Fish Population Dynamic and Stock Assessment

6.3.2. Swept area method

Trawl net is a conical bag with a wide mouth fitted with otter boards for horizontal opening and floats and weights for vertical opening. This net sweeps the sea bed over a wide area. The tail end of the gear where the fishes are collected is called “cod-end”. Trawl nets dragged on the sea floor are called “bottom trawls”. The mean catch (either in weight or in numbers) put unit of effort or per unit of area is an index of the stock abundance. This index is converted into an absolute measure of biomass. This technique is called “swept area method”.

The trawl sweeps a well defined path, the area of which is the length of the path, times the width of the trawl called “Swept area” or “the effective path swept”.

The swept area ‘a’ can be estimated by following formula

a= v * t * h * x2

Where

V = Velocity of the trawl over the ground when trawling

h = length of head rope

t = time spent for trawling

x2 = wing spread as fraction of the head rope length (0.4 - 0.6)

When exact position of the start and end of the haul are available, the distance covered can be estimated by

D = 60 x $$\sqrt{{(Lat1-Lat2)^2}+{(Long 1-Long 2)^2}.\cos^2(0.5x(Lat 1+Lat2))}$$

Where D = Distance in nautical mile,

Lat 1 = Latitude at start of haul

Lat 2 = Latitude at end of haul

Long 1 = longitude at start of haul

Lang 2 = longitude at end of haul

Exact latitude and longitude values can be obtained from global positioning system (GPS), a simple electronic device.

If exact positions are not available, the distance covered for an hour can be calculated from

D = $$\sqrt{VS^2+CS^2+2*{VS}*{CS}*\cos{(dir V - dir C)}}$$

D = distance in nm

VS = velocity of vessel (knots = nm/hr)

CS = velocity of current (knots)

dirV = course of vessel (degrees)

dirC = direction of current (degrees)