GILL NETS

GILL NETS

Fish can be caught in a gill net by becoming tangled, wedged in, or gilled. The most common way is gilled. The fish swims in but can only get its head in, as it struggles, the net slips behind the gill covering and traps the fish. Gill nets can be either lethal or non-lethal. To have the net non-lethal, it is dragged through the water and then removed. If the net is left in the water for long periods of time, it becomes lethal.

A gill net is produced by taking a single wall of webbing and holding it vertically in the water column with a lead line weight and cork floats. In BC, the lead line must be 75 g/m. The floats must be cork, and the cork must be braided into the corkline (15 to 20 cm apart). The distance between the cork should not vary, the size can be varied to allow the net to float or sink. The netting material is a light green multi- or monofilament nylon. The net must have nylon gables of approximately 18 kg test. The mending lines must be of monofilament nylon. The common design for an experimental net is six different panels, 2 m deep and 8 m wide, Table 1. The different mesh sizes are used to reduce the effect of size selectivity.

Table 1: The mesh size used to catch various types of fish in BC.

Order	Mesh Size (mm)	Filament Size (mm)	Fish Fork Length (mm)
1	25	0.20	114
2	38	0.20	178
3	51	0.20	228
4	64	0.25	280
5	76	0.25	345
6	89	0.30	380

The manner in which the net is hung (hanging ratio) is important. The hanging ratio is the ratio between the length of the float line and the length of the stretched mesh hanging on the float line. In BC, the hanging ratio is 2:1. The mesh size of the gill nets can be expressed in two ways, bar measure-the distance between the knots and the stretch measure-the length of a single mesh when the net is stretched taut. In general, the bar measure is half the stretch measure.

Gill nets are most effective when they are set among an aggregation of fish. They are set depending on the species desired and the habitat involved. The most common way to deploy the nets is to set the netting along the bottom. In shallow water, the net is set by driving stakes into the bottom. Other ways that the net can be deployed is to have it drifting, floating, or encircling. To retrieve the net, start at the downwind end, and pull the net up over the side of the boat. The net is usually set at dusk and retrieved in the morning. The net works best when set on even, regular substrate.

The largest bias is size selectivity, they do not capture the small or really large fish. Another bias is that only the fish that come into physical contact with the trap are caught.