I have a real problem getting good mackerel. What I consider to be "real" mackerel is that served typically in sushi bars, which I assume is Atlantic mackerel on the east coast of the USA. This fish is characterized by its strong, distinctive oily taste and large amount of brown fat (glycogen). Normally when I buy "mackerel" from "Spain", which I assume to be Scomber scombrus (Atlantic mackerel), it looks and tastes identical to sushi mackerel.  I have read that in Japan there are over 30 kinds of mackerel, but only 3 of these types are considered Saba (good mackerel).  The problem arises that a different kind of "mackerel" is frequently sold which has no distinctive taste, no brown fat, and tastes more or less exactly like tuna. I notice, for example, that one maker when it says on their package "product of Spain" it is the good mackerel, but every year around the same time it changes to say "product of Thailand", and this indicates the bad mackerel tasting like tuna.  What are the good species and how can I figure which is which before buying it (assuming I can't see it inside can)?  (Just to illustrate how tough this problem check out this fish website, it shows a picture of Atlantic mackerel (Scomber scombrus) but identifies it as Spanish mackerel (Scomberomorus maculatus)! Not even the fisherman seem to know what they are catching.) 
Searing on a grill to "seal in juices" has largely been disproven.  Meat loses juice at roughly the same speed regardless of searing the meat first.  Searing does produce the Maillard reaction and caramelization which enhances flavor; however, searing first doesn't produce better results.  A test performed by Alton Brown in 2008 demonstrated that searing at the end of the cooking process loses less water than searing at the beginning.  For more information see this link.  For the Alton Brown results see this link (video). 