1. In large bowl, mix tuna, 3 eggs, and remaining ingredients except piecrust mix until well blended.
2. Prepare piecrust mix as label directs for 2-crust pie. Divide dough into 2 balls, one slightly larger. On lightly floured surface with floured rolling pin, roll smaller ball into 16" by 4" oval; place on large ungreased cookie sheet.
3. Spoon tuna mixture on top of pastry; with hands, mold tuna mixture to form fish shape. Roll remaining ball into 18" by 7" oval. Gently place over tuna mixture. With knife, trim pastry edges to about 1/2 inch all around tuna mixture; set scraps aside. With floured 4-tined fork, press pastry all around edge to seal.
4. Preheat oven to 375°F. Reroll scraps; use to cut out shapes to resemble fins and tail of fish; tuck under fish slightly; press to seal. With small, sharp knife or kitchen shears, cut eyes, mouth, gills, and scales on fish.
5. In small bowl, beat remaining egg. With pastry brush, brush egg over entire surface of fish. Bake 45 minutes or until browned. To serve warm, with metal spatula, carefully loosen fish from cookie sheet and gently slide onto platter. Or refrigerate to serve cold later.