瀬戸内海の環境を健全に保全し、それのもたらす自然景観、水産資源を有効活用し、瀬戸内圏を活性化することを大きな目的として、瀬戸内海の(1)栄養塩管理と豊かな海の実現、(2)水産資源とその持続的発展、(3)観光資源としての価値の創造、の3つの小課題を設けて香川県海域で研究を実施した。
(1) 瀬戸内海の栄養塩管理と豊かな海の実現
香川県沖の定点で2020年4月から約2年間、毎月観測と水質分析を行い、栄養塩と呼ばれる海水中の植物プランクトンの栄養分である溶存無機態窒素（DIN）、ならびに溶存有機態窒素（DON）、粒状態窒素（PN）濃度の時空間分布を調べた。このデータを瀬戸内海の高漁獲期（1989-1992年）における同定点の様子と比較し、季節ならびに深度分布における現在と過去（2020年代と約30年前）の違いを詳細に解析した。また、今回（2020年代）の調査では、海底直上海水と海底泥の間に取り込まれた海水中のDIN濃度も測定した。これらの結果を統合して解析することにより、現在瀬戸内海の漁獲低迷の原因として挙げられている瀬戸内海の栄養塩濃度（DIN濃度）減少機構の究明に定量的視点から当たった。また、陸域からの栄養塩負荷を調整して海域によっては栄養塩をある程度人手で管理するという、現在進められている豊かな海の実現に向けた瀬戸内海環境管理政策を進めるうえでの課題の抽出を目的とした。
(2)瀬戸内海の水産資源とその持続的発展
現在、瀬戸内海における漁獲量低下の原因として、「海中の栄養塩が減ったことにより食物連鎖の出発点である海産植物（植物プランクトン）が減少し、魚介類の漁獲量も減少している」というシナリオが強調されている。しかし瀬戸内海域では他にも漁獲量低下への関与が疑われる事象が挙げられており、その最たるものの1つが干潟やアマモ場に代表される浅場の減少である。そこで、高松市新川春日川河口に現存する干潟域と隣接するアマモ場を対象として、干潟・藻場を利用する魚種とそれらの出現時期を明らかにすること、またその餌料生物も含め化学的手法を用いて調査干潟・藻場の食物連鎖構造を解明することを目的に調査観測を実施した。また、この小課題では、激減する水産資源の漁獲回復の具体的な試みとしてアサリに着目し、その種苗確保と水中垂下養殖に関する基礎事項についても検討した。さらに、漁獲される水産物の季節性とその旬について、一般市民にも広く知ってもらうべく、香川県域で漁獲される水産資源についての情報を整理した。
(3)観光資源としての瀬戸内海の価値の創造
海岸線や浅海域を中心にして環境マネジメントという新たな視点から瀬戸内海の観光資源を開拓すると共に、ツアーガイドの養成、ツアーの商品化に取り組んだ。本小課題の主導助成研究者と共に、(1)、(2)の小課題を進める助成研究者が協働でプロジェクトを進めた。助成1年目に観光資源の開拓とガイド養成を行い、2年目にツアーの商品化を行うことが計画された。1年目の観光資源の開拓とガイド養成は、一般社団法人かがわガイド協会と連携し、ガイドの役割等について解説すると共に、瀬戸内海の水質や浅場についての研究成果等をわかりやすくレクチャーする「瀬戸内海観光ガイド研修会」を実施した。そして、2年目では、1年目に実施したガイド研修会に基づいて、ツアーを作り、実際にモデルツアーを実施した。さらに、そのモデルツアーを踏まえて、ツアーを商品化した。

Aiming for environmental conservation of the Seto Inland Sea and leveraging of its natural landscape and fisheries resources for the development and regional activation of this area, we carried out our research projects on this sea under the following three themes: (1) nutrient management and implementation for a bountiful sea; (2) fishery resources management and sustainable development; and (3) creation of new value as a tourism resource.
