Walking in the market you’ll see from time to time fishmongers spiking fish: https://mesubim.com/2014/03/26/hirame-video/
Flounders are available all the year round but the best season is from autumn to winter while others have a season from May to July.
Pictured below is hoshi karei one of the prize flounders (rather huge) and is considered more expensive costing as much as $300 per fish. It is a late spring-summer flounder with a slightly deeper flesh due to its size.
Many types right eye flounders are caught off the coasts of Japan and include various species of flathead flounders, all with a personality of their own. Ishi-garei is also a large stone flounder when compared with hoshi-karei, a distinctive dotted flounder. The chef shows off the fish by holding it up and this gives you a sense of its size.
Meita-karei is best in winter and has a distinctive small mouth and is caught in the northern seas of Japan and has a small mouth. Baba-karei is another species, yet its slimy and has a dirty appearance, sometimes called baba-karei a granny flounder.
Mako-garei is caught in the Bay of Beppu in oita beneath the hiji castle and feeds in the fresh waters beneath Hiji Castle. These species are vary diversified and while many are similar, it is dependant on the origin and season when it comes to nigiri or sashimi.