From what I see, most creatures stick around for a couple of months before they're unavailable. It's just that this game released close to a season change so everyone had less time to catch the winter/March fish. You can also visit someone who has a southern hemisphere island and catch creatures that are unavailable in the northern hemisphere.
Well I might have to wait until at least May or so. Plus I refuse to get a Lite and the regular Switch is sold out. So possibly even longer.
---Gamertag: Kegfarms, BF code: 2033480226, Treasure Cruise code 318,374,355, Steam: Kegfarms