Question

Selection curve expected from a standard gillnet is :

• A. Sigmoid
• B. Asymptotic
• C. Linear
• D. Bell shaped

Hint:

Remember: Passive entangling gears (like gillnets) have a Bell-shaped selection curve, while selective filtering gears (like trawls) have a Sigmoid selection curve.

Answer 1

The Correct Option is D

Step 1: Understanding the Concept:
Gear selectivity refers to the capacity of a fishing gear to capture certain sizes or species of fish while allowing others to escape.
Different types of fishing gears exhibit distinct selection curves depending on their catching mechanism.

Step 2: Detailed Explanation:
A gillnet is a highly size-selective, passive fishing gear. It captures fish when they swim into the net and get caught by their gills (gilled), wedged behind the operculum (wedged), or entangled by their spines or mouthparts (entangled).
The selection curve of a gillnet is typically Bell-shaped (representing a normal probability distribution):
- Small Fish: Can easily swim through the mesh openings without getting stuck, so their probability of capture is zero.
- Optimum Sized Fish: Have a head size that passes through the mesh but a body size too large to pass, resulting in them being securely caught at the gills. This represents the peak of the bell curve (maximum selectivity).
- Large Fish: Are too large for their snout or head to penetrate the mesh at all, so they simply bounce off the net without being caught, making their capture probability drop back to zero.
In contrast, active gears like trawls show a sigmoid or asymptotic selection curve, where all fish above a certain minimum size are retained.

Step 4: Final Answer:
Therefore, the selection curve expected from a standard gillnet is Bell shaped.