Production amount of Japanese littleneck clam Ruditapes philippinarum in Japan is affected by many factors, such as the fishing ground area, fishing effort depending on the density of adult clams, demand for clams and harvests abundance of other kinds of fisheries or aquaculture. These factors for the clam production are also different in each bay, and the cause of the decrease in clam production can not be discussed similarly. In consequence, the factors for the decrease of clam production should be researched in each fishing ground. The density of adult clams in fishing grounds is attributed to the recruitment of juvenile clams and the survival rate of those juvenile clams, and 17 factors affecting the survival rate have been listed up. It is well accepted that the density of clams mainly is reduced in winter, and the main factors for the reduction in winter were assumed to be low temperature, lack of food, movement of bottom sand by wave action, and prey mortality. The movement of bottom sand increased with the decrease in laver culture structures off Chiba prefecture, in Tokyo Bay and it resulted in the reduction of the clam survival rate. Retention rate of clams with 26〜28mm shell length was lower at the bottom friction velocity of over 4cm/s and Shields parameter of over 0.2 on the tidal flats of the Kisarazu area. In recent years, the bottom friction velocity was estimated to 1.2〜1.5 times and Shields parameter was 1.4〜2.3 times of those in 1984 in the Kisarazu fishing area by the hydraulic model test on the base of the decrease in laver culture structures, although those may be under estimated. Accordingly, the movement of bottom sand is considered to be an important factor for the decrease in clam density, associated with the decline in the clam resources. On the other hand, the movement of bottom sand caused higher clam mortalities in the northern area of Chiba in winter than that in the Kisarazu area, and therefore the ratio of density in February to that in October was lower than 5 percent in the northern area while that was 40〜80 percent in the Kisarazu area.

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Biotelemetry is a powerful technique that provides continuous and simultaneous monitoring of fish behavior in their natural environment. The interaction between fish and fishing gear was monitored by biotelemetry. This paper presented three examples of applying biotelemetry to fish behavior around fishing gear. First, three set receivers were installed on leader net of a set net in Lake Biwa, and largemouth bass implanted with a coded ultrasonic transmitter were released near the set net. After northeasterly winds, southwestward currents, and declines of the ambient water temperature, the fish was away from the set net. Catch efficiency using passive fishing gear is affected by environmental factors. Next, a receiver was installed on an anchor line of a Payao in the Philippines. Three yellowfin tunas and two rainbow runners were implanted with the transmitter and released near the Payao. Almost of the fish remained around the Payao for over 60 hours. One of the aggregative keys of the FADs might be the emergence of prey. Third, when a diamond squid was captured by a jig with a data logger in the Sea of Japan, accelerations and depths were recorded in the logger. The results suggested that the jig lured the squid by moving vertically, vibrating, and changing its posture. Biotelemetry, in collaboration with fishing gear telemetry, is helpful to make clear the capture process of the gear.

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Here we discuss the establishment of the brown macroalgae, Ecklonia cava, on a submerged breakwater in Sagami Bay along the Miyuki-no-Hama coast. The small number of available spores for E. cava was the principal factor underlying the localized distribution of this alga. In an attempt to promote the establishment and subsequent transfer of E. cava, the following transplantation technique was employed: during the maturation period of E. cava, 14 40-ton concrete blocks were temporally placed on a natural E. cava community off Nebukawa, 3km west of the coast in October 1993. The E. cava colonized the concrete blocks within a year. All of the concrete blocks were then transferred to the submerged breakwater in December 1994. The same procedure was conducted from November 1994 using an additional 14 concrete blocks, which were transferred to the submerged breakwater in January 1996. No reduction in the E. cava population was observed on the concrete blocks during transplantation. The number of E. cava sporophytes transplanted in this way was estimated at 11,805 in December 1994 and 15,042 in January 1996. While several other seaweed taxa were observed after transplantation, E. cava was dominant and was observed to have colonized the submerged breakwater at depths of 3〜9m in the areas adjacent to the two transplanted zones. High recruitment of juvenile E. cava was observed from 1996 to 1997 in response to lower than average water temperatures, which contributed to range expansion in E. cava. Grazing pressure by herbivores such as sea urchins and fish was low. The climax phase of the E. cava community was reached in March 2001, when the colony occupied an area covering 8000m2. Density and expansion rates of the E. cava community were 16〜34 indiv./m^2 in November 1997, and 20〜48m/yr from 1997 to 2001, respectively. These rates are comparable with those of a neighboring endemic E. cava community. Distribution of E. cava on the crest of the submerged breakwater was limited by the shallow water depth (D.L. 1m). The E. cava community on the eastern section of the submerged breakwater was washed away when the powerful Typhoon 0115 hit Sagami Bay in September 2001. It is suggested that kelp forest creation on submerged breakwaters should be planned with respect to strong wave activity.

