Melt the butter in a large heavy saucepan. Add the onion and celery and stir over moderately high heat for about 5 minutes. Add the potatoes and thyme and cook until just heated through, about 1 minute. Add the clam juice and 11/2 cups of hot water and bring to a boil. Reduce the heat to moderately low, cover and cook until the potatoes are just tender, about 7 minutes. Add the trout, corn and cream. Cover and cook over low heat until the vegetables are tender, about 5 minutes. Add the parsley, season with salt and pepper and serve.