I read the previous posts about a family trying to determine the source of a fishy smell in their daughter's bedroom. Turns out it was connected to the wiring in her overhead light fixture.
We have a HORRIBLE fishy smell in our backyard. Like the previous post, it seems to come and go. To go along with the smell, we have an over abundance of flys; millions of them. We have fly traps (Lowes) in the trees and the amount in there after a week turns my stomach.
There are many overgrown trees on the property and I'm thinking, based on previous posts regarding "fishy" smell, that it might be rodent, or Possum nests in the trees.
Has anyone else encountered this problem, and can offer a solution, other than completely ridding the property of trees??

You need to find the source. It sounds as if something has died and is decomposing in your yard. Skunks, raccoon and possum nests do not attract flies. But dead animals do.

If you don't find anything, check out where your sewer pipes go. There might be a clean out or leak somewhere in your yard.