イサキの資源生物学的研究

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  • イサキ ノ シゲン セイブツガクテキ ケンキュウ

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イサキは一本釣や定置網の対象魚種として非常に重要であり，また栽培漁業の適種のひとつにもあげられている。ところが，近年漁獲量の減少や，魚体の小型化など資源の安定性に問題がおこり，本種の資源特性や資源状態を解明することが必要となってきた。このような現状から，著者は特に三重県熊野灘沿岸のイサキについて種々の資源生物的特性を明らかにする目的で研究を行ってきた。本報告は9章から成り，まず第1章では全国的なイサキ漁業の概観と三重県前島地方のイサキ漁業の実態について述べ，第2章では本種の地理的分布や移動について検討した。第3章から第6章までは資源生物学的知見として最も重要な年齢・成長，年齢組成・生残率，成熟・産卵，および食性について論述した。第7章では人工種苗生産に関する諸問題について検討を加え，この研究で得られた卵，仔稚魚を使用して第8章では本種の初期生活史，特に卵から若魚に至る期間の形態形成過程について述べた。最後に，第9章ではこれまでに得られた知見を総合して本種の資源解析を行い，資源の維持・管理および増殖を行う施策について検討した。これらの研究によって得られた知見を要約すると次のようになる。 1．漁 業 (1)漁場は黒潮および対馬暖流域でかつ天然礁がよく発達した海域に形成される。代表的な漁法は一本釣で，このはか定置網や刺網などでも混獲されている。主漁期は全国的に4月，5月から10月，11月であった。 (2)三重県前島半島沿岸では年間150～160トン程度の漁獲量があった。一本釣漁獲量は1982年をピークにして減少傾向を示し，魚体の小型化も認められた。 2．分布・移動 (1)わが国における通常の分布域は太平洋沿岸では千乗県～鹿児島県，対馬暖流域では新潟県～鹿児島県であった。国外では韓国南部や台湾，中国大陸東シナ海，南シナ海沿岸に分布が認められた。分布の特徴として，暖流性，外海性があげられた。また本種は沿岸性が強く，いわゆる「大陸棚固有種」であると考えられた。 (2)本種は大規模回遊は行わないが，季節的に局所的な向岸一向沖移動は行っていると考えられた。 3．年齢・成長 (1)年齢表示形質として鱗、耳石，椎体，主鰓蓋骨，上後頭骨背側隆起板の5硬組織を観察した。 (2)本種の鱗は典型的な櫛鱗で，被覆部には明瞭な輪紋が形成されていた。体各部の鱗には相似的に輪紋が形成され，どの部位の鱗であっても年齢形質として利用できると考えられた。 しかし，実際には体中央部(胸鰭後端付近の側線直下)の鱗が最も大形で，かつ外形の歪みも少ないことから，輪紋の読取りはこれが最良であった。鱗の輪紋は年2回，1月～2月と6月～7月に形成された。 (3)耳石は比較的大きく，明瞭に透明帯と不透明帯が観察された。この不透明帯の幅は著しく狭く，線状であった。このような耳石の帯構造は，15％中性ホルマリンで固定した場合，数ヶ月後でも観察可能であった。耳石の輪紋（不透明帯の外縁）は年1回6月～7月に形成された。この期間は産卵期と一致しているため，輪紋数は直接満年齢を表していると考えられた。 (4)椎体の後方陥人面には不明瞭ではあるが透明帯と不透明帯がみられた。しかし，この輪紋は相似性が悪く，また輪紋が欠落している場合も認められた。 (5)主鰓蓋骨の帯構造は極めて不明瞭であった。また，上後頭骨は明瞭な帯構造が認められるが，不透明帯数は大形魚でも2帯しか認められなかった。 (6)このような観察結果から，これら5硬組織のうち年齢形質として適当であるのは鱗と耳石で，他の3硬組職は不適当であると判断された。鱗と耳石を比較すると，高年齢魚での輸紋の明瞭さや複輪，偽輪の出現率の点で耳石の方が優れていた。 (7)耳石の輪紋から年成長を推定した。平均的な年成長は(13)式で表され，大部分の個体がこの式で表した値の前後 3cm以内の成長を示すと考えられた。 (8)耳石の縁辺成長率から，季節的な体長の増大は8月から10月にかけての期間が最大であった。 (9)成長補償作用が確認された。 (10)飼育実験の結果から，孵化後1日から407日までの初期成長は(28)式，(29)式で表された。 4．年齢組成・生残率・自然死亡係数 (1)一本釣で漁獲されたイサキの年齢組成をいくつかの方法で推定し，比較した。その結果，年齢査定法と繰返し計算法によって得られた値が最良であった。 (2)年齢組成から最近の生残率は0．25程度であると推定された。 (3)自然死亡係数は0.5～0.6と推定された。 5．成熟・産卵 (1)本種の生殖腺は左右不相称である個体が全体の半数強を占め，このうち石偏の個体が左偏の個体よりも多かった。 (2)生殖腺の成熟状態は部位に関係なく，はぼ均一であった。 (3)2歳魚以上の卵巣内には卵黄胞期以下の成熟段階の卵が周年存在した。第1次卵黄球期以上に成熟した卵は5月から8月まで出現し，特に胚胞移動期以上の卵は2歳魚では6月に，3歳魚以上では6月から8月にかけて出現した。 (4)精巣の成熟段階を精子形成準備期，精子形成期，精子放出期，精子吸収期の4期に分けた。精子形成準備期は12月あるいは1月から4月，精子形成期は4月から5月，精子放出翔は6月から8月，精子吸収期は9月から12月であった。 (5)生殖腺組織の観察やGIの変化および水槽内での産卵実験の結果から，産卵期や産卵時刻，最小成熟年齢などについて次のように推定した。産卵期は6月から8月で，若齢魚の産卵期間は高齢魚よりも短い傾向が認められた。日本国内での産卵期に顕著な地域差は認められなかった。産卵水温は通常20～28℃であった。産卵時刻は20時をピークとして前後1時間程度であった。最小成熟年齢は雌雄とも満2歳で，生物学的最小形は雌で尾叉長16cm，雄で15cm程度であった。 (6)性比は成長にともなって雌よりに偏る傾向を示し，これは雌雄の生残率の差に原因していると考えた。各年齢群の性比を(34)式，(35)式で表した。 (7)産卵前後の卵巣の重量差と卵巣卵数から産卵数を推定し，(40)式に表した。また個体群としての産卵量は3歳魚が最大であると推定された。 6．食 性 (1)釣と定置網で漁獲された標本の胃内容物を比較した結果，定性的には明瞭な差は認められなかったが，空胃率は明らかに釣獲標本の方が高かった。 (2)摂餌活動には季節的な周期性がみられ，夏季には活発になり，冬季には活動が低下した。 (3)未成魚・成魚の主餌料はキビナゴやマイワシ，カタクチイワシなどの魚類および橈脚類や端脚類などであった。これらのことから，本種の食性は幅広い肉食性であると考えられた。 (4)後屈曲期仔魚の餌料はすべて橈脚類であった。稚魚期になると，これに加えて二枚貝幼生や橈脚類などを捕食するようになり，さらに若魚期では，これらのほかヤムシ類や尾索類およびカタクチイワシのシラス期仔魚などを餌料としていた。 (5)餌料動物の大きさを捕食者体長との相対値αで表した。αの最大値は0.6前後で，典型的な魚金魚と比較するとかなり小さい値を示した。 (6)摂餌量に関する問題について飼育実験を行い，次のような結果を得た。