Hisashi Kurokura

High-performance liquid chromatography of 5,8,11,14,17-Eicosapentaenoic acid in fatty acids (C18 and C20) by labelling with 9-anthryldiazomethane as a fluorescent agent

Abstract A sensitive high-performance liquid chromatographic method using 9-anthryldiazomethane has been developed for the separation and determination of 5,8,11,14,17-eicosapentaenoic acid (EPA) from C18 and C20 fatty acids, with a view to its application to the HPLC determination of EPA in fish and human blood or serum and in plankton body fluid. The limit of detection of EPA is about 300 pg and the coefficient of variation is 2.1%.

Cryopreservation of the euryhaline rotifer Brachionus plicatilis embryos

Abstract A method for the separation and cryopreservation of Brachionus plicatilis embryos is described. Juveniles with uniform development were collected from a cultured stock by passing them through a series of nets. Collected juveniles were cultured and the embryos separated by vigorous vortex mixing as soon as the majority had laid their first eggs. Separated embryos at stage I (cleavage stage), stage II (invagination stage), stage III (symmetrical embryo stage), or stage IV (“eyed” stage) were frozen to −196°C using various concentrations of DMSO and a two-step freezing procedure. No stage I embryos survived freezing and the highest post-thaw survival was obtained with stage III embryos. A DMSO concentration of 10% of the freezing medium resulted in high post-thaw survival while concentrations higher than 10% appeared to be harmful to embryos. Prolonged incubation in 10% DMSO for up to 30 min before freezing increased post-thaw survival. Incorporating the above results, stage III embryos from a single batch culture were incubated in 10% DMSO for 30 min and frozen to −196°C. Post-thaw survival rates of 63%, 62%, 53%, and 55% were obtained after 3, 7, 15, and 30 days of storage in liquid nitrogen, respectively. Survivors fed actively on marine chlorella and started to lay eggs 2–3 days after thawing.

Effects of mangrove deforestation on fish assemblage at Pak Phanang Bay, southern Thailand

Daytime sampling using a seine net was conducted at Pak Phanang Bay (Nakhon Si Thammarat Province, Thailand) in February and July 2006, to determine differences in fish assemblage structures between a mangrove site and an adjacent site completely cleared of mangroves. The overall numbers of fish species and individuals were significantly higher at the mangrove site than the cleared site in both months. Although benthic crustacean feeders showed more species and individual numbers at the mangrove site in both months, the opposite was found for zooplankton feeders. A cluster analysis, based on the abundance of each species, demonstrated that the fish assemblage structures were distinctly different between the two sites. In addition, significant differences in length frequency distributions for each of the four most abundant species were found between the sites in February and/or July. Small individuals of Scatophagus argus, Ambassis nalua, and Tetraodon nigroviridis were more common at the mangrove site, and of Chelon subviridis at the cleared site. These results suggest that mangrove deforestation exerts marked effects on fish assemblages.

Role of small-scale fishing in Kompong Thom Province, Cambodia

Household surveys of small-scale fishing were conducted in Svay Ear and Srey Rangit villages in Kompong Thom Province, Cambodia. Data were collected from 104 of 183 households and 105 of 140 households in Svay Ear and Srey Rangit, respectively. In both villages, almost all households conducted rice cultivation and approximately 75% of them also conducted fishing. Three fishing groups: (i) inside lake; (ii) around lake; and (iii) around village were observed in both villages, according to their main fishing grounds. The inside lake group conduct fishing only in Tonle Sap Lake which is located 30 km away from the villages. The around lake group fish in lakes, marshes and flooded forests adjacent to Tonle Sap Lake, which are more than 5 km from the villages. The around village group collects fish from rice fields and ponds located inside the villages and also in some streams, rivers and lakes adjacent to the villages. The distance to the fishing grounds is less than 5 km from the villages. Most fishing groups were the inside and around lake groups, which occupied 76.0 and 65.7% of Svay Ear and Srey Rangit, respectively. These two groups produced 80.6 and 69.7% of the respective village annual total fish catches. These two groups sold a large portion of their fish catches. Although previous reports indicated that small-scale fishing is for subsistence, present results indicated that small-scale fishing was commercially conducted and fishing was important as an income source.

Influence of a microhabitat on the structuring of the benthic macrofaunal community in a mangrove forest

Habitat heterogeneity contributes to the maintenance of species diversity; however, little is known about the influence of microhabitat heterogeneity on the diversity of mangrove invertebrates. We tested the hypothesis that the structures of benthic faunal communities differ among microhabitats because each microhabitat function differs widely according to their physical properties. The descriptors of invertebrate assemblages were determined every March (dry season) and August (wet season) of 2005, 2006, and 2007 in three different mangrove microhabitats—tidal creek, deep forest, and canopy gap—of an intertidal mangrove forest around the Sikao Creek in the Trang Province of southern Thailand. The physical properties of the selected microhabitats were studied in 2005. The properties of each microhabitat varied, especially with respect to the light intensity and sediment moisture content: the tidal creeks were wet and well-exposed to sunlight, forest interiors were moist and dark, and the canopy gaps were dry and well-exposed to sunlight. Seasonal differences in the microhabitats were also noted. The numerical abundance and biomass of benthic macrofauna differed among the microhabitats; both values were the highest in canopy gaps, and these values increased in the wet season. Despite seasonal differences, species composition differed in the three microhabitats; the majority of animals were members of the epifauna; these were surface deposit feeders. Each microhabitat functioned differently because their physical properties were disparate. A diverse benthic faunal community is formed by a combination of different functional microhabitats in mangrove ecosystems, thereby indicating that microhabitat heterogeneity should be considered as an important factor influencing the structuring of a benthic community.

