For most fish, the term “seasonal” doesn’t really apply: they are grown commercially in fisheries or available year round. One exception is mahi mahi, which hits its peak in the winter months and will probably leave the fishmongers’ stock in a month or so. Coastal Seafoods had some lovely fish in recently (for about $10/pound), and will surely point you in the right direction.
As for sustainability (an important question when it comes to ocean fish) The Monterey Bay Aquarium “Seafood Watch” program lists domestic troll/poll caught mahi mahi as a “best choice.”
Flavor wise, mahi mahi (or mahi-mahi) is a mellow white fish, not quite as firm as swordfish but certainly more steak-like than not. Because it is mild but a little flaky, it is wonderful simply prepared and paired with something acidic and fresh, like a mango salsa or vinegar reduction.
Quick and simple mahi-mahi in February:
Heat a pan to medium/high heat. Rub a little oil, salt, and pepper onto the fish, then place in the pan, flesh side down to get a nice sear. For thick cuts of fish, give this a few minutes to crisp up, then put on a lid and cook until the fish is opaque on the sides (about another 2-5 minutes). Serve with fresh mango salsa — chopped mango, red onion, mint or parsley, lime juice and salt.