how many times you see trout rise everywhere on the surface and yet deliberately and carefully ignore the fly you have eagerly offered? many many times here! trout said "sorry, wrong fly again!" usually a very small thing and very often is almost invisible to the frustrated angler: that a blunt big meaty offer often put the feeding activities off. They only want to eat certain thing, and focus on this very image. It has been called "search image" feeding strategy in trout. why? if you have better food items (bigger and meatier) why ignore them? Most of the books will tell you that if fish are "lock in" some pre-request image, it will reduce the research effort even though ignore the nutritional rich item could be even more "costly". Search image strategy has been applied successfully in many predator and prey interaction. see LINK http://en.wikipedia.org/wiki/Prey_detection.
However, for trout feeding specifically on certain insect type or certain stage of life cycle in insects(larva, pupa, emerger etc...). Should we call this behavior is the exercise of the search image feeding behavior? or this is just another alternative feeding behavior that "looks like" search image strategy? Base on the fundamental definition, my answer is a NO, if save energy is the main goal (which is the center of the optimal feeding strategy), trout should NOT avoid big meaty item when it appear in front of them. This feeding behavior will not yield a net extra gain in feeding specifically on certain thing, as we all know trout are opportunitiest general feeder. Trout should pick up everything that looks edible. Even during the peak hatch. But why trout lock them self in feeding certain thing and ignore big flies? predation avoidance behavior is my guess. I argue this is conditioned behavior which is responding to high angling pressure. Only keen on certain thing will ensure their feeding won't be disturbed by angler. This is a evolved alternative feeding strategy that responded to high pressure from angling. How to test it in a scientific way? First, if feeding specifically on certain food items is conditioned we can make some predictions and test it. 1) trout live in the high angling pressure area will more likely to feed specifically compare to the area lacking of angling pressure. 2) In the evolutionary perspective, young trout should also have develop this "search image" behavior to get a higher yield in feed activities. Young trout should have being as selective as old trout (as optimize foraging energy purposed by searching image)... well... I am eager to test this hypothesis, stay tuned, let see if I have gain more information from literature search or field observation..
