Selectivity index - shows selectivity of individual food items of components by fish. Selectivity index is provided by morphological and physiological features of fish. It is calculated according to the formulas of Shorygin and Ivlev.

SI = r % / b %,

Е = (ri - pi) (ri + рi)

where r % is the value of a group of food organisms in the food; b % is the value of the same group in the water body, r1 is the share of individual food ingredients in the fish diet, %, p1 is their share in the food base, %.

According to A. A. Shorygin Selectivity index is the ratio of the percentage of a certain group of food organisms in a food clump to the percentage of the same group in the fauna of a water body. The value of the Selectivity Index ranges from 0 to several hundred. A Selectivity Index greater than one indicates fish selecting a food organism, while a Selectivity Index less than one indicates avoidance.

According to V.S. Ivlev, the Selection Index is the quotient of the difference between the percentage values of nutritional components in the food and in the forage base divided by their sum. The maximum value of the index is +1, and the minimum value is 1.