Any pictures of that?

. . . how are you gonna keep them out of your pool?

I say that because my wife and I were swimming in a large mountain stream several years ago . . . until we discovered the bottom was COVERED with 'deposits' from another large rodent: beaver!

You might also want to investigate what human pathogens your ducks (what species are they?) and nutria may carry. Beaver notoriously carry Giardia, which is very, very chlorine resistant.

You may also want to consider replacing or supplementing your existing filter with a large DE filter. DE filtration will remove many pathogens and most protozoan cysts (like Giardia). Another option is to regularly treat your pool with polyquat (http://pool9.net/polyquat/) since it is a weak supplemental biocide, but an strong filter aid, and will enhance sand or cartridge filters capture rate on pathogens.