魚体の大きさと最大摂餌量との関係は(44)～(46)式で表された。摂餌量は水温と密接に関係し，12℃以上では有意な正相関が認められた。また，生殖腺の成熟にともなって摂餌量が減少する傾向がみられた。 (7)摂餌後の胃内容物の減少速度に関する実験を行った。その結果，胃内容物減少率は水温が高いほど，また魚体が小さいぼど高くなる傾向がみられた。 (8)捕食・消化器官の形態は全体的に特化の程度が低く，これは食性自体の特化程度が小さいためであると考えた。ただ，鰓耙上の針状突起はよく発達し，これによって，橈脚類などの捕食が可能になると考えられた。 7．種苗生産 (1)本種は漁獲直後の親魚から自然採卵することが可能であった。 (2)孵化率は通常50～100％であった。孵化率と浮上卵率や卵径の変動係数との関係には有意な相関がみられ，これらは卵質判定の指標になると考えられた。 (3)胎盤性性腺刺激ホルモンは．畜養親魚を用いる限り，有効性は認められなかった。 (4)孵化率は卵収容容器の表面積に対する卵密度と逆相関するが, 20粒/cm²以下の範囲内では密度の影響を受けなかった。 (5)最適孵化水温・塩分はそれぞれ22.6℃，34.2‰と推定された。孵化率と水温・塩分との関係は(65)式で表された。 (6)水温18℃～28℃の範囲内では，孵化に要する積算温度は一定で，孵化時間と水温との関係は(67)式で表された。 (7)孵化後30日から407日までの体重の増加を(68)式で表した。 8．初期生活史 (1)受精卵は球形の分離浮遊卵で，卵径は0.78～0.85mm，卵黄は無色で表面に亀裂があった。油球は単一であった。受精後40分で2細胞期になり，28～30時間後に孵化が始まった。 (2)孵化仔魚は全長1.50～1.65mmで，3日後に卵黄を吸収し終り，開口した(全長2.71～2.80mm)。全長約5.5mmで脊索の屈曲が開始し，約12mmで鰭条総数が定数に達して稚魚に移行した。全長約35mmで体がはぼ完全に被鱗し，若魚になった。 (3)仔魚期の形態的特徴は，前期仔魚では油球が卵黄の後下方に位置していること，肛門が卵黄から離れていること，膜鰭縁辺に黄色素胞が発達することなどで，また前屈曲仔魚でほ全体に黒色素胞が少ないこと，後関節骨後端や肛門直前の膜鰭に塊状黒色素胞があることなどであった。 (4)本種特有の暗色縦帯は稚魚期に発現し，若魚期に完成した。この縦帯は人工種苗の方が天然稚魚に比較して小さな体長で形成された。 (5)仔魚期から若魚期における体各部の相対成長を(70)～(86)式に表した。人工飼育魚と天然魚の相対成長を比較した結果，人工飼育魚の体形は本質的には天然魚と大差ないと考えられた。 (6)鰭の形成過程を観察し，次のような結果を得た。原基が形成される全長は胸鰭2.8mm，尾鰭4mm．背鰭，臀鰭6mm，腹鰭8mm前後であった。全長12mm付近で鰭条総数が定数に達し，18mm前後になると棘数も定数に達した。各鰭軟条の分枝過程が完了するのは全長140mm前後であった。 (7)鱗の形成は全長16mm程度から開始され，35mm前後で吻以外は被鱗した。吻まで完全に被鱗し，鱗形成が完了するのは全長130mm程度であった。 (8)腸管の施回は全長3.1mm付近から開始され，3.7mm前後で完了した。胃の伸長や幽門垂の形成は全長12.7mm付近から開始された。全長22.5mm程度で幽門垂数が定数に達し，消化管の形態は成魚とほぼ同様になった。 9．資源の現状と管理および増殖 (1)これまでに得られた生物学的知見と漁獲資料から，三重県志摩町沿岸のイサキ資源の解析と診断を行った。1歳魚以上の資源尾数は約660万尾と推定された。現状の資源における雌親魚数と産卵数はそれぞれ処女資源の41.2%，16.4%に減少し，この資源は濫獲状態にあると判断された。 (2)現在の濫獲状態を是正し，最大持続生産を行うためには漁獲努力量を3690 boat・day程度に減少さすか，あるいは漁獲最小尾叉長を17～18cm程度まで引上げるという2方法の漁業規制が考えられた。 (3)さらに積極的に資源を増殖させるためには種苗放流も効果的であると予想された。

The threeline grunt Parapristipoma trilineatum (Pisces : Perciformes : Haemulidae) is a valuable species as an edible fish along the coasts of southern Japan. The present report deals with the fishery biology of this species, with particular reference to its fishery, geographical distribution and migration, age and growth, age composition and survival rate, maturity and spawning, feeding habits and morphology of feeding and digestive organs, artificial spawn taking and larval rearing, embryonic and larval developments, and stock assessment. The materials used in these studies were mainly caught from coastal areas of Kumano-nada (Kii Peninsula, Mie Prefecture) during the years from 1978 to 1984. The rearing experiments were carried out at the Fisheries Research Laboratory, Mie University. The results obtained are summarized as follows : 1. Threeline Grunt Fishery Fishing grounds of this fish were formed in the rocky reefs along the warm currents, or Kuroshio (Pacific coasts) and Tsushima Current (coasts of East China Sea and Sea of Japan). This species was caught representatively with hook and line, and allso with set nets and gill nets. The fishing season generally began in April or May, and continued until September or November. The commercial landing of the threeline grunt with hook and line, set nets, and gill nets was totally estimaled at 150 to 160 metric tons per year at the Pacific corst of Sakishima Peninsula, Mie Prefecture in 1982 and 1983. The catch in weight with hook and line increased rapidly during the years from 1979 to 1982, but it decreased in 1983. The average length of the fish caught with hook and line became smaller during the years from 1979 to 1983. 