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With a choice between high risk and high reward , versus low risk and low reward .Results demonstrate a neatly graded foraging response among instars in regard to changing risk and reward .
A similar experiment was conducted with minnows in an artificial stream where the presence or absence of predators , cover , and food were manipulated ( fraser and cerri , 1982 ; cerri and fraser , 1983 ) . Adult creek chub were the predators , while juvenile chub and blacknose dace Rhinichthys atratulus were the prey . Avoid-ance of risk was clearly demonstrated : prey avoided patches with predators , and were more likely to take risks during the day and when structure was present .Offered a choice among four alternatives ( food versus no food cross-classified with risk versus no risk ) , juvenile R.atratulus showed no indication of balancing risk against reward . Instead they appeared to choose a patch based on food , and reduce their use of a patch proportionate to predation risk and independently of reward .
Juvenile coho salmon , Oncorhynchus kisutch , feed on stream drift by making short excursions from a holding position . Using house flies as prey and a model of a rainbow trout as threat , Dill and Fraser ( 1984 ) asked whether risk reduced foraging , and whether the reduction was proportionate to risk . Exposure to the model trout reduced reaction and attack distances and shortened attack time compared with young salmon foraging in the absence of threat ( Figure 7.10 ) . The investigators then varied the frequency with which they presented the model before the salmon , thereby varying the level of risk , and found that attack distance decreased proportionately ( Figure 7.11 ) .
Moreover , responsiveness to the model was reduced by higher hunger levels and the presence of a competitor . This behavioral flexibility evidently allows young coho salmon to adjust their foraging in a quite complex fashion .
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類似的實驗，進行魚的人工流的存在或缺乏天敵，封面，糧食被操縱（弗雷澤和cerri ， 1982年; cerri和弗雷澤， 1983年） 。成人河鰱魚是食肉動物，而少年鰱魚和blacknose鯪魚Rhinichthys atratulus的獵物。迴避風險是清楚地表明：獵物避免與天敵補丁，並更有可能採取的風險在白天和結構時，本。提供了一個選擇的四個備選方案（糧食與食物沒有交叉分類與風險與無風險） ，少年R.atratulus顯示，沒有跡象顯示風險的平衡對報酬。相反，他們似乎選擇一個補丁的基礎上的食物，並減少其使用補丁相稱捕食風險和獨立的獎勵。
少年銀鮭，銀大馬哈魚，飼料就流漂移，使短期旅行從一個控股地位。用房子作為捕食蒼蠅和模型，虹鱒魚作為威脅，蒔蘿和弗雷澤（ 1984年）詢問是否覓食的風險降低，以及是否減少相稱的風險。曝光模型鱒魚減少的反應，攻擊距離和時間縮短，攻擊比較年輕的鮭魚覓食在沒有威脅（圖7.10 ） 。研究人員然後不同的頻率與他們提出的模型前，鮭魚，從而不同的風險程度，發現攻擊距離比例下降（圖7.11 ） 。
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