These are species of fish native to Earth around Voi. They are possibly Bass or Tiger Fish.
These fish can be found on the Halo 3 level Sierra 117, at the chapter Charlie Foxtrot, where the Pelicans are attacked by Covenant Loyalist Banshees. You can also see them on your radar at the main dam (rally point beta). If a player jumps in the water, that person may walk around and see them. They will appear as small yellow dots on your radar screen, similar to allies but smaller.
It is possible to kill them; if you do, they float to the surface and then drift to shore before eventually respawning. Throwing a grenade in the water will result in the whole "school" of them dying and floating to the surface.
They are obviously different from the type of fish seen at High Ground, as they are much larger, differently colored, and lack the odd snout that the High Ground species appears to have.
A fun thing to do is to go "fishing" with a friend and have a contest to see who can kill the most. When "fishing", for a challenge, try turning the fog skull to remove the radar, making fishing harder.
