Fish (High Ground habitat)
From Halopedia, the Halo wiki
A species of fish native to Earth and the Ark.
This species appears to be an Elephant fish, a group native to Africa. They are available as pets for experienced aquarium's in the 21st century. The modeling of the fish is fairly accurate.
One of their defining characteristics is an odd snout, distinguishing them from other fish.
On the level High Ground, one can find small fish in the water. 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.
Similar fish can be seen in the waters of Valhalla map.