I'm speaking of the fish stalls just by the port. The price per kilo of the fish, shellfish etc are all marked on cards - you will be shown them by the stall-holders. Some stalls will immediately offer a lower price-per-kilo (we had lobster at 200 MAD per kilo instead of the marked price of 350 MAD). But most of the stalls will start putting assorted fish, squid, etc on a tray and will say "two people, only 150/200 dirhams". With fish at 25-40 dirhams per kilo, you'd need at least 4 kilos on the tray! But there's usually less than one kilo. So, my suggestion is - choose one fish, have it weighed, get a price, choose the next, ditto. The stall will add a salad, bread and, maybe, frites. The quality will probably be first class - but just keep an eye open that the fish you chose was the one that goes on the grill. And, if you buy wine beforehand, decant into a mineral-water bottle, they'll give you glasses.
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