In my world, there is a species of pike, like a muskellunge, but it grows to massive proportions. They are 9 to 18 ft and weight 235-1,900 lb, with a maximum size of 26 ft and 3 tons. They live in deep lakes and rivers and feed on large fish like catfish, bass, carp, salmon, and smaller pike species. Is a pike this size feasible? If not, how large could they get? What evolutionary and environmental factors would be necessary for it to achieve such a size?
I worry that existing freshwater fish are not prey enough.
Pike are fish catching specialists. Occasionally they grab a small mammal or bird. They swallow prey whole.
There are killer whales that are the size you describe; in general these are the smaller ones and they are fish specialists. Larger killer whales are a different group and seals can tell the two groups apart.
So you could sustain a giant predator fish on a diet of smaller fish.
Could a bony fish get that big? Xiphactinus was a predator and close to what you propose.
But the problem is food density. Really big aquatic fish eaters are in the ocean where there is more prey. That said, the biggest lakes like the Great Lakes might offer comparable food density especially in the pre-Columbian days.
Then here is my (longstanding!) question: why are there not giant freshwater piscine predators? Catfish get big but they scavenge a lot. Alligator gar live like pike but they max out at 100-150 lbs. Alligators and crocodiles get big but they eat land animals. If the Great Lakes could support giant fish that lived like killer whales, where are those fish? Even at sea, the largest fish eaters are things like tuna - big but not whale size. Great white sharks eat mammals.
I think there is some factor at work today (and maybe for much of history) that limits the size of predatory bony fish.
All that said: yay giant pike! You have much mythologic precedent!
ADDENDUM Maybe your big pike could be anadromous, splitting time between salt and freshwater?
Ecology, evolution, and management strategies of northern pike populations in the Baltic Sea. There are pike in Northern Europe that spawn in freshwater then move out into the Baltic to feed, returning to freshwater to breed. Maybe your real giants are only seasonal visitors?
You'd need a reasonable population size for these fish to maintain genetic health/diversity which means you also need and a large volume of fresh water (and hence a large prey base) to support that population. So that would mean a continent with large, interconnected fresh water lake systems and or river systems. At least as far as adults go.
Juveniles might avoid intra-species predation once hatched by either: A) Migrating down rivers to the sea or B) Swimming 'up-stream' into smaller streams/riverlets and ponds where then back down again as they outgrow their feeding grounds.
Also while smaller juvenile pike would be small and agile enough to capture various large fish I doubt the adults would be able to - at least not easily or frequently enough to make their large size an evolutionary advantage. Pikes are ambush predators if I recall and use a burst of speed to catch their prey.
Another problem - pike eat their prey whole (unlike some sharks for instance). So your pike have to either evolved specialized dentition and hunting behaviors for larger prey types including mammals that let them either gab and 'bite/chew' inflicting lethal injuries and rapid death due to blood loss or alternately the pike backs off and follows the prey waiting for it to bleed out (or both depending on the size of the prey and situation) before consuming the carcass in chunks. Either that or they do what crocs do. Grab the prey and try to drag it underwater while they hold on until the animal drowns.
So 18 feet might be a little to large although salt water crocs do exceed this length. If the aim is to have a local peak predator that is a potential threat to humans living beside/on the same water systems 10-12 feet or thereabout would be more than enough of length to be a serious threat. Finally If they don't normally eat terrestrial mammals then I suggest you add species fresh water dolphins/whales and or seals to the ecology. That would provide a prey source that could justify a larger body size on their part. (I'd still stick to an upper limit about 12 feet though.)