粗放的育成池におけるマダイ仔稚魚の生態観測

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自然海域における資源添加のため放流するマダイ種苗を半野外的条件にある廃止塩田改造池(2m深、9,000㎡)で粗放的に育成している現場において、これらが浮遊生活から底層へ定着する過程の生態を潜水観察を中心に調べた。(1) 地には2・3の体長群に分れて成長するマダイ仔稚魚が、種苗として放流されるまで地中でかなり高密度(10～30個体/㎡)で共存して生残り成育する。(2) 孵化直後仔魚の浮遊生活は全長10㎜を境に終るが、水深2mの池底の生活に直ちに移るのでなく、素早い全長10～15㎜の段階をへて、成育のよいものから逐次、底層生活に移る。(3) 全長6㎜より腹鰭第1軟条(li)が相対的に(li/S.L)特異的に伸長し、この伸長した腹鰭を体軸に直角に立てて底面などに定位する。全長15～35㎜の段階で底面に単独solitaryに定着するマダイが観察された。(4) 定着しているマダイに近づく個体があるとこれを追い払う攻撃行動が、全長15～25㎜個体のほぼ1/3、25～35㎜のほぼ1/4で示された(1978年の渚部観測)。しかし、より高密度(15個体/㎡以上)ではこの行動はみられなかった。(5) 夜間、水中ライトの近くに集まるマダイ稚魚の観測(1980)によると、おそらく池中の大型餌生物の不足により、全長20㎜をこえても、なお小型餌生物であるプランクトンに依存し続けるマダイが、夜間も燈光に集まる小型動物を摂食するものと考えられる。そしてまた、相対的に成育のおくれたマダイが追加的に投与される魚肉ミンチに餌付くことなく、最後まで池の中で増殖する餌生物に依存し、かなりの数健全に生残る。(6)マダイ稚魚による池底空間の利用の観点からみると、単独(solitary)、集まり(aggregation)、群れ(school)といった集団形態の違った存在が、狭い空間を高度に分け合って共存している。自然水域において柔軟性と優位性を発揮するマダイが、単一種・高密度という"実験"条件においても、柔軟性を発揮しているものとして注目に価しよう。(7)粗放的に育成されるマダイ仔稚魚は、池の空間・食物nicheを分け合って利用する複数の存在によって生残っているが、放流後も含めて、これらが、定着 "persist with" したり、移動 "shift from" したりする現象も、生き残りのための摂食戦略の観点から解析する可能性について論じた。The systematic underwater observations on the larval red sea bream Pagrus major, cultured for liberation in the semi-wild extensive farming saline pond (Fig.1 ) 1),2) were carried out at 160 stations by using scuba during a period from 6th of June to 16th of July in 1980. The fish grew in the No.1 pond as shown in Fig.2 during the observation. In turbid water, where prey organisms, pelagic and demersal, were propagated, the observable range were limited to 0-50 cm in surface layer, and to bottom-30 cm above in bottom layer, beside to horizontal 30 cm on both side of diving course. The OCCURRENCE, numbers of the fish observed along 2 m line of each station, was recorded (Tab.1 : surface layer; Tab.2: bottom layer).Results: 1) The total of OCCURRENCE (Fig.3) was larger in surface layer than in bottom layer when the larvae were 6-8 mm long in their total length, and then it gradually increased in bottom layer during middle and late June when they grew to 15-35 mm, and finally leveled off in July. Horizontal distribution of fish, expressed by rim total of OCCURRENCE for each observation (Fig.4), was shown to be uniform over the bottom, except the structural shallower portions of the pond (b, o: near shore and k; upper bank). Frequency distributions of the OCCURRENCE could be splitted into 4 components of Poisson distribution (Fig.6): means of each component ("A" to "D") were < 1,4,10, and 30-40, respectively, implying an existence of different types of grouping of the fishes with different density. Temporal change in compositions of those Poisson components (Fig.6), might show the change of appearance of "groups" over the bottom, as following: at first sparsely ("A") then gradually densely inhabited by "B", "C" and "D" in turn, presumably each corresponding to "solitary", "aggregated" and "school"10). Values of MORISITA's Iδ- Index 3), as measure of dispersion of individuals of the fish in flat portion of the pond (Fig.5), drop at the first step of appearance in bottom, then level off about Iδ=1.2-1.5, and finally go upward. Those imformations tell us the 2-3 size groups growing in the pond (Fig.2) might inhabit efficiently the bottom space niche. changing their types of grouping.2) The morphological characteristic feature on elongation of 1st soft ray of ventral fin (Fig.11) between S.L. 6-20 has presumably certain behavioral significance relating to the opening of sedentary life of the fish 4)5). At the similar phase, the aggressive behavior of solitary individual, observed on the beach portion of the pond in 1978 (Tab.3, Fig.10 and Fig.12), was shown by about 1/3 of individuals which were 15-25 mm long and by about 1/4 of those being 25-35 mm long in total length. In the night observation in 1980, the OCCUIXRENCE of the fish crowded in the light of underwater lamp, was counted (Fig.13). The OCCURRENCE decrease gradually by time, and size of those fish (Fig.2, black bar) were also differing gradually from the previously grown ones. Those fish crowded in the light would appear as the feeder of light attracted plankton from necessity, sharing food niche of the pond with other size groups.3) Those size groups in the pond utilize their niche, adjusted each other, space and food. And "persist with" or "shift from" their habitat after liberation is discussed from viewpoint of feeding strategy of fish for surival 12)-23)

粗放的育成池におけるマダイ仔稚魚の生態観測

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