Archival Tags On Blue Fin Tuna

I wanted to fully under stand the procedure that NOAA uses to track and calculate stocks of species and what I found was a tag method used on Blue Fin Tuna in the Atlantic. The trackers are quite valuable so rewards are offered for retrieval of the tags. 
What are archival tags? Archival tags are electronic data-logging devices that provide location estimates by measuring light intensity through a light sensor. They also provide data on swimming depth, water temperature, and body temperature of the fish. This information is collected on a daily basis and stored in the tag for several years.
How do you determine that a blue fin tuna has an archival tag? Archival tags are implanted in the body cavity of the tuna and only the light sensor protrudes out of the body.  However, these specially equipped blue fin tuna also carry unique external conventional streamer tags, with two-tone coloration, to help fisherman recognize these fish and return the archival tags. The external tags are placed about an inch off the dorsal midline on each side of the fish. On the white portion of the streamer tag it says “electronic tag inside cavity” and on the green side it says “Big $$$ reward”.  The reward offered is a $500 to $1,000 reward, and special instructions must be followed because  research depends on the successful recovery of these tags.  This is critical that these tags are recovered because it stored useful information on stocks like the blue fin tuna that can accurately measure the impact of overfishing or rebuilding.

The tag referred to in this article is B in the picture.