- 著者
-
小林 裕
Kobayashi Hiroshi
- 出版者
- 三重大学水産学部
- 雑誌
- 三重大学水産学部研究報告 (ISSN:02875772)
- 巻号頁・発行日
- no.13, pp.p25-133, 1986-11
- 被引用文献数
-
1
この研究は,深海性サメ類と深海底延縄との関わり,および,分布と生態的な特性を解明するため,熊野灘海域(本州南東岸)において、主として底延縄による漁獲試験を行い,つぎの諸点を明らかにした。1.漁獲試験を行った水域は,長鳥沖,尾鷲沖,および,阿田和沖に存在する海底谷,あるいは,その近傍を主とする水域(3漁場)である。これらの海域の深海(200~1100m)における海洋環境(水温,塩分量)は,周年安定していて,漁場間の相遵もきわめて微少である。また,溶存酸素極小層の酸素量は,1.6~1.7mL/Lで1500mに至ると約2.1mL/Lまで増加する。長島沖,および,尾鷲沖漁場の海底地形は,海底谷をやや離れた比較的平坦な水域であったが,阿田和沖漁場は海底谷の真上,および,その近傍の海底傾斜の急峻な凸凹のある複雑な水域であった。海底を覆う底質は3漁場とも細かい砂を含んだ泥土であった。2.底延縄の海底における縄成り(枝縄の水中形状)は,漁具性能を決定する重要な要素の一つであり,浮子とロープの浮力,その海水抵抗,沈子の固定力とその流水抵抗,および,海底における流速などによって決まる。理論計算の結果,枝縄は,海底においてほぼ直立状態にあると考えてよいことが分かった。3.この研究によって,熊野灘海域から生息が確認された底層性サメ類は,これまでに,この海域から確認されている種,この海域で初めての種,稀にしか漁獲されない種など8科20属31種(底延縄が3科10属19種,底刺網が7科13属17種)に達した。これらの中から,ツノザメ科サメ類9属18種を,これまでに,他海域(銚子沖と駿河湾)において記録されているツノザメSqualidae科のサメ類(TANIUCHI、田中、TANAKA,YANO and TANAKA)と比較すると,隣接海域の共通性は駿河湾を挟さんだ銚子沖と熊野灘海域の共通性に比べて高い。4.長島沖,および,尾鷲沖漁場ではへラツノザメDeania calceaが,阿田和沖漁場ではタロウザメ Centrophorus acus が最も多獲され最優占種の地位にある。漁場によって第2優占種以下の種が異なる。種組成を類似度指数Cλ(Morisita)を適用し比較すると,長島沖漁場と尾鷲沖漁場の類似性はきわめて高い。しかし,これらの2漁場と阿田和沖漁場の類似性は低く,サメ類相は種組成においても量的組成においても明瞭な相違がある。5.底層性サメ類(主としてツノザメ Squalidae 科のサメ類)の深度別釣獲率(分布)を種に関係なくみると,どの漁場においても釣獲率の一つのピークが,300~500mに存在する。この深度を越えると漸次減少傾向に移行するが再び増加し,長島沖漁場では,1000mを超える深海に,きわめて高い釣獲率域の存在が確認された。これは1000m以探における高い密度域の存在の示唆と考える。また,魚種組成は異なるが釣獲率の変化傾向は,どの漁場においてもほぼ同じであった。さらに,この結果はインド洋西部で Foster et al. が,行った漁獲試験結果とよく対応する。釣獲率を種別にみると,分布水域は種間で重なりを持つが,高釣獲率域は,種によって異なり,多獲される種は,漁場と深度によって特定される。また,漁獲種数は深度によって異なる。これらの結果は、深度によってサメ類相が質的にも量的にも相適することを示すものと考える。6.タロウザメ Centrophorus acus は、等深線の尾根付近の比較的傾斜の緩やかな水域を,モミジザメ Centrophorus squamosus は海底谷の傾斜の急峻な水域を主分布域とする。両種の混獲関係には,負の相関性が認められた。また,フトカラスザメ Etmopterus priceps とニセカラスザメ Etmopterus unicolor とは,分布の重なりは全くなく,明瞭にすみ分けていることが分かった。さらに,タロウザメ C. acus は、海底地形の急峻な水域で,へラツノザメ Deania calcea は比較的平坦な水域で多獲され生息域に地形的な相違があることが分かった。このように,深海性サメ頬は、地形や深度によって独自の生活場所を形成している。7.延縄に沿った漁獲傾向を調べた結果,一回の操業で延縄が敷設された限られた水域における,深海性サメ類の水平的な分布のパターンは,ランダム分布で近似できるが,種によって所々に濃密な分布域が存在することが分かった。8.枝縄に沿った漁獲傾向から,深海性サメ類は,必ずしも,常に着底生活を営むものでないこと,どのサメ類も海底から,10数メートルを越えて上昇し摂餌していることが推定された。9.海洋環境と漁獲の関係を検討した結果,深海性サメ類の海洋環境への適応の幅は,比較的広いことが推定された。また,タロウザメ C.acus, へラツノザメ D.calcea, および,カラスザメ E.pusillusなどの探海性サメ類は,急激な海洋環境の変化に対する耐性が強いことが推定された。 このような特性を持つ深海性サメ域の構成種を,制限し決定する要因の一つとして,海洋環境が,強く関与しているとは考えにくい。地形的な相遼が結果したものと考えられる。10.夜縄と昼縄の漁獲を比較したところ,釣獲率は昼縄に比べて夜縄の方が著しく高い。この結果は,深海性サメ類の摂餌活動が,昼間より夜間に活発化することの現われであり,夜間に摂餌活動が活発化する種の多いことを物語る。11.深海性サメ類の加害生物は,食害の痕跡から腐食性生物(Scavenger)が目立つ。サメ類も加害魚として関わりを持つ。しかし,サメ類の漁獲と食害魚との間に相関性を認めることばできなかった。これは,サメ類が食害を受けにくいことを示すものと考える。サメ類の食害率は,低く平均5.2%であった。12.生物調査結果 a.フジクジラ E.lucifer,および,へラツノザメ D.calcea の全長組成のモードの位置は,雌雄で明らかに相違(雌の方が大型である)する。また,タロウザメ C.acus,モミジザメ C.squamosus,へラツノザメ D.calcea,ニセカラスザメ E.unicolor,および,ビロウドザメ S..squamosus では,複数の位置にモードがみられ,モードの位置より小さい側で漁獲個体数が少ない。これは,漁具の選択性によるものと考えられる。へラザメ A platyrhynchus とフトカラスザメ E. princeps は資料が少なくモードの位置は明確でない。 b.全長と体重の問には,明瞭な曲線関係が存在する。両者の関係は,Table14の通りである。へラツノザメ D.calceaの雌では,調査した月(5月と10月)によって体重の相違が認められたが,どの種についても全長と体重関係の調査月による相違は肯定できなかった。 C.肝重比(体重と肝臓重量の比)は種によって異なる。また,同種であっても個体間のバラツキが大きい。へラザメ A platyrhynchus の平均肝重比は10%に満たない。フジクジラ E.lucifer は約12~13%であるが,アイザメ Centrophorus 属のサメ類では20数%に達し,この海域で漁獲されたツノザメ Squalidae 科のサメ類の申で最も大きい。サメ類の肝臓機能(Baldridge,Castro,Compagno)を考えると,肝重比の大小は,種による行動能力の強弱を強く反映しているものと考える。また,へラツノザメ D.calceaとフジクジラ E.luciferの肝重比は,明らかに調査した月によって相違が認められた(Fig.32参照)。 d.胃内容物の目視観察を行った結果、深海性サメ類は,底層を基盤として生活するサメ類であるが,底層から表層付近に至る幅広い生物を捕食していた。アイザメ Centrophorus属のサメ類,および,へラツノザメ D.calceaからは,表層および中層性の魚類が観察された。落下物を捕食した可能性もあるが,これらのサメ類の肝重比は大きく,行動能力との関連が考えられる。 e.一般に,この海域で漁獲されたサメ類の性成熟の大きさは雌の方が大きい。フジクジラ E.lucifer,および,ニセカラスザメ E.unicolorなどで明瞭な相違が認められた。年を通してみた,この海域における大部分のツノザメ Squalidae 科のサメ類の性比は1:1であるが,へラツノザメD.calceaとフジクジラE.luciferでは調査した月によって違いが認められた。 