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6.3.3. Biomass Estimation
6.3.3. Biomass Estimation
(i) Catch in weight per unit area
$${Cw(kg/nm2)t}\over a$$ = $${Cw/t}\over{a/t}$$
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
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 =
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.