abstract

Original Paper
Title Biological investigation on two types of bill internal structure of swordfish (Xiphias gladius) and genetic differentiation between the North and South Atlantic stocks
Authers Kenji NOHARA*1, Hiroshi OKAMURA*2, Motoki NAKADATE*2, Kazuhiko HIRAMATSU*2, Nobuaki SUZUKI*2, Makoto OKAZAKI*3 and Seinen CHOW*2
Keywords swordfish, bill internal structure, genetic stock structure
Citation Bull. Fish. Res. Agen. No.7, 1-13, 2003
Abstract
It has been claimed that the frequencies of two distinct bill internal structures (chamber and channel types) of swordfish (Xiphias gladius) may vary among localities and be an index for the stock structure. Genetic analyses have revealed stock structuring in this species, and genetic variation at calmodulin gene locus (CaM) indicated presence of two genetically separated stocks (north and south) in the Atlantic. In this study, frequency of the bill types was compared between localities, sexes and body size classes, and genetic variation in nuclear and mitochondrial gene loci were compared between the bill types, and genetic differentiation in these loci within and between local samples were investigated. There was no significant difference in the frequency of two bill types among samples from Atlantic, Indian and Pacific Oceans as well as between sexes. No genetic difference between chamber and channel types was observed in all gene loci. In all samples, chamber type was more frequently observed in smaller size class than in larger size class. These suggest that the frequency of the two types is independent of stock structure and sex. Considerably large genetic differentiation at CaM locus was observed between north (north of 20°N) and south (south of 10°N) Atlantic samples. On the other hand, differentiation among samples within locality was very little among years (1990 to 1998 in north, and 1994 to 1999 in south). The results of genetic analysis indicated that the north and south Atlantic stocks of swordfish were genetically isolated with little gene flow and migration.

Received on October 15, 2002
Contribution No.A 33 from Fisheries Research Agency
*1 Faculty of Marine Science, Tokai University, 3-20-1, Orido, Shimizu, Shizuoka 424-8610, Japan
*2 National Research Institute of Far Seas Fisheries, 5-7-1, Orido, Shimizu, Shizuoka 424-8633, Japan
*3 Computer Center for Agriculture, Forestry & Fisheries Research, 2-1-2, Kannon-dai, Tsukuba, Ibaraki 305-0856, Japan



Technical Report
Title Temporal variation of surface wind-drifted current observed by drifting buoys in the East China Sea
Authers Kosei KOMATSU*1 and Kiyoshi KAWASAKI*2
Keywords drifting buoy, East China Sea, Kuroshio, Ekman drift
Citation Bull. Fish. Res. Agen. No.7, 14-23, 2003
Abstract
Surface and upper layer currents were observed by drifting buoys in the East China Sea from February to March in 2001. Two buoys deployed at the same position 120 nautical miles northwestward from the Kuroshio made different trajectories each other: one without a drogue was drawn in eastward to the Kuroshio, on the other hand another one drogued at 15m depth drifted northward to the Japan Sea. In the region further from the Kuroshio where currents were comparatively weak, the buoy with no drogue drifted to the direction which gave good agreement in synoptic time scale with the surface current direction inferred from the Ekman drift. These results suggested that a small difference of the vertical distribution of eggs, larvae and plankton let their future routes change drastically due to the effect of the Ekman drift.

Received on December 3, 2002
Contribution No.A 35 from Fisheries Research Agency
*1 National Research Institute of Fisheries Science, 2-12-4, Fukuura, Kanazawa, Yokohama 236-8648, Japan
*2 Research Promotion and Development Division, Fisheries Research Agency, 2-12-4, Fukuura, Kanazawa, Yokohama 236-8648, Japan



Doctoral Thesis
Title Physiological and ecological studies on harmful dinoflagellate Heterocapsa circularisquama - I Elucidation of environmental factors underlying the occurrence and development of H. circularisquama Red Tide
Authers Yukihiko MATSUYAMA*
Keywords Heterocapsa circularisquama, dinoflagellate, red tide, shellfish, environmental factors
Citation Bull. Fish. Res. Agen. No.7, 24-105, 2003
Abstract

