A fascinating article by Jules Silverman and Brad W. Taylor in Fly Fisherman looks at why and when trout react to certain insects on the water. Two trips to the Missouri River to fish the Trico hatch left Silverman perplexed about why trout would eat the spinners one week and not at another. Rather than just chalking it up as one of the mysteries of the sport, he decided to do some scientific experiments, and the results are fascinating:
In conclusion, trout develop a search image for a certain (energetically favorable) prey type and potential prey occurring outside this search image are not detected. However, this search image can be expanded over time to include the initially rejected prey by exposure to intermediate-sized prey. In the case of Missouri River trout and their PMD and Trico prey, greatly enhanced continuous exposure to Tricos (more than the small prey we could deliver in our experiments), as well as possible exposure to fewer, smaller, later emerging PMDs may have triggered the feeding shift to Tricos.
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