様式：課程博士用2015 年
環境・生命工学専攻
申請者
氏名

今村

学籍番号080601

教員

和志

論 文 要 旨
論文題目
(要旨指導1月16 日

加藤 茂
井上 隆信

(博士)

アカウミガメの繁殖活動に影響を与える砂浜環境に関する研究―表浜海岸を対象として―

1,200 字程度)

本研究では，良好な砂浜環境の指標種としてアカウミガメを設定し，その繁殖活動に焦点を当て，砂浜幅や海浜植生などといった砂浜環境との関係を調べた．これらの結果をもとに，環境と調和した砂浜海岸の保全・再生について提言した．
第 1 章では，研究対象種であるアカウミガメの生態，減少要因，保全の重要性および研究対象海岸の概要等について整理した．
第 2 章では，現地調査の概要について整理するとともに，2008 年～2014 年の期間における豊橋・湖西市域で確認されたウミガメの上陸回数についてとりまとめた．
第 3 章では，表浜海岸に訪れるウミガメの繁殖活動を「上陸」・「産卵」
・「ふ化・脱出」の 3 ステージに分類し，現地調査からウミガメの繁殖活動に適した砂浜環境について検証した．上陸ステージにおいては，潜堤が設置された正面の砂浜ではウミガメの上陸密度が粗になる傾向や潜堤群と潜堤群の間に上陸が集中する傾向が明らかとなった．産卵ステージにおいては，2008 年～2010 年の期間，豊橋市域に上陸したウミガメ 327 例の移動痕跡を元に各個体の挙動を分析した結果，未産卵で帰海したウミガメ 110 例のうち，半数の 55 例は人工構造物（消波堤・消波工）が原因で，帰海したことが明らかになった．また，産卵場として選択した環境を分析した結果，海浜植物の存在やそれらの地下茎や根の発達に伴う地形変化（急峻な砂浜勾配の形成）が産卵のトリガーになる可能性を示した．さらに，上陸後の移動距離の計測結果から，表浜海岸に上陸するウミガメにとって産卵のために必要とされる砂浜幅は，最短でも 40m 以上必要であることが示された．ふ化・脱出ステージにおいては，砂中温度が高温化し，胚が 32℃以上の温度に長時間曝されると，ふ化率の低下を招き，特にふ卵期間後期の暴露時間に左右されることが示された．一方，砂中温度の低温下がふ化率に及ぼす影響については，初期に 24℃以下の低い砂中温度を経験しても，中期～後期にかけて砂中温度が上昇すればふ化率の低下は避けられることが明らかとなった．
第 4 章では，表浜海岸のうち，地形変化等の情報が蓄積されている豊橋市内の 3 海岸を対象に，砂浜幅や標高といった環境要素の現況を分析し，ウミガメの産卵成功率等を向上させるための改善策について検証した．その結果，ウミガメにとって好適な産卵・ふ卵範囲は消波ブロックの陸側に存在し，ブロックを越えられないウミガメには利用不可能であることや，ブロックの海側エリアは標高が低く，台風等の高波浪時には産卵巣が水没したりする可能性が高いことが分かった．ブロックのセットバックや堆砂を促し，ブロックを埋没させる等の対策を講じることで，ウミガメが容易に浜奥へと移動できるようになり，産卵成功率が向上すると考えられる．
第 5 章では，ウミガメの産卵に配慮し，既設の消波ブロックを陸側に移設し，緩傾斜堤へと改良したエコ・コースト事業について検証した．当該事業地内におけるウミガメの産卵状況については，同期間における豊橋市全域の産卵成功率よりも高い値を示し，産卵個体の平均移動距離も豊橋市域の平均移動距離よりも長いことが明らかとなった．
第 6 章では，本研究で得られた知見を体系的にとりまとめ，環境的な側面から海岸の保全目標を設定するための諸量を明らかにした．

