Within the fish population, variations exist in color, size, gamete production, and swimming speed. a variation that would most likely be passed on to future generations of the species is

I think that swimming speed would be the variation that would be least likely to be passed on to future generations of the species.
This is because color, size, gamete production, are traits that are influenced by the genetic make up of the fish and thus they have a higher likely hood of being passed on to the next generations. On the other had swimming speed may be influenced by the environment as a adaptation for survival by the fish, thus the likely hood of being passed to the next generation is low.