Appendix 10
A NOTE CONCERNING FISHERIES MANAGEMENT IN THE WESTERN SAHARA REGION

by

F. Mennes
Associate Expert FAO
Institut Scientifique des Pêches Maritimes
UNDP/FAO Project MOR/81/002
Casablanca, Morocco

A multispecies yield per recruit model, based in Beverton and Holt, is presented in Figure 1.
It is valid for : Boops boops
Dentex gibbosus
Dentex macrophthalmus
Dentex macroccanus
Dentex canariensis
Diplodus vulgaris
Diplodus bellottii
Pagellus erythrinus
Pagellus acarne
Pagellus bellottii
Spondyliosoma cantharus
and Octopus vulgaris
Loligo vulgaris
Sepia officinalis

The model includes the biological parameters for each species :

• maximum weight
• growth coefficients of von Bertalanffy
• natural mortality
• age at first recruitment
• selectivity

  As well as economical parameters for each species :

• Dollars/ton at Las Palmas, 1984

The biological parameters were determined based on cruises in the region 22–26°N (Western Sahara, 34.1.3) during 1980–1981–1982–1983 (Mennes, 1984 and Idelhaj, personal communication)

The economical parameters were determined in Las Palmas (Gréboval, 1982).

The yield per recruit is thus expressed in value.

Two cross sections are presented in Figure 2. The first cross section (A) shows that a reduction of fishing by 40% corresponds to MSY, if a mesh size of 60 mm is assumed.

The second cross section (fishing effort = 100%) (B) shows that 2 optima exist : the first one at 124 mm mesh size, the second one at 204 mm mesh size.

LITERATURE

Mennes (1984), CECAF/TECH/84/62
Gréboval (1982), CECAF/TECH/82/45

Figure 1 : Plot. Beverton and Holt yield isopleths, valid for 11 Sparids and 3 Cephalopods. Values for each s[ecoes are included in the model.
Actual fishery effor : 100
Actual mesh size (mm) : 60.
Casablanca, night of 11.12.84 - 12.12.84

Figure 2 : Plot. Beverton and Holt yield isopleths, valid for 11 Sparids and 3 Cephalopods, values included.
Cross sections on 60mm mesh size and on 100 relative fishery effort.
Casablanca, 17.12.84.