I'd go with the mono. Cecil has used it to deter Great Blue Herons from his ponds.
It's something that can be in place 24/7, and you aren't at the pond every hour of daylight. Like you said, take away one and another will show up.
A friend down in Texas has a permit to shoot cormorants on his ponds. I remember a few years ago I shot the last one of the year, and it was #200. They have to be retrieved and buried. 17HMR from 150 or so Yds works just fine.

He has to keep a log book with the date, name of shooter, time, and what pond.