Melt butter in a large saucepan. Add onion and fennel; cook until onions are soft but not browned. Add potatoes, broth and clam juice; bring just to a boil. Lower heat and simmer 20 minutes or until potatoes are tender.
Remove half of soup mixture and puree it.
Add salmon to the portion remaining in the pan and simmer until the salmon is just cooked through and flakes (about 5 minutes.)
Stir in pureed mixture, milk and dill. Cook until warmed through. Do not boil.