White flesh
Rainbow trout (Nijimasu)
【Nigiri sushi: Shiromi】
Wild rainbow trout are found only in limited areas like rivers in Hokkaido and rarely appear in the general market. They possess the moderate fat and sweet flesh characteristic of freshwater fish, making them delicious as sushi toppings. However, I've never heard of a sushi restaurant in Tokyo serving wild rainbow trout. And it's also true that many sushi restaurants avoid using rainbow trout raised on land or in sea-based farms.
Wild populations of rainbow trout exhibit different life-history strategies within the same species. Individuals that migrate from freshwater to the ocean to grow before returning to rivers to spawn are known as steelhead. In contrast, those that remain in freshwater habitats such as rivers and lakes throughout their entire life cycle are generally referred to as rainbow trout.
Steelhead are therefore the anadromous form of rainbow trout. They are not a separate species, but rather an ecological variant of the same species, differentiated by life-history strategy in response to environmental conditions.