Hello mhmacw and welcome to Pond Boss. Trespassing and poaching are huge issues for pond ownership but unfortunately if the "visitors" are invited on the property then they are neither trespassing nor poaching. Time to have a talk with M-I-L.

Anyhoo welcome to Pond Boss, we're glad you found us.

JHAP
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"My mind is a raging torrent, flooded with rivulets of thought cascading into a waterfall of creative alternatives."
...Hedley Lamarr (that's Hedley not Hedy)