Fish Population Dynamic and Stock Assessment (2+1)

Solution

(i) The distance covered per hour can be calculated from

D² = VS²+CS²xVSxCos (dir V – dir C)(nm)

D= distance in nm

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

CS= Velocity of current (knots)

dir V= Course of vessel (degrees)

dir C = direction of current

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

(i) a = V.t.hr & (hr) x 2

Where

A = area of swept, hr=length of headrope, V-velocity of the trawl over the ground when trawling. t – time spent for trawling, x₂ = wing spread as fraction of the head rope length (0.4 – 0.6).

Biomass estimation

(ii) Catch in weight / unit area =    CW/t

                                                  ---------------

                                                          a/t

CW = Catch in weight of a haul

CW/t = Catch in weight per hour

t = time spent for trawling

a = area of swept

a/t = the area swept per hour

iii. Average biomass

Where,

A = Total size of the area under investigation (nm²)

cw/a = mean catch per unit area of all hauls

X₁= Fraction of biomass in the effective path swept by the trawl, actually retained in the year (usually 0.5-1.0)

Total biomass

B =

B = Total biomass

A = Total size of the area under investigation.