Anyone replies, (fishing experience, fishing for beer in the fridge).
Most of the Island is surrounded by a ring reef, within the lagoons there isn't much fish life as the indigenous will harvest anything that moves irrespective of size. The lagoons are mostly barren of fish life, most of the coral within the lagoons has been destroyed through dynamite fishing which was finally outlawed in the late seventies.
Mahebourg has a large river estuary, perhaps there are some fish there, but seeing as all waste is dumped in inland ravines by the MOE and then there are the upstream villages which also treat a river as a refuse receptacle, the river and estuary will be polluted. "Old Grand Port" or Mahebourg has a channel through the ring reef to the open sea, winds are predominantly easterly and south easterly during winter so seas will be rough on the windward side of the island. Blue Bay which is around the corner from Mahebourg is a protected reserve and fishing is prohibited within the reserve area. If you have good sea legs and you can find "a local" brave/skilled enough to take you out through the channel at Blue bay, go right , past the Shandrani beach and past the coast guard training centre until you get to le souffler area, note this is quite a ways along the coast, you'll come across areas which the locals don't frequent due to their inaccessibility and are more abundant with fish.
Expats don't venture out for deep sea fishing in the south due to the rough seas during winter, most of the deep sea fishing boat charters operate in the north west and western side of the island, Flic en Flac (leeward) has several deep sea charter operators, you can also hire a local fishing boat to take you out to potter about in the deep sea which is calm, if it's pelagics you're after I would suggest Flic en Flac during the winter season.
but hey, what do I know about fish, fish swim everywhere right ?, make the fish come to you, chum the water with your gf's panties, hang a line in the water, and hey presto something fishy will jump onto your hook