Pools spawn faster if they're fished faster.
You have a number of spawn locations, that could have pools at them:
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Then you have a maximum number of pools that can be spawned at any time (the number there would eventually be if no-one was fishing, allowing them to respawn over time):
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You also have a minimum number of pools that will be spawned.
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If a pool is fished out and it would reduce the number of spawned pools below the minimum, then a new pool will instantly be spawned in one of the available locations (sometimes even in the location you just fished out):
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This is called 'forced respawn'.
The same system is used for herb and ore nodes, as well as mob spawns, because it allows the game to adapt to different player numbers, without having to do much coding to handle it.
In a lot of cases though, the minimum number of mobs/spawns will be set at 0, so you have to wait for timed respawns. If a mob/node is needed for a quest though, there's a good chance there is a minimum number of spawns for it, and you can force fast respawn rates.