Why is a freshly caught tuna fish so expensive yet canned tuna is relatively cheap?
Almost all the canned tuna (I have not really examined, but at least the ones I have seen) are Albacore Tuna (OK, there are some Skipjack Canned Tuna as well).
Now when you say Freshly caught tuna, I am guessing it is Yellowfin, Bigeye or Bluefin tuna, whose meat is red.
Given this, Albacore Tuna (also known as White Tuna) costs a lot less than Yellowfin, Bigeye or Bluefin Tuna.
As of this writing, when in season, Fresh albacore in the San Francisco Bay Area costs 50% - 45% of Yellowfin Tuna, which explains the reason why you find canned tuna relatively cheap.
Also, note that each can weighs around 5oz/142g, which is a small portion.
They sell for $2-$4/can - that is $6.4 - $12.8/lbs.