|Image of cat enjoying fish, taken from a popular series of nature documentaries. Apparently mice like fish too, but that's beyond the scope of this post.
Your basic felis catus (and its closest wild relatives, bobcats and lynx and such) is built for hunting birds and small mammals on the ground and up trees in more wooded areas. That's pretty standard across the whole cat family; with one odd exception (the fishing cat, one of those niche-evolved oddities that does exactly what its name advertises) cats are pretty exclusively terrestrial predators. So domestic cats' similar enthusiasm for things like chicken makes some sense, since "poultry" to "unfortunate songbirds" isn't really a huge leap. I imagine if we ate a lot of rabbit in our house the cats would be into that too (full disclosure: as a vegetarian, I mostly just get to point and laugh as the cats steal my wife's stuff).
|A fishing cat doing what it does. How it has the patience to fish without a six pack is not clear.
Fish smell and taste quite a bit different from any normal cat prey though. Moreover, most cats don't even like getting their paws damp, which tends to preclude any kind of major underwater hunting activity (there are exceptions, because cats are all special snowflakes, but they're not common). So where did they pick up a taste for fish?
The answer to this one is obvious when you think about domestic cats a little bit. They're what wildlife experts like to call "opportunistic feeders" (a fancy way of saying "they'll try to eat any goddamn thing," a philosophy my dumber cat takes to its logical extreme) and they've also had thousands of years to get good at hanging around humans and scavenging their leftovers. So in a really half-assed way, this is a natural-selection/co-evolution thing: the cats who figured out that all the weird-smelling meat the humans down by the docks were throwing away was both tasty and good for their coats had a basically limitless, zero-effort food source available and could reproduce to the point of ridiculousness. There's a story I ran across a few times in my extensive research for this post about how this all started back in ancient Egypt, when the first crazy cat people domesticated the first cats by luring them into their houses with bits of fish. That may be apocryphal (I couldn't verify it at even the Wikipedia level of rigor I usually use for these things) but it does make some sense; the cats who ended up domesticated were the descendants of the cats willing to be suckered into someone's hut by a chunk of some kind of meat they'd never seen before.
It's worth mentioning that, since cats' digestive systems haven't really caught up with their relatively recent discovery that fish tastes awesome, fish really isn't that good for them. Even ignoring all the mercury/lead issues that come with seafood in general, too much fish in the diet can leave cats susceptible to UTIs and other issues. Speaking from experience the combination of a cat, a UTI, and any furniture you aren't willing to burn is one of the worst things there is, so make sure your cats only get fish in moderation, no matter how much they try to convince you otherwise.
It's easy to forget that humans have had both intentional and unintentional effects on the evolution of all kinds of other species, from corn all the way to house cats. Cats liking to eat fish makes no sense if you think about it in terms of what they did before humans were around (hunt small land critters and stay the hell away from water) but all the sense in the world if you consider it in terms of how they've lived with us for the past several thousand years (as unapologetic, indiscriminate scavengers). And speaking of "unapologetic, indiscriminate scavengers," I'm totally going to bust my cats' chops about how my species evolved their species when I get home later. I'm sure that will make me feel much better when I'm sleeping in the living room because they successfully crowded me out of the bed again.