THE FACTS: As with most varieties of Sushi, the Japanese have a very strict pairing between a given breed of fish and it’s true name. Aji, in that sense, is known in Japan as the Horse Mackerel (and may be known in English as the Saurel or Scad), but Western Sushi-ya regularly interchange the name Aji as a reference for the Spanish Mackerel (which is actually Sawara). Often, these types of substitutions are made due to the availability of a given variety of fish; so don’t be afraid to ask your Itamae (Sushi Chef) what he is really serving.
But for a “True Aji” (Formally known as Maaji in Japan), this should be exclusively a variety of Horse Mackerel.
Adding to this confusion, the Horse Mackerel really isn’t a Mackerel at all. In fact, the Horse Mackerel belongs to an entirely different genus, and is at best a distant relative of the Mackerel. More accurately, Aji is a member of the Yellowtail Family (see Hamachi). Strangely, a detailed Google comparison between the Horse Mackerel and the Spanish Mackerel will easily lead one to conclude that these fish are indeed identical twins. Therefore, it is easy to understand why Sushi-ya may be quick to make a substitution in the face of limited availability.
Aji typically ranges in size from 2 inches up to the more common adult size of 12 inches or more which is typical in Sushi. While Aji can be found year round, in the summer months of June and July, Aji is at its finest. During this time, Aji are at the beginning of their spawning season, which in turn means that the fat content and light-sweet flavor of the meat is at its peak. Moreover, this is the time that the very rare Shimaaji (Trevally Jack) start to appear, which is the most sought after among the 20+ varieties of Aji found in the waters surrounding Japan.
Notably, Aji is a blue backed fish most often prepared for sushi with part of the shiny skin left intact (similar to Sawara and Saba). Aji is a very healthy choice as it contains high levels of DHA and EPA and is low in Mercury. The sustainability of the Aji is thought to be of less concern by comparison to other species as stocks worldwide are considered to be quite strong.
Fonte: Sushipedia https://itunes.apple.com/us/app/sushipedia/id326316017?mt=8