What are the indicators of tuna grading?

Each tuna is graded based on five indicators:

• Initial appearance (freshness) is where the grading process starts. Most tuna is traded headless, and one of the most apparent signs of freshness is the collar, where the head has been removed. Then the fish’s skin, scales, and fins are visually inspected. Once the outside is evaluated, then the inspector looks inside, at the belly wall, to grade based on what is visible from the cut made to gut the fish.
  

• Size and shape – The size of a fish and its marketable yield are directly proportional. Larger fish produce larger loins and fatty portions, which increases their value.
  

• Color - Tuna’s meat is generally red but the specific level of redness determines the grade. The tail color is a good indicator of the condition of the fish since that area tends to change color first. The color of the core sample and the bloodline are also good indicators of health and general quality.
  

• The texture is best determined by physically feeling the core sample and the tail cut. The stickiness of the core sample and the smoothness of the cut of meat are good indicators to determine tuna’s grade.
  

• Fat – A tuna’s grade depends also on its fat content. The best places to determine fat content are the core sample, belly wall, tail cut, and the nape (collar).
  

Source: 

Tuna Grading System