Holes in pond banks

Question
I have a small pond (30 x 50) and I am seeing holes about an inch or two in diameter in the banks.  They go in the ground at an angle.  I am sure it is some varmint but I never see anything.  Perhaps it is a nocturnal animal.  Any ideas as to what it might be and how to get rid of it?
I also have duckweed on the pond that I am trying to clear up.  I have used a mixture of weedtrine and cutrine that I have sprayed on the surface three times.  The weeds turn brown but within a week, they seem to be back as green as ever.  This is what my Agway store recommended.  What would you use?

Answer
Small pond?  If it's 30 feet by 50 feet and even just a foot deep, that's 10,000 gallons or more!  That's big compared to the ornamental ponds about which I normally field questions.  The holes you are seeing may be from muskrats which are small rodents that den in the bank. Muskrats are nocturnal but sometimes can be seen during the day.  Crayfish make mounds above the water line.  Their holes are normally only about an inch in size with a lot of globby dirt pilled around the opening.  Muskrat holes are normally closer to 2 inches in diameter.  Neither is a big threat to your pond but muskrats will eat some plants (and damage the liner but it sounds like your pond is dirt bottomed).  They can be trapped but I advocate living in harmony with nature.

I also don't believe in herbicides.  Not only do they harm wildlife and people, as you've found, the plants can simply return.  The following animals eat duckweed:  goldfish, koi, grass carp, turtles, ducks, muskrats, and more.  You can add some of them as one option.  Physical removal will reduce the population of duckweed.  Also, duckweed does NOT like moving water.  Your pond should have an aeration device if it does not.  This will not only irritate and kill off the duckweed (at least by the moving water) but it will oxygenate the water giving a healthier pond.  I have some links to suppliers of fountains, aerator devices, and chemicals if you're into those on my farm pond page at http://www.fishpondinfo.com/farm.htm

Good luck!
Robyn