Melt the butter in a large pot over medium heat. Add the onion, celery and garlic and cook until tender, about 5 minutes. Mix in the flour and cook 2 minutes more. Slowly stir in the stock or clam nectar. Add the potatoes, bay leaves and clams and their liquid. Bring to a gentle simmer, and simmer until the potatoes are just tender, about 10 minutes. Stir in the salmon, shrimp, crab and dill or tarragon. Simmer 3 to 4 minutes more, or until the salmon is cooked. Stir in the cream and season with salt and pepper. Heat the cream through a minute or two and the chowder is ready.