Yes you can,
If you have completed the game you will be assigned the national pokedex and once you have that you will be able to catch every Pokemon ever made in the wild, except legendary's but some might be harder to find than others.
corphish is a Pokemon that is native to the hoenn region
You have to evolve Corphish.
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Corphish evolves into Crawdaunt at level 30.
You go to Celestic Town, Register your Super Rod and fish. You can catch the Corphish with a Rarity of 30%.
Use the super rod in Celestic Town.
You need the Good Rod.
Corphish can be caught in the Rocky Beach area in the Safari Zone when you place certain objects in that area. Corphish can only be caught by a Super Rod, though.
Corphish is a Water type pokemon.
you have to use the super rod in celestic town after you get the national pokedex
you can go to petalburg city and use the good rod to catch a corphish :)
Use "Good Rod" on Route 102, 117, and Petalburg City, and you'll get 20% chance to catch Corphish. If you use "Super Rod," it is certain that only Corphish will be caught.