The rats definitely like vegetation. I have had problems with them coming up the overflow pipe, using it as a protected tunnel from hawk predation, to enter my main pond and also from my main pond to forage pond. A trap right at the end of the pipe seems to do the trick. They make such things called "rat guards" that prevent them from using the pipe, but the way my pipes ends are makes it hard if not impossible for me to install the rat guards.
The old Field of Dreams movie saying, "If you build it, they will come", comes to mind. I have all sorts of herrings and cormorants that just love my main pond where I have high concentrations of fish. Not to mention a Bald Eagle. The more we try and make the ponds thriving with fish, the more attractive we make it to predators too.