2. Geographic Distribution and Migration The threeline grunt was distributed in southern Japan, the southern coasts of the Korean Peninsula, Taiwan, and along the East and South coasts of mainland China. But this species did not inhabit the sea along the Ogasarwara Islands, Ryukyu Islands, nor the Philippines. This fish was one of the representative submarginal continental endemic species. ln Japan, the fish normally inhabited the area from Kagoshima Prefecture to Chiba Prefecture along the Pacific coasts, and from Kagoshima Prefecture to Niigata Prefecture along the coasts of the East China Sea and the Sea of Japan. This species preferred warm water temperature, high salinity, and rocky shores. The threeline grunt did not make a large-scale migration which was observed in the tunas Thunnus spp., the yellow tail Serlola quinqueradiata, or the saury Cololabis saira, but made a seasonal offshore-inshore migration in limited areas. 3. Age and Growth The scale, otolith, centrurn, opercle, and supraoccipital crest were observed in detail in order to select the most suitable hard tissue for age determination of the threeline grunt. The scale was a representative ctenoid, and ring marks were shown clearly on the embedded part. The ring marks formed similarly on scales taken from any part of the body. Although any scale on the body could have been used for age determination, the scale taken from a row beneath the lateral line just posterior to the tip of the pectoral fin (B-region in Fig. 10) was the most suitable for mark reading because of having the largest scale radius and less distorted shape. Ring marks on scales formed twice in a year, both in summer (June or July) and in winter (January or February). The otolith was fairly large and was removed very easily from the cranium. Hyaline and opaque zones were shown clearly without grinding or sectioning. The opaque zones were very narrow and shown as opaque lines. Otoliths preserved in 15 % buffered formalin for some months were sufficiently readable. Ring marks on the otoliths (outer margins of opaque zones) formed once a year in June or July. Because such months coincided with the spawning period of the fish, the number of ring marks on the otolith represented the full age of the fish. Ring marks on the longitudinal sections of the centra were very obscure, and lack of ring marks frequently appeared. Zonation of the opercle was extremely indistinct. The supraoccipital crest had distinct hyaline and opaque zones, but only two opaque zones could be recognized in any large fish. From such observations, the author concluded that only scales and otoliths could be used for age determination of this species among those five hard tissues. The otolith was more suitable than the scale because the ring marks were shown distinctly even