A juvenile coho salmon holds its position in the flow of a brook. To conserve energy, it positions itself in the lee of a small rock. Distinctive blotches of color on its sides, called parr marks, provide effective camouflage. As long as it holds its position, it is virtually impossible to see. The simple strategy ofkeeping still hides it from the prying eyes of potential salmon-eaters. Kingfishers and herons threaten from above, and cutthroat trout, permanent residents ofthe stream, seldom reject a meal of young salmon. The threat posed by these and other predators is ever present.
The clear water flowing past the salmon presents a stream of food items: midges struggle on the surface; mayfly nymphs drift in the current. But, here's the rub: to capture a prey item, the salmon must dash out from its station, potentially telegraphing its position to unwelcome observers. When the salmon feels safe, it will travel quite a distance to intercept a food item, making a leisurely excursion to collect a drifting midge as far as a meter away from its location.
Detecting a predator changes the salmon's behavior. Depending on the level of the perceived threat, the salmon has several options. It may flush to deep water or another safe location. It may stop feeding altogether, but hold its position. It may continue feeding, but dramatically lllllllllllllllllllllllllllllllllllllllllllllllllllllllllllllllllllllllllllllllllllllllllllllllllllllllllllllllll
reduce the distance it will travel to intercept food. This series of graded responses represents a sophisticated and often effective strategy to avoid predators. Sophisticated or not, all of these responses reduce the salmon's feeding efficiency. The salmon's problem is far from unique; virtually all animals face a trade-offbetween acquiring resources and becoming a resource for another.
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