missing koi

Question
QUESTION: We had 13 koi, 3 died this winter and we found them floating.  The pond was covered with a net fastened to the ground all winter.  We just uncovered it and the remaining 10 koi are gone, totally.  The pond is 7000 gallons and the koi were 12-20 inches long.  No animal could have gotten in and the net was not disturbed.  There is no sign of any of the koi.  It is a mystery.  any suggestions?

ANSWER: I'm so sorry about your koi.  Are you absolutely sure the koi are gone?  Are there any places for their bodies (alive or dead) to be hiding?  How deep is the pond?  Are there pots on the bottom?  Is the net secured absolutely all the way around?  Check the edges for gaps including under it and check for holes in the net itself on top.  If there are any holes, some predator may have gotten in.  Few predators will take out 10 adult koi.  One that comes to mind is an otter.  Some predators like herons can kill fish through the net but then their bodies would be under the net.  There's only one predator who can get large koi from under a net and not disturb the net at all.  That's because us humans put the net back.

I hope you can solve the mystery!

---------- FOLLOW-UP ----------

QUESTION: We leave in NW MAss.  The pond is about 4 feet deep.  There are no otters around.  The net was secure on all sides but one and that side had a small opening of about 18 inches.  These fish were large, 18 inches or so.  A fisher cat has been suggested, but we can't see how it could get 10 large fish out without a sign.  We are sure there is no sign of any fish.  Not a trace, and the bottom of the pond was not disturbed.  It really is a mystery.

Answer
That opening may have been large enough for some animal.  What mammals do you have there?  Raccoons normally make a big mess and wouldn't go into water 4 feet deep.  If you have fishers, they may have been able to get in there.  You'd think there would be some sign though of scales or parts.  I don't know enough about fishers though; they are pretty rare, and we don't have them here.  Did the fish disappear all at once or were you not able to tell because it was frozen over?  That would seem to indicate they were taken while the pond was frozen over?  I check my fish daily so I'm confused how you wouldn't notice fewer and fewer fish.  I guess there's no way they were koi-napped by a human?  There are some pond mysteries we never can figure out!  If you get more fish, be sure the net has no openings.  If the fish are expensive koi, you might want to set up a night camera that's activated by motion or something to get photos or video of potential culprits.  Of course, if you find out it's some animal, and you love animals like I do, there's not much you can do that won't harm them but net the pond and/or install an electric fence.