How to Get Carvanha in Sapphireby Timothy Baron ; Updated September 22, 2017
In addition to the hundreds of terrestrial Pokemon that you will encounter while wandering the Hoenn region, "Pokemon Sapphire" features several aquatic Pokemon that you can catch only while fishing. Closely resembling real-world piranha, the Carvanha are the most vicious of these species. To catch them, you will need a Good or Super Rod. As luck would have it, the Good Rod is available in the same area where the Carvanha live.
Travel to Route 118, which is east of Mauville City, west of Route 123 and south of Route 119.
Walk to the middle of the map, where you'll find a river. On the eastern shore is a fisherman. Speak with him, and he'll give you the Good Rod.
Activate the Good Rod on the river to start fishing. You have a one in three chance of catching a Carvanha. Keep fishing until you catch one. You will then enter into a battle with it.
Attack the Carvanha until its health bar turns red or yellow. Once it's weakened, throw Poke Balls at it until you catch it.