for older specimens, and lack or duplication of ring marks were very few. From otolith reading, average length growth of the fish was expressed by the following equation:Lt=357.5(1-exp (-0.2855( t +0.2909))), where L=fork length in mm and t=full age. Range of the length growth was estimated by the mean values and standard deviations of calculated fork lengths at the time of each ring formation. The growth for about 95% of the fish was within the limits of the following two equations:Lt=362.1(1-exp (-0.3377( t +0.3622)))) and Lt=355.8(1-exp (-0.2427( t +0.1283))). The length growth was more rapid during the months from August to October than the other months. The growth compensation was obsered in the growths of 2‐ to 4‐age‐groups. The early length growths were estimated from rearing experiments and these were expressed by the followilg two equations:TL=0.00986t² + 0.0734t + 2.43(t≦30) and TL=324(1-exp (-0.00983(0.127t+5.26+8.53 sin (0.0172 t -20.0))))(t>30), where TL=total length in mm and t = days after hatching. 4. Age Composition, Survival Rate, and Natural Mortality Coefficient Age composition of the threeline grunt caught with hook and line was estimated by otolith reading for the sampled fish, by analyzing the polymodal frequency distribution of the fork length, by solving simultaneous equations based on the length compositions of each age-group and the length composition of commercial catch, and by an iterative calculation method based on the market size or length compositions of each age-group and the length composition of commercial catch. Good results were obtained by otolith reading and the iterative caluculation methods. Survival rate was estimated at 0.25 from the age composition of the fish by LEA's (1930) and DOI's (1975) methods. Natural mortality cosfficient was calculated at 0.5-0.6 by DOI's (1977) and PAULY's (1981) methods. 5. Maturity and Spawning The larger half of speciments had a pair of gonads bilaterally asymmetrical in weight. Of these, the speciments with dextral gonads dominated over the specimens with sinistral ones. The developement or maturity of ovarian eggs proceeded uniformly throughout the ovary. According to the degree in advancement toward ripeness, the ovarian eggs were divided into the following nine stages ; (1) peripheral nucleolus stage, (2) yolk vesicle stage, (3) early yolk globule stage, (4) middle yolk globule stage, (5) later yolk globule stage, (6) migrating nucleus stage, (7) pre-maturation stage, (8) maturation stage, (9) ripe egg stage. The ovarian eggs at peripheral nucleolus and yolk vesicle stages were present throughout the year in the ovaries of the groups older than age2. The eggs at yolk globule stages or more mature stages appeared during the period from May to August, especially the eggs at migrating nucleus stages or more mature stages were found only in June in 2-age- group and between June and August in the groups aged older than 3. From the histological observation, the process of testicular maturation was divided into the following four phases ; (1) preparative phase : the testis was occupied mainly by spermatogonia, (2) spermatogenesis