While I cannot answer the question of crayfish decline, I can state that I have observed the rise and fall of competitive species of fish elsewhere.
My experience is with the CT river. The three species that I target are smallmouth bass, walleyes and rainbow trout. Over three decades of fishing there have led me to believe that a species will rise in dominance as the others diminish. This seems to change every five years or so.
I realize the above is not a scientific study but this is the internet so I shall propse the following theory:
Something has changed in the balance of predatory fish. The crayfish are being eaten in greater numbers than they were in the past. What species of fish has been rising over the last few decades? Could it be white perch or rock bass?
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