Mortality rates
Jones and van Zalinge’s (1981) method was applied to investigate
the annual instantaneous total mortality (Z) using the
cumulated catch curve of the length composition information
as follows: ln(Cn) =a + (Z/K) ln(L1 L), where ln(Cn) is
the cumulative frequency. A linear regression between ln (Cn)
and ln (L1 L) was done and the slope = Z. Pauly’s (1980)
equation was used to calculate the natural mortality (M).
Pauly’s equation depended on the annual mean sea surface
temperature of the fishing area, that equal to 25 C. The fishing
mortality (F) was expressed as F= Z M and the exploitation
rate (E) was estimated as F/Z according to (Gulland,
1971). The fishery resource status was assessed according to
Patterson (1992) by the comparability of the current fishing
mortality rate with the optimum or the target (Fopt) and limit
(Flimit) biological reference points (BRP) which were delineated
as: Fopt = 0.5 M and Flimit=2/3 M.