繁殖生態に関する調査と観察の結果,オシザメ P.microdonlの産仔様式が非胎盤性胎生(卵胎生)であること,胎抒の栄養吸収方は卵食性でないことが確認された。 また,交接器,子宮,生殖巣を観察した結果,タロウザメC.acus,および,モミジザメ C.squamosusは,未熟個体に限られた。大部分のへラツノザメ D.calceaは未熟であったが,成熟した雌2個体が確認された。へラザメA platyrhynchus(卵性)の輸卵管内には周年卵殻が観察された。また,フジクジラE.luciferは8月に胎仔が,どの季節にも大型卵巣卵,子宮内下降卵が観察された。ニセカラスザメ E.unicolorでは4月に胎仔が,5月と10月に大型卵巣卵が観察された。これらのサメ類の雄の精巣は膨大していて,周年精液が確認された。このように,種によって繁殖生態の一端を知ることができたが,多くの種については,断片的な資料にとどまり,多くの未解明な事項がのこされた。今後の課題として他梅域を含む調査と研究が望まれる。 13.深海底延縄漁業は,他の漁業との競合が少なく,地形的制約を受けることも少ない。また,小人数で小型の船を用いて行うことができる。したがって,現在,利用度のきわめて低い熊野灘海域における深海漁場の開発とその利用は、現漁業の延長として十分考えることができる。また,深海性サメ類は,残された数少ない未利用水産資源の一つであり,その適正な漁獲と利用は,今後の地域漁業の発展を考える上に取組まなければならない重要な課題の一つと考える。This study was performed in the Kumano-nada region along the Pacific coast of Japan, mainly, by fishing tests using bottom longline, in order to clarity distribution and ecological characteristics of deep-sea shark and their relationship with deep-sea bottom longline. The following are the results obtained. 1. Areas of fishing tests are located at 5 - 15 miles lrom the coasts of Nagashima, Owase and Atawa. Marine environment (water temperature and salinity) of deep-sea in these regions (200 - 1,100m) was stable the whole year, and difference among fishing grounds were also quite small. The dissolved oxygen was 1.6 - 1.7mL/L, and it, reached to 2.1mL./L at 1,500m - depth. Bottom topgraphy of the fishing ground of Nagashima and Owase offshores was relatively plateau at a distance from a submarine valley, but the region of the fishing ground of Atawa offshore was just over the valley or over uneven steep slopes around the valley. Sea bed was covered with fine mud. 2 . Shapes of branch-lines of bottom longline on the sea bed is an impor tant facter for the evaluation of efficiencies of fishlng gear, and it is determined by buoyancy of floats and ropes, their current-resistance, the fixing power of sinker and its current-resistance and the speed of current on the sea bottom. Theoretical calculation based on our data showed that branch-lines on the sea bottom would be almost in a vertical position. 3 . Deep-sea sharks, confirmed for their inhabitation in Kumano-nada region through this study, reached to 31 species covering 24 genera of 8 families ( 3 families, 10 genera, 19 species by bottom longline and 7 families,13 genera, 17 species by bottom gill net) including known, new and rare species in this region. Among them, dogfish sharks (18 species of 9 genera)were compared with the sharks of squalidae reported so far in different region (Choshi-offshore and Suruga-Bay; Taniuchi, Tanaka and Yano ).Commonness of the species in adjoining region were high compared to those in the separated regions (Choshi and Kumano, separated by Suruga-Bay). 4 . Needle dogfish, Centrophorus acus, and birdbeak dogfish, Deania calcea, were the highest catches and the first dominant species in fishing grounds of Nagashima and Owase offshores (former species ) and Atawa offshore (latter one ), relatively. Second and less dominant species varied depending on the fishing ground. When content of the species were compared by the Morisita's Cλ fishing ground of Nagashima and Owase offshores showed quite high similarity, while Atawa offshore showed a low similarity compared to the former two grounds. Consequently, the shark biota were clearly different depending on the fishing grounds in terms of both the contents in their species and amounts. 