Press 2 cups potatoes in 9 inch glass pie plate to form pie shell. Put cod in sauce pot, cover with water, bring to boil, lower heat and simmer 5 minutes. Add catfish and shrimp and simmer until catfish flakes and shrimp are about done. Drain off stock and reserve. Flake fish; add crab to seafood , place mixture in potato shell and set aside. Melt butter in skillet. Add flour and stir constantly; do not brown. Add milk and a half cup reserved fish stock. Whisk until smooth and creamy. Add salt, Tabasco, worcestershire and parsley. Mix well. Pour sauce over seafood mixture. Place remaining potatoes in pastry bag, then pipe ring of potatoes around pie, with rosette in the middle. Sprinkle pie with cheese. Bake at 375 degrees for 40 minutes.