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The reef affinity defined as the degree that fish gather at fish reefs has not progressed beyond qualitative evaluations. So, quantitative evaluation is needed. For this reason, indexation of the reef affinity was tested using the fish catch examination results of artificial reefs in comparison with areas without reefs. This fish catch examination was carried out 181 times in nine areas around the Hokkaido coast. First, the existence the reef affinity of each species of fish was tested. Consequently, of 29 species of fish, 15 species were considered to show reef affinity at a statistically significant level. Furthermore, a reef affinity index was defined. The reef affinity index is a numerical value reflecting the fish catch frequency and the number of individuals in a fish reef. And, the reef affinity index was computed for the 29 kinds of caught fishes. As for the reef affinity index of flounders including barfin flounder or halibut, the tendency of ranking was accepted for this result. Moreover, the reef affinity of fishes other than the flounders including rock trout or Atka mackerel was classified into three groups. As stated above, the reef affinity index is considered to be an effective index, in order to quantitatively evaluate the reef affinity of fishes.

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漁具選択性の解析において,しばしば重要となる魚休胴周長の測定効率と精度を改善するために,伸びの無い巻尺とアクリル板を用いた胴周長測定器(以下,測定器と呼ぶ)を考案した。ニジマス(Oncorhynchus mykiss)を対象として,この測定器と従来から用いられてきた糸を用いた胴周長測定の比較試験を行い,測定器の効率と精度を検討した。測定効率の試験(30尾の胴周長の測定時間の計測,5名によって実施)において,胴周長の測定に要する時間は,糸に比べて測定器の方が短かった(1.2-3.6倍)。同じ個体を繰り返し測定した値(20尾の胴周長を10回測定)の変動係数を比較したところ,糸に比べて測定器の方が小さい値であった(CV:測定器0.46-0.98,糸1.18-3.20)。このことから測定器での測定値は,より安定しているといえる。また,2名の測定者が,あらかじめ尾叉長を計っておいた100個体の胴周長を測定器と糸で測定し,測定者間と測定方法の組み合わせの間で尾叉長と胴周長の関係を比較したところ,ひとりの測定者における糸による測定から求められた関係式に有意差が認められた(p=0.02,重回帰分析)。ここで開発した測定器では測定者間に有意差が認められなかった(P>0.05)。測定器を使用した場合,測定者間における測定値の変動は,糸に比べて小さいことが示唆された。本研究で考案した胴周長測定器は,糸の使用といった従来の測定方法に比べて,より早く,正確に魚の胴周長を測定することが可能である。

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Vertical distribution of the sea urchin, Strongylocentrotus nudus, and vegetation in Sakazuki, Tomari, Hokkaido, Japan, was observed monthly during from December 2002 to September 2003. Sampling points were established in the kelp forests, Laminaria religiosa, (St. A) and the Sargassum forests, Sargassum confiisum, (St. B), respectively. Individual number of the sea urchin and coverage of the L. religiosa or S. confusum were recorded, along a belt transects (65m long×1m wide, 0 to 2.9m depth in St. A, 0 to 2.3m depth in St. B). Dominant bottom quality in two stations was rock with large angulations. At deeper zone rather than kelp or Sargassum forest, where curstose coralline dominates, individual density of the sea urchin does not increase over year. Individual density of the sea urchin in kelp bed decreased during from December to next May. In Sargassum bed, the density during from April to September is lower than that of other months.

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海岸防御を目的とした人工的な静穏域である離岸堤背後域(D.L.-2.5m)で,1997年12月にアマモ種子袋で播種し,発芽後,2005年3月まで継続してアマモの生長と成熟をモニタリングした。試験区内のアマモは,天然区と同様の季節消長を示したが,播種試験区外へはほとんど拡がらなかった。試験区のアマモは,株密度の減少が著しい時期が認められ,人工的な静穏域である本地点において,1)台風による厳しい波浪や砂面の変動(2001年,2004年),2)アイゴ等の植食性魚類による過剰摂食(1998年),3)浮遊性アオサ等の寄り藻(2002年)やハスノハカシパン,アナジャコ等による生育場所の競合(1998年〜2005年)等の条件が,近接する天然アマモ場よりも厳しいことが考えられた。

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The production amount of wakame seaweed (Undaria plnnatiflda) in Japan has declined with wakame aquaculture households decreasing. The wakame aquaculture tasks, that are harvesting on board and processing of the salted seaweed product on shore, are concentrated in a period of one month. Especially the harvesting operation is a heavy task, so labor saving measures will be required. In this report, workload analysis methods, such were Ovako working posture analyzing system (OWAS) and low back compression values for human body, were applied for the examples of the harvesting on board. The OWAS workload indexes were approximately in good agreement with the low back compression values. Comparison of workload analysis by OWAS among three types of fishermen's harvesting tasks at Sanriku district, the work efficiencies were strongly affected by AC scores from the OWAS method.

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