(1) Nutrient management in the Seto Inland Sea and implementation for a bountiful sea
During an approximately two-year period since April 2020, monthly investigations were conducted offshore at a site off Kagawa Prefecture in the Seto Inland Sea to determine the following nitrogen compounds and their spatio-temporal distribution in the water column: dissolved inorganic nitrogen (DIN), which is the nutrient for phytoplankton growth most deficient in the Seto Inland Sea; dissolved organic nitrogen (DON); and particulate nitrogen (PN). The data obtained during the last two years (2020-2022) were compared with those obtained from the same site during 1989-1992 (the period around the highest annual fisheries yield recorded in this sea) and the differences in seasonal and depth distributions between then and the present were analyzed in detail. During the monthly survey since April 2020, DIN in the sediment pore water and the overlying seawater were also determined. In the Seto Inland Sea, it has been considered that a decrease in nitrogen input from the nearby land was responsible for the reduction of seawater DIN, and subsequent decrease in fisheries yield. The Japanese government revised its policy with the intention of controlling the nutrient status artificially to some extent in this sea by regulating pollutant load to the sea from terrestrial sources. One aim of the present study was to reveal the mechanisms of DIN reduction at the study site based on quantitative measurements. Using the results, the intention was to better inform these attempts at artificial management of nutrient status in the Seto Inland Sea.
(2) Fishery resources management and their sustainable development in the Seto Inland Sea
Currently, the annual fishery catch has been decreasing in the Seto Inland Sea. It has been argued that the decrease of nutrients in the seawater has reduced the amount of phytoplankton (marine plants) that are the foundation/starting point of the food chain system, which causes the decrease of fish. However, other recent phenomena, such as the decrease in shallow water areas including tidal flats and seagrass beds, have also been regarded as possible causes for the decreasing annual catch. Therefore, field observations were carried out to determine 1) the fish species using tidal flats and seagrass beds and fluctuations in their occurrence; and 2) food chain structure including the occurrence of the organisms on which these fish feed, using chemical isotopic analysis of trophic levels. The target area was the tidal flats and adjacent seagrass beds in the estuary at the mouths of the rivers Shinkawa and Kasugagawa, at Takamatsu, Kagawa. In addition, to contribute to the recovery of fishery catches, which have severely decreased, this research focused on the Manila clam, Ruditapes philippinarum (Mollusca: Bivalvia), to examine the basic methods of reliably obtaining seed individuals (juvenile shells settled shortly after their planktonic stage) and their subsequent aquaculture. Moreover, information about the fishery products caught in Kagawa Prefecture has been published to enhance public awareness of the seasonality of this fishery and concepts of the best seasons in terms of both quantity and quality.
(3) Creation of new value of the Seto Inland Sea as tourism resource
From the perspective of the environmental management of coastlines and shallow coastal regions, tourism resources were developed by providing tour-guide training and commercializing of guided tours developed during this project. The research leader of theme (3) benefitted from the co-operation of the leaders of themes (1) and (2). The plan was to develop tourism resources and offer guide training courses for trainees in the first year, and to commercialize the guided tours in the second year. During the first year, in collaboration with the Kagawa Tourist Guide Association, a Seto Inland Sea Tour-Guide Training Course was devised (based on the outcomes of this research project) in which trainees were provided with a series of lectures including topics such as the roles of a guide, water quality, and the tidal-flats and shallow water areas of the Seto Inland Sea. During the second year, model guided tours were provided based on the first-year training course and these tours were then commercialized.

