"Yeah, but at least two live fish (a male and female) being dropped into the same remote lake, then finding each other at spawning time seems increasing implausible. Never say never, but ..."
It just needs to be one pregnant fish. (or many eggs)
It just needs to be one pregnant fish. (or many eggs)
There are many internal or quasi-internal fertilization oviparous species.
And, in sharks at least, ovoviviparous and even placental species