There are some areas that you will only pass through once or twice in the entire game. Killing the dinos in these areas is pointless; more importantly, it is a waste of ammo. Use the weakest tranquilizer dart on it/them, and move on. This way, the bullets/shells/slugs/grenades are available when you need them. Once you get unlimited grenades (by completing the game three times under the normal difficulty setting), you can kill those dinos, without wasting ammo.