phase : primary and secondary spermatocytes were dominant in the testis, (3) discharging phase : spermatozoa filled up in the testis, (4) resorptive phase : spermatozoa resorbed by the seminiferous epithelial layer. The testes of such phases appeared in the following months ; (1) preparative phase : December or January to April, (2)spermatogenesis phase : April and May, (3) discharging phase : June to August, (4) resorptive phase : September to December. The spawning season began in early June, and continued until mid-August. The younger fish (2- and 3- age-groups) finished their spawning earlier than the older ones. The water temperature of the spawning ground ranged from 20 to 28℃. The time of actual spawning in a day usually fell at around eight p.m. The group maturity rates of both sexes for each age-group were estimated as follows ; 1-age-group : 0% in both sexes, 2-age-group : 43% in females and 83% in males, 3-age-group : 95% in females and 100% in males, 4- to 8-age-groups : 100% in both sexes. The biological minimum sizes were about 160 mm in fork length in females and about 150mm in males. The sex ratio of males to females declined with their growth, and this charge was attributable to the difference of the survival rates between both sexes. The percentage occurrence of females in each age-group (Pf) were expressed by the following equation : Pf = 100/(1＋1.921・0.788t), where t = age. The number of eggs spawned in a year by a single female (= fertility) was estimated from the difference of the ovary weight between samples of pre-spawning and spent conditions. The fertility (Fe) was given by the following equaion: Fe = 4.759L⁵.²⁸¹・10⁻⁸, where L = fork length in mm. The total amount of eggs spawned by the 3-age-group was the largest among the all age-groups. 6. Feeding Habits Empty stomachs occurred more frequently in the samples collected with hook and line than in those with set nets. The feeding activity was intense during the period from May to September, and declined after October. Main foods of the adult and subadult fish, larger than 100 mm in fork length, were　clupeoid fishes (Spratelloides gracilis, Sardinops melanostictus, Engraulis japonicus, etc. ), but crustaceans also were fed on abundantiy in March, June, November, and December. Clupeoid fishes appeared as the prey animal in almost all seasons. In crustaceans, copepods chiefly appeared in spring and autumn, while amphipods in sumrner. Postflexion larvae fed on only copepodspecies (Corycaeus spp., copepodid larvae, etc. ). Juveniles became to feed on branchiopods and larval bivalves in addition to copepods. The main foods of young were an arrow worm Sagitta sp., copepods (Eucalanus spp., etc.), larval decapods (mysis and zoea larvae, etc), protochordates (Oikopleura dioica, etc.), and larval anchovy Engraulis japonicus. Maximum value of the size preference (αmax=PLmax / L, where L = fork length of the predator and PL.max = maximum size of the prey animal) was 58% in the fish of 50 to 100 mm in fork length. The value of αmax decreased with growth for the fish larger than 100 mm. This value of the