Heterocapsa circularisquama Horiguchi (Dinophyceae) is a causative agent of red tide, and appeared in 1988 for the first time and then rapidly dispersed off the costal waters of western Japan. The red tide due to H. circularisquama was associated with massive killing of commercially important bivalve species: manila clam Ruditapes philippinarum, Pacific oyster Crassostrea gigas, pearl oyster Pinctada fucata, blue mussel Mytilus galloprovincialis, etc. Until 2000, 43 cases of H. circularisquama red tide (including 18 incidences leading to fisheries damage) had been recorded in western Japan. Economic losses in shellfish aquaculture by direct killing of marketable products were estimated to be at least 10 billion-yen in the last decabe. Although H. circularisquama blooms mainly affect bivalve aquaculture, no harmful effects on wild or cultured fish, other marine vertebrates, or any public health hazard were recorded. Therefore, this phenomenon is referred to as a "novel red tide." The red tide due to H. circularisquama has damaged shellfish aquaculture in most of the region, in terms of mass mortality in farming organisms, cost of measures to prevent the damage, adverse affects on the development of shellfish aquaculture, and secondary damage, i.e. decline of demand due to misinformation. Incidence of this species has increased recently, and the economic losses in aquaculture have been a cause for concern for the industry and society. In the present study, environmental conditions conducive to the red tide occurrence was conducted, in orther to clarify the environmental factors causing H. circularisquama red tide.

 

Frequent field surveys were carried out for four years in two representative locations (Ago Bay and Hiroshima Bay) of recurring H. circularisquama red tides. As a result, dense assemblages of H. circularisquama cells were found in the innermost part of the inlets, but not offshore or in the channels. The distribution pattern of H. circularisquama appears to be strongly affected by the water exchange rate. The red tide of H. circularisquama occurs between July and November in the embayments of western Japan. Generally, H. circularisquama blooms appear mainly under high water temperature (>23℃) and salinity (>30psu) conditions. The results of field surveys suggested that H. circularisquama grows well under high water temperature and salinity conditions. In Ago Bay, strong disturbances of the water stratification by typhoons during the development of highly anoxic water at the bottom had preceded the large-scale red tide outbreaks of H. circularisquama in summer. Temporary or sustained water mixing throughout the water column may provide large amount of nutrients and growth-promoting substances from the bottom to the photic layer, which trigger the H. circularisquama red tide. Furhter, H. circularisquama sometimes caused red tides during an ecological niche opening, e.g. decline of predominant diatom populations due to several hydrographic events such as development of highly stratification associated with anoxic water in summer, decreases of light intensity and increase of vertical transportation to the non-photosynthetic layer from late autumn to early winter. In some cases, populations of H. circularisquama were affected by the predation of heterotrophic dinoflagellate Gyrodinium dominans and oligochin ciliates.

 

The red tides of H. circularisquama has rapidly increased since the late 1980s. Relationships between the recent proliferation of H. circularisquama and changes of water qualities were analyzed covenanting various environmental factors. First, laboratory study showed that maximum growth was observed at water temperature of 32.5℃ and no growth was observed under 11.2℃. H. circularisquama is presumed to be the tropical dinoflagelltate, so that winter water temperature is considered as the most limiting factor for the prolifelation of this species. On the Japanese coast, however, the water temperature in winter has increased about 1.0~1.2℃ since the late 1980s, probably due to global climate change. This long-term hydrographic change may be conducive to the overwintering of H. circularisquama, which has led to the recurring of red tide in Japanese coastal waters. Second, H. circularisquama blooms are sometimes observed in non-entrophicated areas, therefore, this dinoflagellate is likely to be adapted to oligotrophication, especially in low phosphorus conditions. In Japan, administrative efforts have been carried out to reduce nutrient loading from land to the coastal sea in the last two decades. The selective improvement in the phosphorus load brought about a change in the nutrient balance, such as an increase of the N:P ratio in coastal waters. Nitrogen and phosphorus demands from H. circularisquama are much lower than other causative red tide species such as Chattonella spp. and Karenia mikimotoi, and can utilize various dissolved organic phosphorus (DOP) for their growth using alkaline phosphatase in their cell surface. This physiological characteristic is important to clarify the growth strategy of this specie. Third, simulated experiments have revealed that H. circularisquama could easily migrate to distant areas in association with shellfish transportation. The rapid development of H. circularisquama since the late 1980's appears to be caused in part by the artificial dispersal due to aquaculture operations.

 

The developments of monitoring systems for H. circularisquama are important in order to reduce the fisheries damage. The key factors affecting long-term prediction of H. circularisquama bloom is winter water temperature, precipitation in early summer, increase of water temperature, and the development of anoxic waters at the bottom. Further, temporary mixing of the water column by typhoon events and/or niche opening due to decline of predominant diatom populations is also important for the red tide development.

Received on February 6, 2003
Contribution No.A 36 from Fisheries Research Agency
* National Research Institute of Fisheries and Environmental of Inland Sea, 2-17-5, Maruishi, Ohno, Saeki, Hiroshima 739-0452, Japan