While negitoro is heavily associated with tuna, some supermarkets or chain sushi restaurants use opah instead. The flesh of opah is very similar to that of tuna and cheaper. In fact, it is similar to tuna in taste and texture, and is actually very tasty. Personally, I consider opah to be far superior compared to making negitoro with "akami" red meat (lean part) of tuna. If you buy negitoro premade in the supermarket, it is usually made by mixing the cheaper red meat with either fat from the tuna or sometimes with opah (which is fattier than tuna), albacore, or yellow tail to keep costs down. This is also why the dish is simply known as "negitoro", rather than "tuna negitoro", it might not be 100% tuna!