イワシ棒受網漁業の水揚げ量と種組成が選別作業時間に及ぼす影響を明らかにすることを目的とした。阿久根漁港において各漁船の選別開始・終了時刻と選別作業人数を記録した。北さつま漁協の水揚げ伝票から調査対象船の魚種およびサイズ別の水揚げ量を解析し，水揚げ物組成をシンプソン指数で示した。水揚げ量と選別作業時間の関係式から，水揚げ量が2,000 kgでシンプソン指数が0.1未満，0.1以上0.2未満，0.2以上0.3未満，0.3以上0.4未満，0.4以上0.5未満，0.5以上0.6未満，0.6以上の時，選別作業時間はそれぞれ0.7, 0.8, 1.1, 1.2, 1.9, 2.2, 3.4時間になると考えられた。
The Pacific bluefin tuna (PBT) Thunnus orientalis is an important commercial species. The life cycle aquaculture of PBT has been widely studied in Japan. Collisions or contact with the net cage leads to mortality in juvenile PBT. Therefore, it is important to understand the schooling behavior of juvenile PBT to reduce contact and collisions.
In this study, we developed a fish schooling behavior model based on a Boids model to understand the schooling behavior of PBT. The parameters used to simulate PBT schooling were optimized using a three-dimensional movement track. The optimized model results agreed well with the actual measurements. Next, the proposed model was applied to circular, octagonal, and square net cages. Simulated PBTs swam in a circular pattern inside the circular net cage. In the octagonal and square net cages, schooling behavior was less obvious than in the circular net cage. In addition, the modeled PBT swam nearer to the wall in the octagonal and square net cages compared with the circular cage. Consequently, the collision risk was higher in the octagonal and square net cages.
Using the proposed model, we can simulate the behavior of PBT. Ultimately, a net designer can use the model to assess collision risk. We believe that our model will enhance fish aquaculture.
本研究では，カオス時系列データからノイズを除去する方法について論じる。対象とするのはUedaモデルである（Ueda 1973, 1978, 1979, 1985, 1991）。Uedaモデルはパラメータが特定の場合のダフィング方程式の解として得られる。本研究でUedaモデルを用いた理由は，船体横揺れ運動方程式がダフィング方程式で記述される場合があるからである。解がカオスとなるようにパラメータの設定は，Kan and Taguchi（1990）の論文を参考にした。Ohtomoら（1995, 1996）は，カオス時系列データのパワースペクトル密度関数が指数関数的に減衰することを示している。本研究では，この特性を利用した。本研究で提案した方法を用いることにより，高い精度でノイズの除去が可能であることが明らかになった。
Understanding the capture process of fishing is important for the sustainable use of fishery resources. In this study, we observed the entering and escaping processes of coon stripe shrimp, Pandalus hypsinotus, at the shrimp pot fishery in Funka Bay, southern Hokkaido, Japan. The survey was performed during spring and autumn fishing seasons for the shrimp pot fishery in 2019 and 2020. Experimental pots equipped with an underwater camera and a red LED light were placed on the seafloor at 80–100 m with other pots for fishing. A total of 32 shrimps were observed in eight surveys. Eleven shrimps did not enter the pot and just escaped at the funnel, and the other 4 individuals escaped through the funnel after entering the pot and making contact with the bait. Twelve individuals fell to the bottom of the pot and 2 of them escaped through the funnel after falling. No shrimp escaped through the mesh of the pot. The mean duration of contact with the bait was short, 41 min, indicating that the shrimp’s time allocation for activity in the pot was mainly occupied by moving and remaining in the pot. Therefore, it is suggested that the main escape behavior is escaping from the funnel, and if the pot is installed with escape gaps, encountering the gaps may occur while moving in the pot.
The foundations of offshore wind power generation facilities are said to have reef effects such as collecting fish. However, some fishermen are of the opinion that the reef effects are to be expected, while others are concerned that it may promote recreational fishery and lead to problems with the coastal fishery. There are currently few research or survey results on the specific reef effects in Japan. While the foundations of offshore meteorological masts installed by Japan Wind Development Co., Ltd., off Kodomari and in Mutsu Bay in September 2020 are smaller in scale than the foundations of actual seabed-fixed offshore wind power generations, but they have almost the same installation depth and the same structure.
The authors have therefore continuously surveyed and analyzed the habitat and growth of fish, fish feed organisms, seaweeds and benthic animals in and around the monopile and scour protection of offshore meteorological masts. As a result, their reef effects as well as the protection and growth effect of fishery resources as a nursery ground for young fish and juvenile fish have been clarified. It has also been found that the planning and construction of offshore wind power generation facilities can be linked to the development of fishing grounds (fisheries public works projects), such as the release of seedlings, the creation of seaweed beds and breeding grounds, and the creation of artificial reefs, based on the life history of fishery organisms, to achieve sustainable fisheries.