Fish Population Dynamic and Stock Assessment

6.3.3. Biomass Estimation

(i) Catch in weight per unit area

Catch in weight per unit area can be calculated by

$${Cw(kg/nm2)t}\over a$$ = $${Cw/t}\over{a/t}$$

Cw = catch in weight of a haul

Cw/t = catch in weight per hour

t = time spent for hauling (in hours)

a = area swept

a/t = area swept per hour

(ii) Average biomass per unit area

Average biomass per unit  area can be calculated by

$$\overline{b}$$ =$$\overline{(Cw/a)}\times 1 Kg/nm^2$$

$$\overline{b}$$ = average biomass per unit area ($$(Kg/nm^2$$ )

$$\overline{(Cw/a)}$$ = mean catch per unit area of all hauls

x1 = Fraction of biomass in the effective path swept by trawl which is actually

retained in the gear (usually 0.5 - 1.0)

(iii) Total biomass for total area under investigation

Total biomass could be estimated for total area under investigation by 

B = $$\overline{(Cw/a)}\times A \over x1$$

B = Total biomass

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

Though swept area method is not very precise, higher precision can be obtained by increasing the number of hauls Another way of reducing errors is to apply stratified sampling. Suitable stratification may reduce the variance considerably for the same number of hauls and thus improve survey efficiency. The distribution of many species is determined by depth and bottom type.