Question

If the food item does not occur in the fish stomach, but occurs in the environment where the fish lives, then electivity index will be

• A. Zero
• B. 0.5
• C. 1.0
• D. -1.0

Hint:

Ivlev's Electivity Index range is always between \(-1.0\) and \(+1.0\):
- \(E = +1.0\): Perfect positive selection (preferred food).
- \(E = 0.0\): Non-selective feeding (random feeding).
- \(E = -1.0\): Complete negative selection (avoided/ignored food).

Answer 1

The Correct Option is D

Step 1: Understanding the Concept:
Ivlev's Electivity Index (\(E\)) is widely used in fish biology to determine food selection or preference.
It measures the relationship between the abundance of a prey item in the diet of a predator relative to its abundance in the natural environment.

Step 2: Key Formula or Approach:
Ivlev's electivity index formula is:
\[ E = \frac{r - p}{r + p} \]
where:
- \(r\) is the percentage composition of the food item in the stomach (diet).
- \(p\) is the percentage composition of the same food item in the environment.

Step 3: Detailed Explanation:
According to the problem:
- The food item does not occur in the fish stomach, which means \(r = 0\).
- The food item does occur in the environment, so \(p > 0\).
Substituting \(r = 0\) into the formula:
\[ E = \frac{0 - p}{0 + p} = \frac{-p}{p} = -1.0 \]
An electivity index of \(-1.0\) indicates complete avoidance or total exclusion of the food item by the fish, despite its availability in the environment.

Step 4: Final Answer:
The electivity index will be -1.0, which corresponds to option (D).