Thaw and pat dry the cold water shrimp, scallop and salmon separately with paper towels to remove excess moisture.
In a large saucepan, melt butter over medium heat; brown the celery, onion, bay leaf, dill and ¼ tsp (1 mL) pepper, about 5 minutes, or until onion begins to brown. Stir in the potatoes; brown for 2 minutes.
Increase to medium-high heat; add the fish stock and bring to a boil. Cover, reduce heat to medium and boil for about 5 minutes, or until potatoes are almost tender.
Whisk the flour into the milk and pour into the saucepan; bring to a boil, stirring often. Stir in fish and scallops, simmer, stirring often, for 5 minutes, or until seafood is opaque.
Discard the bay leaf. Add the cold water shrimp and lemon juice and season with salt and pepper.
Serve in bowls and garnish each serving with fresh dill leaves.