@FantasticDorf: Are you sure this applies to vermin "fish"? It contradicts what I've heard so far that vermin fish presence is tied to 16 * 16 tiles (plus water, of course), and that they don't replenish. I don't have much first hand experience, though.
I've yet to try the practical methods myself (after a year back i was doing some skullduggery trying to tame large creature fish at much time and effort expended making pens), but i am quite sure that lots of vermin together retain presence wherever they spawn and move to if in numbers, they simply spawn in river tiles from world populations the same way cave spiders spawn in the caverns, but can be took out of that enviroment and pernamently settled if ground vermin reliably proves by plenty of
cave spider guides on the wiki and my anecdotal testing by trapping female hamsters inside a fixed cage that multiplied rapidly from males entering breeding range.
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The industrial cave spider use section here is wrong within the scope of my experience, and the spiders can be controlled by use of a Kennel & a weaver (for collection) in a area while cats are kept out, if the writer was true, all the spiders would be dead simultaneously or within a short space of each other by yearly checks but while unidentifable they persist. Mayflies for example expire naturally almost as soon as they appear.
That is unless the code for vermin fish is intangibly different to other vermin, pond turtles don't count but amphibious mog-hoppers (for production purposes of mog juice) are a ideal candidate from fishery caught sources this way or the more traditional (throw them in a hole). Egg layers only reproduce in worldgen and hence wouldnt appear outside of their existance within the appropriate biome.
> The fish also have to be opposing male & female genders additionally for practical reasons so moon snails and clams will not work (see:Hamsters).
> Theoretical fish breeder
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Screw pump pulls away water from the aquarium tile with female fish inside so they can manage & add and relative to the rest of the pool 2 tiles of water are left remaining for male fish to live in, water is sent to a channel to resupply (little difficult to try and show diagonals so i left it to imagination) or the user simply lets dwarves refill the pond with buckets.