Hi! Despite France is not the best place for fishing, there are somme great oppportunities, in Normandy in particular.All other rivers in the area have a good reputation for brown trout.
Most of fishing is private but you can get info there, even if not well updated:
http://www.federationpeche.fr/
Thick on the district ("departement") requested; hope somebody can reply in english...
If you can, there is a really pretty river, called "Touques" that runs in the Calvados:
BIENVENUE sur le site de la Fédération de Pêche du Calvados
You can expect here nice brown and huge sea trouts, up to 17-16 pounds (annual run is currently about 6000/7000 fish; fishing is worth from mid-may). I don't know if there are good guides here. Some friends of mine used to fish this river but it needs lots of knowledge to land a "blanche" (local name for sea trout). Have a look to the above link, may sure they can provide you the information you need.
Regarding river Sienne, even if salmon population is depleted, some grilses can be catch by the second half of summer. Brown trouts population seems to be healthy. It is not in the same district (Manche -50) as river Touques.