Minecraft – What ocean biome spawns the most fish

Technically, placing torches will decrease your overall mob spawns; the game does pick a single random point within the eligible chunks to spawn a monster, and if that point is not suitable, the monster doesn't spawn. Therefore filling other caves with torches will reduce the number of spawn-friendly points, reducing your overall monster count. This does technically help your mob grinder because you don't have mobs lurking in neighboring caves counting against the mob cap (and ultimately taking it all up, preventing anything from spawning in your grinder).

The best solution would probably be to use a vast water network to drag all the mobs from all nearby caves into your grinder, so you maximize spawn area, but bring any mobs from all the viable spawn points into a single area.

I know this is an old question, but it came up in a Google.

It has recently been theorised (and to a certain extent proved) that there is a bug related to mobs spawning on the edge of the loaded chunk area, wandering out and still using up the mob limit.

Further analysis and experimentation is available in this video.