Which of the following is an incomplete count to estimate population density?

Options

A. Number of Parthenium plants in an area
B. Number of fish caught per trap
C. Biomass of bacteria in culture plate
D. Number of birds in a sanctuary

Answer

To determine which of the provided options represents an incomplete count for estimating population density, we need to understand what population density is and how it can be measured.

### Step-by-Step Solution:

1. **Define Population Density**:
- Population density refers to the number of individuals of a particular species living in a unit area. It is calculated by dividing the total number of individuals by the area they occupy.

2. **Analyze Each Option**:
- **Option 1: Number of parthenium plants in an area**:
- This option provides a direct count of the number of plants in a specific area. Therefore, it gives a complete count of the population density.

- **Option 2: Number of fish caught per trap**:
- This option indicates that fish are being caught using traps. The number of fish caught can vary based on multiple factors such as the type of trap, location, and time. Since this method does not account for all fish in the area and only provides a sample, it is considered an incomplete count.

- **Option 3: Biomass of bacteria in a culture plate**:
- This option measures the total biomass of bacteria, which can be used to estimate the population density of bacteria in that culture. This provides a complete count of the population density in that specific culture plate.

- **Option 4: Number of birds in a sanctuary**:
- This option refers to the total count of birds within a sanctuary, which is typically monitored and recorded. Thus, it provides a complete count of the population density.

3. **Conclusion**:
- After analyzing all options, it is clear that **Option 2: Number of fish caught per trap** is the only option that represents an incomplete count for estimating population density.

### Final Answer:
**Option 2: Number of fish caught per trap** is the correct answer.