(1) 瀬戸内海の栄養塩管理と豊かな海の実現
香川県沖の定点（水深43 m）で調査を実施し、最不足栄養塩である溶存無機態窒素（DIN）の過去の高漁獲期（1990年前後）と現在の違いを明らかにした。同定点のDIN 濃度は深度や季節を問わず低下していること、その低下幅は粒状態窒素（PN）や溶存有機態窒素（DON）よりもかなり大きいことを定量的に示した。また、水温の上昇する夏の成層期底層とそれに続く水温の低下する秋の混合期始めのDIN低下が特に顕著なことを見出し、この要因として海底からのDIN溶出低下が起きておりこれが本質的に効いている可能性を導いた。以上より、この間のDIN低下は系内よりも系外からの窒素供給低下によることを明示すると共に、海域栄養塩管理政策の実現上、陸域のみならず他経路からの栄養塩供給に着目する必要性を示した。一方、沖合域の植物プランクトン量はこの間のDINのような大幅低下の傾向はなく、漁獲減少原因を栄養塩のみに帰すことは困難な可能性を示した。
(2)瀬戸内海の水産資源とその持続的発展
高松市の河口干潟とアマモ場を対象にした調査から、水産上の有用生物を含む干潟・藻場を利用する具体種の情報を得た。特に干潟域に出現する稚魚については、その出現時期の季節性や依存形態（定住型、短期滞在型、来訪型，通過型）を明かした。さらに、炭素・窒素安定同位体比に基づく解析から、個々の魚類種の餌料資源の利用性に関する知見を得ると共に、総じて、その場での微細藻類による光合成産物が干潟藻場の食物連鎖を支える物質循環上の要であることを示した。これらを通じて、瀬戸内海の漁業や生物生産、豊かな海の実現に対する干潟・藻場の重要性を示した。本小課題では、漁獲減少著しいアサリの増養殖策も検討した。水中垂下養殖の検討では、高密度飼育でも成育に大きな影響を受けないこと、生存率が高いことから同手法が瀬戸内圏で有用な可能性を示す一方、費用面での課題を指摘した。また、本小課題では香川県域の漁獲物について、季節に応じた様々な水産資源が水揚げされており、それらには “水揚げの旬”、 “味の旬”があること等を整理した。季節の旬の魚を、地域住民や観光客に楽しんでもらう必要があり、瀬戸内海の魅力を伝える一手段として、また、観光資源としての活用を視野に入れ、旬の魚について学術誌「瀬戸内海」に掲載した。
(3)観光資源としての瀬戸内海の価値の創造
「瀬戸内海観光ガイド研修会」の成果として、瀬戸内海の環境と生物に関する素養を備え、機会に応じてその内容を解説できるガイドを10人以上養成した。また、養成したガイド自身による企画内容を外部のプロガイド講師と助成メンバーが適宜ブラッシュアップすることで、瀬戸内海の環境について考える機会を併せ持つ8つのエコツアーを造成した。さらに各ツアーの主企画者により、瀬戸内海観光ガイド研修会を受講した研修生ならいずれのツアーでもガイドできるように、ガイド用マニュアルが作成された。本ツアーの中には、香川県山田海岸における生物観察会等、本小課題（1）、（2）と深く関係する内容も含まれている。これらのツアーは、連携してガイド研修会を進めてきた一般社団法人かがわガイド協会を受付先とする形で、2021年度末に商品化された。今後、本ツアーがシンポジウム等の従来の座学聴講型とは異なる形で、瀬戸内海の環境を学ぶ機会となることが期待される。

(1) Nutrient management in the Seto Inland Sea and implementation of a bountiful sea
At the offshore site of Kagawa Prefecture (43 m depth), DIN concentration in 2020-2022 differed markedly in comparison with measurements around 1990, when fisheries yield was around its peak: almost always lower than DIN levels in the past, irrespective of month and sampling depth, and considerably reduced in comparison with DON and PN levels. This demonstrates that the recent decrease in DIN at the study site is due mainly to changes in the external input of nitrogen, rather than the intrinsic supply indicated by DON and PN levels. A larger reduction of DIN concentration from the past to the present was detected in the lower layer of the water column during the stratification period and in the entire water column after the stratification period. Our estimation of nitrogen budget at the sediment-water interface suggests that this large reduction of DIN in the seawater is attributable to a decrease in DIN input from the bottom sediment rather than from a terrestrial source. The results suggest that, to achieve a desirable nutrient status under the environmental policy for the Seto Inland Sea, it is important to consider origins of external nitrogen input other than those of from terrestrial sources in determining the sources of DIN in the ambient seawater. In addition, phytoplankton biomass at the offshore study site did not show a decreasing trend despite the clear DIN reduction, suggesting that decreasing fisheries yields should not be attributed solely to nutrient reduction.
