Heat the olive oil in a large casserole dish over a moderate heat. Sweat the fennel, onion, garlic for 10 minutes with a little salt, stirring occasionally until they are soft but not coloured. Stir in the reserve prawn shells, tomato purée, fennel seeds, tomatoes, bay leaf and thyme sprigs. Stir well and cover with the fish stock and water. Bring to the boil and simmer for 30 minutes. Strain into a large, clean saucepan, pressing the tomatoes and all other ingredients through the sieve to extract as much flavour as possible. Add the Pernod to the broth, then add the white fish and mussels and bring to a simmer. Once it has reached a simmer add both the shelled prawns and the whole prawns. Cook for a further 2 minutes until the mussels have opened and the prawns are pink, discarding any mussels that haven't opened in that time. Adjust the seasoning if necessary and ladle into a large serving bowl. Sprinkle the chopped tarragon on top and serve immediately.