This has got to be one of the most idiotic reasons to oppose nuclear power that I have ever heard of. There are literally THOUSANDS of fish eggs produced by a single female with most fish, and of that number, only a few hundred would survive whether or not they were caught in a nuclear cooling tower. Fish eggs are tasty to a large number of animals; this is why fish evolved to produce large numbers of eggs to compensate for the amount of predation that their eggs face.
There is also the fact that even if all of those eggs actually hatched into fish fry, most of them would soon be gobbled down by creatures that like to eat baby fish. This also includes their parents in some cases. The fish roe and the fry that hatch from it are doomed in most ecosystems anyway, so it is no big loss to the environment.
Also, why single out nuclear power for this attack, when dams destroy millions of fish eggs and adult fish every year? This is not unique to nuclear power. This also fails to take into account that most LWR sites actually recirculate the water that is taken from an artificially constructed pond or lake, anyway. Finally, if we are going to talk about environmental damage to aquatic ecosystems then surely coal shares a large part of the blame for directly contributing to the phenomenon of mercury and heavy metal contamination of the surrounding water table.