ANNEXE 12
R/V ATLANTIDA SURVEY IN SIERRA LEONE
BIOMASS ESTIMATES FOR SELECTED COMMERCIAL DEMERSAL SPECIES
FROM “SWEPT AREA” METHOD

by P. Showers

The species considered (tables 1–9) are Sepia officinalis + others, Balistes capriscus, Pagellus bellottii, Epinephelus aeneus, Sparus caeruleostictus, Dentex angolensis, Priacanthus arenatus, Pseudupeneus prayensis, and Dentex congoensis. The other species occuring with Sepia officinalis include Sepia bertheloti, Illex coindetii, Octopus vulgaris, Sepiella ornata, Alloteuthis africana, Sepia orbignyana, Todaropsis eblanae. The data treated were from the bottom trawling programmes from 5 cruises on board the R/V ATLANTIDA (06/87, 10/88, 04/89, 11/89, 07/90). In all these cruises all gear and equipment remain unchanged. The gear used is the bottom trawl “Hake-4M” with 75 m headrope and 11 mm stretched codend and mesh size. The cruises cover the shelf area from about 15 m to 100 m. Because the trawling stations were not arranged along transects, the area has been stratified to give better estimates.

Stratum A covers the area above the 8° latitude

Stratum B " " below " "

Stratum I " " shallower than 50 m

Stratum II " " deeper " "

Area of stratum A = 3,500 nmile²

B = 2,700 "

I = 4,100 "

II = 2,100 "

The parameters used in each stratification are:

where (1) Cw is the mean catch in weight per haul

(2) A is the area swept = D * W nmiles²

(2.1) D = V * t is the distance covered by the trawl

(2.2) W, the width of the path swept = h * X2 nmile²

where (2.2.1) h is the headrope length

(2.2.2) X2 is a correction factor for the arc formed by the headrope

(X2 = 0.5 for bottom trawls)

(3) X1 is the proportion of fish retained in the net = 0.5

(4) A is the total/stratified area or area of the stratum

Detailed results are given in the following tables.