The Isole, the Elle and the Naïc flow through Querrien. These rivers still support abundant natural populations of atlantic salmon and wild brown trout but are relatively undiscovered, making the area one of the best fishing destinations in Brittany and all France. The season for trout is from mid-March to mid-September, while salmon has more complicated local rules to help with conservation. Local knowledge is the key to fishing success here, so you may want to track down a guide.
Generally, the best time for salmon fishing here is the grilse, when the adults return to their native rivers for spawning between mid-September and late October. There is a tradition of wet fly fishing for them. Licences are relatively inexpensive and fishing rights relatively unterritorial. A Carte de Vacance (2 week holiday fishing licence) is available from the Mairie in Querrien, and probably the Tabac too. Be sure that you get a class 1 licence for salmon or trout, and while you're there, perhaps they'll know of a guide.
If it's mackerel or sea bass you're after, you can charter small fishing boats in just about all the small picturesque ports along the south coast. At Doëlan for example, check out the notice board in front of the Port Office, and just ring to make a booking. We have seen happy groups of elderly French gentleman returning with sackloads of mackerel, distributing them to anyone who'll have some. Happy days.