threeline grunt was fairly smaller than that of a piscivorous fish as the Japanese bluefish Scombrops boops. Maximum amount of food ingested (Fmax) was given by the following equation : log Fmax=0.842 log W-1.026, where W = body weight in g. The amount of ingestion was positively related to the water temperature. There was a little decrement of the amount of ingestion according to the gonadal development. The gastric emptying rate tended to increase with rising environmental temperature, while the rate varied inversely with the size of fish. The feeding and digestive organs (teeth, jaws, gill rakers, stomach, pyloric caeca, and intestine) of this species were less specialized owing to its unspecialized feeding habits. But the needle ‐like processes on the gill rakers ( = secondary gill rakers) were well developed, and they enabled the fish with only a small number of gill rakers to feed on such small crustaceans as copepods. 7. Artificial Spawn Taking Spawning was observed twenty times or more during the period from early June to mid ‐July. It was possible to collect the fertilized eggs from non‐reared parental fish. Even injection of gonadotropin (HCG) took no recognizable effect. Hatching rate usually ranged from 50 to 100 %. The rate was significantly related to the ratio of floating eggs and inversely to the coefficient of variance of the egg diameter. Accordingly, the ratio and the coefficient of variance could be indicators appraising the quality of eggs spawned. The hatching rate was inversely related to the areal density of the eggs in a hatching vessel, but the rate independent of the areal density within less than 20 eggs/cm² and of the voluminal density of eggs in the vessel. Optimum temperature and salinity for hatching were estimated at 22.6 ℃ and 34.2‰, respectively. The relationship between the hatching rate (Hr,%), temperature (T,℃) and salinity (S,‰) was expressed by the following equation : Hr=-1.984 T² - 4.125 S² + 70.92 T + 269.9 S - 0.5475 TS - 530. The total effective temperature ((T - T₀) Ht, where Ht = time to hatching in h, T = water temperature in ℃, and T₀ = biological zero in ℃ ) was almost constant within the limits of water temperature ranging from 18 to 28 ℃.　Consequently, incubation time was given by the following equation : Ht = 241.5/(T-13.2). The weight growth curve for the juvenile and young stages was expressed by the　following equation : W = 449 (1-exp (-0.00983 (0.127t + 5.26 + 8.53 sin (0.0172t - 20.0))))³.⁰⁷, where W = body weight in g and t = days after hatching. 8. Embryonic, Larval and Juvenile Development Fertilized eggs, measuring 0.78 ‐ 0.85 mm in diameter, were buoyant, spherical, and transparent. The yolk was segmented partly and contained a single oil globule. Hatching oocurred at 28 to 30 h after spawning when incubation temperatures ranged from 20.5 to 21.5 ℃. Total lengths (TL) of newly hatched larvae ranged from 1.50 to 1.65 mm. In yolk‐sac larvae, a single oil globule rested on infra‐posterior portion of the yolk sac, the anus opened away from the yolk, and xanthophores developed on the dorsal and anal margins of the finfold. The yolk