Effect of shrimp farming organic waste on food availability for deposit feeder crabs in a mangrove estuary, based on stable isotope analysis

We have investigated the impact of shrimp farming waste on benthic invertebrates in a mangrove estuary in southern Thailand. Stable carbon and nitrogen isotope analyses were used to assess the contribution of aquaculture feed as a nutrient source for benthic invertebrates (fiddler crabs Uca annulipes, U. bengali, and U. forcipata) and to compare the organic origin of sediments in tributaries with and without a shrimp farm. The isotopic contributions of shrimp feed to crabs varied between the tributaries: in the tributary with the farm, the crabs had higher contributions from shrimp feed, whereas there was only a small contribution from aquaculture feed in crabs from the reference tributary. In contrast, the contribution of shrimp feed to the sediment did not differ between the tributaries, the contributions to sediments from aquaculture feed were, in general, quite low. Based on these results, we suggest that, in mangrove estuaries, changes in the food sources of several surface feeder invertebrates would be an effective indicator of the possibility that aquaculture waste was affecting the benthic ecosystem.

Microbial risk assessment of livestock‐integrated aquaculture and fish handling in Vietnam

Animal manure has been traditionally used to fertilize fish ponds in Vietnam. While this practice effectively reuses agricultural wastes, high concentrations of pathogenic microorganisms in animal manure raise public health concerns. Working in fish ponds and handling of contaminated fish in unhygienic manner were identified as potential factors of occupational risk. Escherichia coli occurred in numbers <103 colony forming units (CFU)/100 mL in irrigation water and <104 CFU/100 mL in fish pond water that uses animal manure. Escherichia coli on tilapia skin in numbers were <103 CFU/100 cm in excreta-based systems and <101 CFU/100 cm in feed-based systems, respectively. The study identified direct use of animal manure as major contributors of the fecal contamination of pond water as well as skin of cultured fish. Estimated risks of enteric infection were 100–1000 times higher than the US Environmental Protection Agency acceptable risk. While these risk values are not likely to accurately predict infection rates in Vietnam, they indicate a potential occupational risk in the long term. Therefore, a need for risk mitigation measures was realized for health protection of future generation in agricultural communities.

Water quality fluctuations under floating net cages for fish culture in Lake Cirata and its impact on fish survival

Vertical profiles of water quality were surveyed before and after mass mortality of fish cultured in cages in Lake Cirata, a man made lake in Indonesia. The first survey was performed 4 months before the mass mortality and the second survey was carried out 4 days after the mass mortality. In the first survey, the conditions below 8 m depth were reductive and anoxic, although at 8 and 10 m depth, the water became oxidative in spite of low oxygen concentration in the second survey. These changes were probably caused by strong mixing of surface water with deeper water. Eight months after the mass mortality, when the redox potential at 8m became reductive again, exposure tests of fish were performed in different layers in the water column around the cage. During the exposure tests the redox potential of the water was less than −100 mV and 93% of carp died within 6 h of exposure at 8 m depth. These results indicated that the mass mortality was caused by the strong mixing of surface water with reductive water in deeper layers.

Incentive for shifts in water management systems by shrimp culturists in southern Thailand

Water management systems and other business situations of shrimp culturists were surveyed in two districts in southern Thailand. There were three types of water management systems in southern Thailand, namely closed, semiclosed and open systems, categorized by the frequency of exchange of pond water. Shrimp culturists in those districts tended to shift their system from the open system to the closed system of their own accord, and the average net income ratio decreased due to the change of water management system in a district from more than 2 to less than 1. The reason for this shift, in spite of the decrease in the profit rate, was prevention of infectious disease caused by water exchange. In conclusion, it was proven that independent shrimp culturists, such as medium and small scale shrimp pond owners in southern Thailand, would accept short-term decreases in profit rate in order to stabilize production.

Genetic differentiation of the bronze featherback Notopterus notopterus between Mekong River and Tonle Sap Lake populations by mitochondrial DNA analysis

Although the bronze featherback Notopterus notopterus is an important fish in South-East Asia, the population structure has not been investigated. In this study, genetic diversity and population structure of the bronze featherback were examined using nucleotide sequence analysis of the mitochondrial DNA control region for 332 fish collected from Tonle Sap Lake and the Mekong River in Cambodia. The average nucleotide diversity (π) of all samples was 0.033, and the Mekong River samples had higher nucleotide diversity (0.034) than Tonle Sap Lake samples (0.028). The nucleotide diversity between the lake and river samples varied from 0.029 to 0.037. The genetic differentiation between the river and lake populations was also supported by the pairwise F-statistic values and hierarchical analysis of molecular variance, indicating that the Tonle Sap Lake population is genetically isolated from the population in main stream of the Mekong River.