Boil linguine according to the package instructions or make it fresh. Cool under cold running water and set aside. Flour the prawns and salmon lightly. In a large saute pan, heat the olive oil over high heat, then add the prawns and salmon and saute for 1 minute. Add the leeks and mushrooms (one at a time), and saute for 1 minute after each addition. Next, add wine, marjoram, parsley, garlic, salt, pepper, and butter. Saute lightly for 1 minutes. Add broth, lower heat and simmer for 2 minutes, adding more broth if the mixture becomes too dry. Add the drained linguine and mix well, then simmer for 1 minute. To serve, place linguine on a plate first, decorate with prawns, then pour the remaining sauce over both. Garnish with parsley.