(2) Fishery resources management and their sustainable development in the Seto Inland Sea
Based on field observations at the study area, the species present were identified and those harvested as fishery products were noted. The juvenile fish species obtained in sampling at tidal flats were investigated to clarify their seasonality and habitat types (habitats used for settlement, short-term stay, visiting, or in transit). Based on stable carbon and nitrogen isotope ratio analysis, the organisms used as food sources for each fish species were identified. In addition, it was shown that microalgal photosynthetic products at the site are the key to nutrient circulation and thereby provide significant support of the food chains of tidal flats and seagrass beds. This research has shown the importance of tidal flats and seagrass beds for the productivity of the Seto Inland Sea, especially regarding improvements in fishery production to recover a bountiful sea. Under this research theme, the focus was on the Manila clam to determine how best to effect recovery of this important resource, the production of which has severely declined. The results reveal that high-density aquaculture is possible, with higher survival and growth rates. However, also pointed out were a number of challenges for Manila clam aquaculture, especially the high financial investment required. We have collated seasonal information concerning various fishery products caught in Kagawa Prefecture, identifying two kinds of “best season”: one best season for catch quantity; and another for quality, when the fish have an optimal amount of tissue lipid load and consequently have the best taste. To enhance the awareness of, and to enjoy the seasonality of the fishery products, we published two articles in the academic the magazine “Seto Inland Sea” for the benefit of both tourism and local residents. Without doubt, enjoying the best seasons of various fishery products can be local tourist attractions to encourage people to visit Kagawa and the surrounding regions of the Seto Inland Sea.
(3) Creation of new value of the Seto Inland Sea as tourism resource
The Seto Inland Sea Tour-Guide Training Course was used to train more than ten guides in knowledge of the environmental and biological backgrounds of the area sufficient to provide information of interest to future participants of the tour. Based on the revision and feedback by the project members, as well as from active professional guides, the newly-trained guides themselves planned and produced the eight currently available ecological tours, which could also lead the expected participants to thoughtfully consider the environment of the Seto Inland Sea. The chief planner of each guided tour also created a guide manual, so that all the trainees of the Seto Inland Sea Tour-Guide Training Course can guide any of these eight tours. The tours include field observations of the animals of the habitat at Yamada Shore in Kagawa Prefecture, which are highly relevant to themes (1) and (2). These tours were commercialized in March 2022 and people can book a place on the tours through the Kagawa Guide Association, which has been collaborating with us to promote the guide training course. It is expected that these tours, which differ greatly from conventional classroom learning, can offer a different opportunity to learn about the environment of the Seto Inland Sea.

• 讃岐の旬の魚Ⅰ（D19-R-0050）
• 讃岐の旬の魚II（D19-R-0050）

• 39号号　座談会「多様な連携の形が新しい変化を生み出す」

プログラム名（Program）	2019 研究助成 Research Grant Program
助成番号（Grant Number）	D19-R-0050
題目（Project Title）	きれいな海から豊かな海への実現戦略 ―瀬戸内海の環境と資源のマネジメント Implementation Strategy for a Bountiful Sea from Clean Seawater: Management of the Environment and Natural Resources in the Seto Inland Sea
代表者名（Representative）	山口　一岩 / Hitomi Yamaguchi
代表者所属（Organization）	香川大学農学部 Faculty of Agriculture, Kagawa University
助成金額（Grant Amount）	￥5,500,000
リンク（Link）
活動地域（Area）