed water treatment was shown to be an effective treatment for BCWD, since F. psychrophilum can not grow and maintain colonies at this temperature. However, it was unclear whether the bacteria might resuscitate after the treatment was over. Therefore, we investigated colony formation, membrane potential and pathogenicity of F. psychrophilum at various temperatures (15°C, 23°C, 28°C and 33°C) in sterilized underground water. Within 2 days at 28°C, F. psychrophilum completely lost their colony-forming abilities but still maintained their membrane potentials. It seemed that these cells entered into viable but non-culturable (VBNC) state. However, experimental ayu infection revealed that VBNC F. psychrophilum cells were unable to cause BCWD, suggesting that the cells were progressing towards death at 28°C. © 2010 The Japanese Society of Fish Pathology. Source


Sugahara K.,Shiga Prefectural Fisheries Experimental Station | Sugahara K.,Kinki University | Fujiwara-Nagata E.,Kinki University | Eguchi M.,Kinki University
Fish Pathology | Year: 2010

Bacterial cold-water disease (BCWD) causes heavy mortality of ayu Plecoglossus altivelis in Japan. To control BCWD of ayu, warmed water treatment at 28°C has been used. However, the mechanism of this treatment is unknown. Hence, we investigated the growth/survival of Flavobacterium psychrophilum at 15-28°C under two nutritional conditions: in modified cytophaga (MCY) broth and sterilized underground water. Within 2 days at 28°C in both the conditions, F. psychrophilum totally lost its colony-forming ability. We also studied distributions of F. psychrophilum in infected fish organs and in rearing water after various treatments. For bacteria detection, the colony-counting method and loop-mediated isothermal amplification (LAMP) method were used. For warmed water treatment, the rearing water was warmed from 18°C to 23°C or 28°C for 3 days. The treatments were started 1 day after immersion infection. In the experimentally infected fish, F. psychrophilum was detected neither in any fish organs nor in the rearing water after the 28°C treatment for 3 days. These results indicate the effectiveness of the warmed water treatment at 28°C against BCWD of ayu. © 2010 The Japanese Society of Fish Pathology. Source


Sugahara K.,Shiga Prefectural Fisheries Experimental Station | Sugahara K.,Kinki University | Sugahara K.,Lake Biwa Museum | Eguchi M.,Kinki University
Fish and Shellfish Immunology | Year: 2012

We investigated the induction of protective immunity against bacterial cold-water disease (BCWD) caused by Flavobacterium psychrophilum by warmed water treatment in ayu (Plecoglossus altivelis). Fish were immersed in a live bacterial suspension (107CFUmL-1) for 30min and placed in 700L concrete tanks. The 28°C warmed water treatment lasted 3 days and began 1, 6, and 24h after immersion in the live bacterial suspension. A naïve control fish group was immersed in a sterilized modified Cytophaga (MCY) broth instead of the bacterial suspension. Fourteen days after the immersion, agglutination antibody titers against F.psychrophilum were measured by using micro-titer methods. Fish were then exposed to a bacterial bath to infect them with live F.psychrophilum, and cumulative mortality was monitored. Fish treated with warmed water at 1, 6, and 24h after immersion in the live bacterial suspension had cumulative mortalities of 36%, 30%, and 18%, respectively, all of which were significantly lower than the cumulative mortality of the naïve control fish (90%). Treated fish also showed high antibody titers against F.psychrophilum in agglutination tests. These results demonstrate that warmed water treatment could not only cure BCWD but also immunize the fish against the causative agent F.psychrophilum. © 2011 Elsevier Ltd. Source


Kikko T.,Shiga Prefectural Fisheries Experimental Station | Okamoto H.,Shiga Prefectural Fisheries Experimental Station | Ujiie M.,Shiga Prefectural Fisheries Experimental Station | Usuki T.,Shiga Prefectural Fisheries Experimental Station | And 7 more authors.
Fisheries Science | Year: 2016

Honmoroko Gnathopogon caerulescens is a critically endangered species and important for commercial fisheries; thus stock enhancement programs are being conducted to restore resources. We evaluated the genetic population structure of field collected samples including spawned eggs around spawning areas in Lake Biwa and the extent of genetic diversity in wild samples and hatchery stocks using sequences of mitochondrial cytochrome b gene. Pairwise Φst analysis and AMOVA clearly showed minimal population structure and haplotype network did not reveal any clear geographic pattern in Lake Biwa. It is probable that strays spawn in non-natal spawning areas, resulting in significant levels of gene flow among spawning areas. Genetic characteristics of hatchery stock F1, F2, and F3 were similar to those of wild samples in terms of haplotype diversity, nucleotide diversity and pairwise Φst values. These results indicate that the relatively high genetic diversity at its initiation was retained due to a lot of broodstock over two successive generations. Accordingly we propose that the current Honmoroko breeding method is appropriate for conserving the genetic diversity of Honmoroko and that the hatchery stock are genetically compatable for release and stock enhancement. © 2016 Japanese Society of Fisheries Science Source


Kikko T.,Shiga Prefectural Fisheries Experimental Station | Usuki T.,Shiga Prefectural Fisheries Experimental Station | Ishizaki D.,Shiga Prefectural Fisheries Experimental Station | Kai Y.,Kyoto University | Fujioka Y.,Lake Biwa Museum
Environmental Biology of Fishes | Year: 2015

Egg size affects larval size, growth rate, survival and fecundity with maternal fitness being maximized by a trade-off between egg size and fecundity. Production of a small number of large eggs maximizes female fitness under poor food conditions, as does a large number of small eggs under rich food conditions. Gnathopogon caerulescens (Honmoroko) spawns over a wide range of water temperature from spring to summer. Thus, we determined whether or not egg size varied with water temperature and how egg size influenced hatchling size at different water temperature. Changes in egg size strongly correlated with seasonal changes in water temperature around the lake, regardless of time and area. An experiment using eggs from the lake indicated that hatchling size has a significant positive relationship with egg size in water temperatures of 24 °C. On the other hand, a lower incubation temperature, similar to that likely to be encountered at the beginning of the spawning season, resulted in a smaller hatchling size, eggs requiring a longer time to hatch. At the beginning of the spawning season, therefore, egg size may have a lesser impact on hatching size in the natural environment of Honmoroko because of the relatively lower temperatures. Although fish larvae below a certain threshold of effective body size are generally not expected to have a high early survival rate due to, for example, a lower competitive ability or a high susceptibility to predation, seasonal egg size variation in Honmoroko may be adaptive so as to enhance the likehood of early survival by establishing an effective body size under changing water temperature conditions. © 2014, Springer Science+Business Media Dordrecht. Source

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