5. Hooked-rate at different sea depths (distribution) were checked on deep-sea sharks (mainly dogfish shark ) independently of the species. A peak ofthe hooked-rate was found at 300 ~ 500m in each fishing ground. The ratio deereased gradually as the depth exceeded this level, but increased again; alayer with quite high hooked-rate was found at more than 1,000m-depths at the Nagashima fishing ground. This could be an indication that there wouldbe a layer (s) with high density of sharks under futher deepend area. These change in the depth were basically the same in all the studied fishing grounds, although the species-content was different. ln addition, these results corresponded well with another result of fishing tests of Forster et al. (1970) performed in a western area of the Indian Ocean. As regarding the species, their distribution areas were overlapping, their areas of high hooked-rate differed and the species with frequent catchingwere spesified by the fishing ground and sea depth. Catches in number regarding species and their varibility differed depending on the fishing ground. It was also found that shark biota changed with sea depth. 6. The main distribution region of needle dogfish , Centrophorus acus, was located around the axes of the valley and leafscalegulpershark, Centrophorus squamosus, were located around the ridg of countourelines, respectively. A negative correlation was found between the two species on their simaltaneous catching. Aiso, no overlap was found with regard to the distribution of great lanternshark, Etmopterus princeps, and Brown lantern shark, Etmopterus unicolor, clearly indicating their habitat segregation. Deep-sea shark, thus, were forming their habitat independent from each other by each own specific conditions of topography and depth. 7. Based on the catching tendency along the longline, it was found that spatiai distribution of deep-sea sharks on the sea bed is approximated atrandom. Based on the catching tendency along the branch-line, it was found that dogfish sharks were not always leading a bottom-clinging life; all speciesshow feeding activity asending over 10 meters from the sea bed. 8 . Results of an examination on the relationship between marine environ ments and catches indicated that the adaptation ability of deep‐sea sharksinto marine environments was relatively high. lt seems reasonable that the topographic characteristics, but not a marine envirornent, are concerned with these specific species or deep - sea shark asfactors which restricted and identified them into one group. 9. Hooked‐rate for night operation was remarkably high compared to day operation. This result indicate that feeding activity of dogfish sharks is morefrequent at night than day. 10. Scavengers were found to be the most frequent natural enemy of dogfish sharks judging from the signs of damage, in addition to some sharks as the enemy fish. No prey‐predator correlations, however, were found among caught sharks. Rate of damaged sharks was low (5.2% in average). This result seem to indicate that sharks are highly tolerant againist natural enemies. 11. Biological examination and observation a) Positions of mode or T.L.. composition were clearly different between male and female (female was bigger) in the case of birdbeak dogfish, Deania calcea and blackbelly lanternshark, Etmopterus lucifer. Two modes were observed for needle dogfish, C. acus, leafscale gulpershark, C. squamosus, birdbeak dogfish, D. calcea, brown lanternshark, E..unicolor,and velvet dogfish, Scymnodon squamulosus,.Their catches in number were low at smaller side of the mode's position, although the results could be explained as matter of selecting the rishing gear. The Position or the mode of spatularsnout catfish, Apristurus platyrhynchus, and great lanternshark, E.princeps, were unclear because of insufficient data. b) A distinct