In a deep saucepan over medium-high heat, saute celery, onions, and potatoes in oil 5 minutes, until soft but not browned.
Stir in flour. Gradually add chicken broth; stir constantly until mixture begins to thicken. Add the evaporated milk; reduce heat to medium. Continue cooking and stirring until mixture is very warm but not boiling. Add fish, corn, salt, and pepper. Cook about 5 minutes until the fish is opaque. Do not boil.
Ladle into soup bowls and serve.