How do I get my dwarves to start fishing? I have enabled the fishing labor, and made sure that the standing orders do not forbid fishing outside of zones, but my dwarves never seem to go fishing. There are a couple of murky pools on the map, but whenever I try to create a fishing zone on or around them, it only says 0 no matter how many tiles I select.
Sounds like there just aren't fish to be caught on the map (Except perhaps in the caverns? Did you discover any underground bodies of water yet?) Perhaps the biome you are in is inhospitable to fish. If that's the case, you probably won't ever be able to fish in that fortress.
That said, fishing is a rather minor part of the food industry, and can be easily ignored.
Fishing happens even in murky pools, as fishermen will catch muscles in murky pools.
Fishermen fish whenever they want. If they don't have other tasks waiting/assigned, they might go fishing. Once a fish is caught, someone with the task
I only zone fishing zones of activity when I need to control where the fisherdwarves will fish (ie, you have an underground pool you want them to fish from, or an enclosed lake that you only want them fishing from, and not all of the possibilities beyond your fortress's safe area in the embarkation site.
Fishing is an excellent way to provide food variety in an early fort. One skilled fisherman can provide a steady supply of fish for several years, allowing more of your gathered or harvested plants to be brewed rather than eaten.