As I see it, they must have entered your premises through gaps from somewhere, maybe in your fence, perhaps a walk around and some repair work and netting could save you the tedious patrol of your pond, I can see your main gate having huge space where they could just hop in.
You can if you want consider toad traps which could mean more digging in your ground near the steps, either than that there's not much you can do to these pest.
Hope, I have help.
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