Fish Population Dynamic and Stock Assessment

5.1.6. Factors influencing the selectivity to Gill nets

Planning an experiment involves prior knowledge of the factors that can affect gear selectivity. Some of them, as those related to the gear, are easy to control, in opposition to those related to the fish or to environmental conditions.

Gear parameters

· Gang and net dimensions

· Mesh size (Use the given one) - The smallest fish caught has a maximum girth equal to the perimeter of the mesh, the largest fish caught has a head girth equal to the perimeter of the mesh. Fish between these two sizes are caught.

· Hanging ratio

· Vertical slack (defined as the ratio between the stretched length of the inner and the outer netting, in trammels and semi - trammel nets). It can be controlled by the height of the walls in a trammel net.

· Twine characteristics (material, construction, thickness, colour, flexibility...)

· Floatation and weight

· Soaking time

· Arrangement of nets in the fleet - sequence and joining between nets; interaction between nets

Parameters related to the fish

· Fish abundance

· Fish availability to the net

· Fish behaviour towards the net

· Fish size

· Fish shape (girth at different body points)

· Presence of by-catch

· Presence of predators (can reduce the soaking time)

· Net saturation

· Patchy distribution in the net (includes attracting effects by individuals caught)

Parameters related to the fishing operations

· Dimension of boats (low-lying vs. high-lying boats)

· Net handling techniques

· Environmental parameters

· Light level

· Sea state and currents

· Seabed type

· Depth

· Occurrence of water/bottom debris