Like the man says, a cannibal eats its own kind. Ferox concentrate on charr for a quite obvious reason, which is that they shoal. I have seen depth/fish finder shots of shoals of, almost certainly, charr being shadowed by larger fish --- according to the size of the symbols and their formation on the screen --- and it seems the larger fish shepherd the shoals.
In trout streams and trout lakes, trout which switch from invertebrates to fish as their main diet will eat whatever fish are most plentiful or easiest to catch. If minnows abound, they will become an important part of the diet of these converts. On Loch Leathan, Isle of Skye, no minnows in my experience, but no shortage of stickle backs, and the bigger trout used to give them some stick along the margins. On our English reservoirs which contain mixed coarse species, big browns and rainbows go fry-bashing. I used to see that regularly at Broom, near Annan. Our Cumbrian ferox in Crummock go for charr; but on some of our smaller streams that don't have minnows or sticklebacks, trout do turn cannibal, and take smaller trout from fry up to almost pan size. These cannibal trout often have large heads and relatively thin bodies. True ferox are a much more beautifully proportioned fish, as a rule, because their prey is generally more abundant than that of the cannibal trout in rivers that contain only trout and no shoal species.
I do not take loach or bullheads into consideration here.
On my home river, the upper South Tyne, we had only loach and bullheads or smaller trout for any trout that turned fish eater, so they would almost certainly turn cannibal. Supply and demand. TC
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