Passive Fishing Gears & Modern Fish Detection: Gill Nets, Trammel Nets, Lines, Traps & Sonar

Gill netting is the most ubiquitous, universally practiced fishing method across marine, estuarine, and inland waters worldwide. Gill nets are passive, highly size-selective, low-cost, fuel-efficient, and easily deployed from small non-motorized canoes to large commercial driftnet vessels.

Gear selectivity characteristics across commercial fishing gears:

• sigmoid (logistic S-shaped) selection curve is characteristic of trawl codends, where retention increases monotonically from 0 to 100% as fish body size exceeds mesh opening.
• Asymptotic curves describe knife-edge selectivity or cumulative asymptotic gear retention.
• Linear curves do not represent physical gear hydrodynamic interactions.
• standard gillnet exhibits a unimodal, symmetrical bell-shaped (normal/Gaussian) selectivity curve. Fish that are too small easily swim through the mesh openings without capture, while fish that are too large cannot penetrate past their opercular girth to become wedged, meaning capture efficiency is maximized strictly within an intermediate, narrow length/girth range.

Classification and regional operation of specialized fishing gears:
Statement (I) is true: A verandah net is a specialized composite barrier trap gear consisting of a vertical blocking barrier net combined with a horizontal floating netting shelf ('verandah'). When jumping fishes (such as grey mullets or milkfish) hit the vertical barrier, they leap into the air and fall onto the horizontal verandah netting where they are captured.
Statement (II) is true: Chinese dip nets (Cheena vala) are giant cantilevered shore-operated lift nets that are an iconic, widely operated artisanal fishing gear throughout the brackish backwaters and estuaries of Kerala (notably around Kochi and Vembanad Lake).

In the Hooghly-Matlah estuarine system (West Bengal), stationary tidal bag nets (locally termed 'Been jal' or 'Behundi jal') are the most dominant and productive fishing gear, contributing over $60\%$ to $70\%$ of the total estuarine fish and shrimp harvest. They are fixed to wooden stakes against tidal currents to capture organisms carried by the ebbs and flows.

Option analysis:

• Trap: Used locally for crabs, but represents only a minor fraction of landings.
• Bag net: Correct, dominant gear ('Been jal') in the Hooghly-Matlah estuarine complex.
• Lampara net: Surrounding net used in marine pelagic fisheries, not in estuaries.
• Drag net: Used occasionally in shallow shores, not the dominant estuarine gear.