relationship on a curved line was observed between the total length and weight. The equational relationship is shown in Table 14. The relationship varied depending on the investigated months (season) in the case of birdbeak dogfish, D. calcea, and blackbelly lanternshark, E.lucifer. c) L/B (ratio of body weight to liver) was species‐dependent, but the rate fluctuated widely even among strain of the same species. Mean valuesof the L/B of spatularsnout catfish, Apristurus platyrhynchus, and blackbelly lanternshark, E..lucifer, were below 10% and 12 ~ 13%, respectively. Sharks of the genus Centrophrus showed over 20% of the rate; this was the highest value among sharks of family Squalidae which had been fished in this region. Based on the liver function of sharks (Baldridge, Castro and Compagno) the size in the L/B seemed to be reflecting strongly each species strength of action ability. In addition, obvious changes in the L/B were observed depending on the investigated month (season) in the case of birdbeak dogfish, Deania calcea, and blackbelly lanternshark, E. lucifer (see Fig.32). d) Macroscopic observation of stomach contents indicated that deep‐sea sharks were praying upon many creatures ranging from the sea bottom to the surface layer despite the fact that they were deep‐layer living sharks. Surface‐living fishes, were found in stomachs of birdbeak dogfish, D.calcea,and sharks of the genus Centrophorus. The L/B of these sharks were high, reflecting their high action abilities. e ) The size of sexual maturation was generally large in females among sharks fished in this region, and this difference was remarkable especially in blackbelly lanternshark, E. lucifer, and brown lanternshark, E..unicolor.Most sharks of fhe family Squalidae in this region showed a sex ratio of 1: 1 for a whole year, although some monthly (season) variations were found in bridbeak dogfish, D. calcea, and blackbelly lanternshark, E..lucifer. The mode of reproduction of false catshark, Pseudotriakis micrdon, was the mode of non-placental type and the nutritional type of embryo was not oophagous. Observation of the clasper, uterus and sexual gland confirmed that all the examined strains of needle dogfish, C. acus, and leafscale gulpershark, C.squamosus, were immature. Most birdbeak dogfish, D. calcea, were immature, and only two matured females were observed. The egg case was observed for a whole year in the oviduct of spatularsnout catshark, Apristurus platyrhynchus, (oviparous ). In the case of blackbelly lanternshark, E. lucifer, embryo was found in Augast and large ovarian eggs and fertilized eggs were observed for a whole year. In the case of brown lanternshark, E. unicolor, an embryo and large ovarian eggs were observed in April and in May and October, respectively. Testis of the male of these sharks was swelling, and seminal fluides were observed for a whole year. But the data on most species were still fragmentary. In order to fulfill these unsettled term, further studies are required expanding the investigation regions. 12. Bottom longline in the deep-sea has very little competitions with other fisheries, topographic limitations are negligible and it can be performed bya small number of people using a small boat. Therefore, development and application of deep-sea fishing ground in the Kumano-nada region, which has very little utility value at present, can be considered as an extention of present fisheries. Since deep-sea sharks are one of the few remaining fisheries resources undeveroped, proper fishing control and application will be the important subjects we have to solve considering future developments of local fisheries.