and oil globule completely absorbed in three or four days after hatching, and the larvae became preflexlon stage. The notochord flexed at about 5.5 mm TL in 19 days after hatching. Aggregate numbers of all finrays were completed at about 12 mm TL, and they became juveniles. Squamation was almost finished in specimens of about 35 mm TL, and they changed to the young. Specific longitudinal bands started to form in the juvenile stage and completed in the young stage. The bands appeared in smaller specimens of reared juveniles than those of wild ones. There was little difference in the relative growths of some body parts between reared juveniles and wild ones. The larvae and juveniles of the threeline grunt bore close resemblance to those of Haemulon pulmieri and Orthopristis chrysoptera rather than those of Plectorhynchus pictus and P.cinctus. The anlages of each fin formed at 6 mm TL (dorsal and anal), at about 4 mm (caudal), at about 2.8 mm (pectoral), and at about 8mm (pelvic). Aggregate numbers of all finrays and the numbers of fin spines were completed at 12 mm TL and 18 mm, respectively. Segmentation of the softrays occurred at 9 to 17 mm TL in dorsal fin, at 8.5 to 15 mm in anal fin, at 6.4 to 9.6 mm in caudal fin, at 9.5 to 60 mm in pectoral fin, and 12 to 16 mm in pelvic fin. Branching of the softrays was observed at 24 to 130 mm TL in dorsal fin, at 20 to 140 mm in anal fin, at 12 to 18 mm in caudal fin, at 24 to 130 mm in the pectoral fin, and at 14 to 34 mm in pelvic fin. Scales appeared at about 16 mm TL. The head and body except the snout were almost covered with scales when juveniles attained to about 35 mm. The snout was covered by scales and squamation was completed perfectly at about 130 to 140 mm. Convolution of the alimentary canal started at about 3.1 mm TL and finished at 3.7mm. The pyloric caeca formed at about 12.7 mm, and completed in number at about 22.5 mm. 9. Stock Assessment Based on the above data, the present parameters of the threeline grunt stock at the coast of Sakishima Peninsula were estimated as follows : natural mortality coefficient(M) = 0.6 , survival rate(S) = 0.25, total mortality coefficient (Z) = 1.39, fishing mortality coefficient (F) = 0.79, rate of exploitation (E) = 0.504, and availability of 1-age-group (Q) = 0.058, catch in number for 1‐ to 8‐age‐groups (C) = 1,275,400, fishable population size in number for 1- to 8‐age-groups (Nc) = 2,966,000, population size in number for 1-age‐group (N₁) = 3,861,500, and total population size in number for 1‐to 8‐age-groups (N) = 6,603,500. From the stock size estimates, the population size of mature females and total number of eggs spawned in the present stock decreased to 41.2 % and 16.4% of those in　unexploited stock, respectively. Accordingly, it seemed that the present fishing intensity more or less exceeded the optimum level. And further, such two methods as reducing the fishing effort to 3690 boat・day in a year, and rising the length at first capture to 17 ‐ 18 cm in fork length, were conceived in order to manage the stock at the optimum level.

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