year month day2015 1 16
Department Environment and Life Engineering
Name

ID

080601IMAMURA Kazuyuki

KATO
Shigeru
Supervisor INOUE
Takanobu

Ａｂｓｔｒａｃｔ

Title

Study on environmental factors of sandy beach affecting the reproductive activityof loggerhead sea turtles (Caretta caretta) at Omotehama Coast（800 words）
The sandy beaches of Enshu Coast (Omotehama), located on the Atsumi Peninsula in the center ofJapan, support a large nesting habitat for the endangered loggerhead sea turtle (Caretta caretta). It is
important that this coast is located nearly at the northern limit of the North Pacific Ocean and that thesandy beaches provide a suitable environment for turtle oviposition.
The reproductive activity of loggerhead sea turtles can be classified into three stages: landing,oviposition, and hatching emergence. The purpose of this study is to investigate the influence ofvarious environmental factors such as beach width, beach elevation, artificial structures, and sandtemperature on the three stages of the reproductive activity of loggerhead sea turtles on the beach.
Submerged breakwaters have been constructed in the western part of Toyohashi area and may affectthe landing activities of loggerhead turtles. The number of landings was found to be smaller on thebeach in front of the submerged breakwaters than in the other areas of the beach without breakwaters.
The breakwaters clearly obstructed turtle landings in this area. Because the breakwaters protect thebeach from erosion, further consideration may be necessary, e.g., shortening the dike and widening thedistance between dikes on coastal managements.
Most egg depositions occur between the mean high tide lines and vegetation lines. In the coastal area of
Toyohashi, many precast armor units (blocks) were placed in the middle of the beach for shore
protection. These blocks sometimes interrupt the turtle landings and ovipositing activities. The field
sign called “turtle tracks” created by turtle landings gives a lot of information on various turtle
behaviors after their landing. Analysis of turtle tracks revealed that among the sea turtles that failed tooviposit, half were unable to oviposit because of the blocks. Investigation of the landing distance of sea
turtles with GPS and measuring tapes also revealed that the necessary beach width is more than 40 mfrom the high tide level on the beach. In general, sea turtles lay eggs around the border between the
sandy beach and vegetation area or deeper in the vegetation zone to protect eggs from wind and waves.
Blocks disrupt the path from the foreshore to backshore and prevent turtles from approaching theirideal nesting area. The frequency and rate of using the vegetation zone for nesting on the basis of thedata from the Kosai area (where no blocks were installed) were investigated in 2009–2011 and2013–2014. Almost 50% of the turtles selected the vegetation area for nesting. In addition, turtles
tended to lay eggs after having climbed over the steep slope formed by an underground stem and theroot of the plant where they decided on a suitable nest site.

To understand the nest productivity of loggerhead sea turtles, we collected sand temperature datausing loggers, measured depths of egg chambers, investigated the hatching/emerging success rate (insitu and in relocated nests), and recorded nest incubation lengths at the Omotehama Coast from 2008 to2010. Our results showed that the distance between the top and bottom of the nests was uniform despite
the various depths of nests. A significant negative correlation of the emergence success rate wasobserved when the sand temperature over 32°C exceeded 150 hours of the cumulative exposure time.
In contrast, no significant correlation was observed when the sand temperature was lower than 24°C.
The beach is restored by reforming the coastal facilities in Toyohashi city. This action is called theEco-coast project. This project was the first trial in Japan and was conducted on the basis of aconsensus among stakeholders such as the local government, community residents, and Non-ProfitOrganizations. After removing the precast units, the shoreline distance has been gradually increasing;
however, the sectional area has been gradually decreasing. The coastal topography and behavior of sea
turtles tended to change toward those of natural beaches and the oviposition rate increased. Vegetation
was damaged by the Eco-coast project because of transferring the blocks. To investigate the recoveryof the vegetation, the belt transect method was performed in this area in 2013–2014. The coastal
vegetation has recovered in less than five years and is stable in the area that is at an elevation of morethan 4 m. It was confirmed that the Eco-coast project restores the function of the sandy beach judging
from the ovipositional viewpoint of the sea turtle nesting. On the other hand, some new problems haveoccurred such as depredation because of the wild animals and light pollution because of an increase incoastal users. It is necessary to take countermeasures for depredation and light pollution with
continuous monitoring because it is known that the depredation increases every year and artificial
lighting obstructs the turtles' ovipositon. To live in harmony with wildlife, it is necessary to continue
monitoring and to give feedback after the project.
By analyzing the reproductive activity of sea turtles, we succeeded in obtaining some data to maintain abetter condition of sandy beaches from an environmental point of view.