A fence keeps large animals (elks, reindeer, bear) from passing through the tile with a fence (although bears can smash fences if trying to get at prey), forcing them to go around to either side. Wolves can jump fences, but unless there's prey, they go around. "Going around" a trap fence means going through the gap, where your trap is (one of multiple traps, typically).
Blocking off a passage (such as e.g. an isthmus) ought to be the most effective trap fence, but even a line in the middle of nowhere should catch things occasionally. The longer the fence, the more wildlife you reroute, so the more you catch.
I used to surround a single tile lake with a trap fence, and that worked well. Currently I've surrounded my farm plot area with one, and it catches things occasionally, but I complement that with active hunting.