In a large Dutch oven or braiser, heat the olive oil until shimmering. Add the fennel, onion, shallots and garlic and cook over medium heat, stirring, until softened and lightly browned, about 8 minutes. Add the tomato paste and fennel seeds and cook, stirring, until sizzling, 1 to 2 minutes. Add the wine and pastis and cook, scraping up any bits from the bottom of the pan, until nearly evaporated, 1 to 2 minutes. Add the clam juice, shrimp, fish scraps, saffron and the additional reserved shrimp shells and bring just to a simmer. Cover and simmer over low heat until the shrimp and fish are cooked through, about 10 minutes.