What if the fish are zombies?
screw the fish, what about giant zombie squids doing "red-allert" on your ships...?
In these situations, the best solution is the profligate use of atomic weaponry to maintain a space launch infrastructure long enough to get the equipment for a self-sustaining human presence off of the Earth. Once that's done, you sanitize the planet.
If you can't secure a 'clean' population and supplies to have an off-world presence, then the option of sanitizing the planet while you are on it is also an option.