EFFECT OF SELECTIVITY OF TRAMMEL NETS UPON GROWTH AND MORTALITY OF TWO TILAPIA SPECIES.

EFFECT OF SELECTIVITY OF TRAMMEL NETS UPON GROWTH
AND MORTALITY OF TWO TILAPIA SPECIES.
A.F.A. LATIF”‘, E1~:-SAYED A. IOIALLAF**, AND A.A ALNE:-NA-:EI**
• Academy of Scientific Research and Technology
··Zoo~~partment1ll’culty of Science, Almenofeya University
ABSTRACT
Trammel net selectivity effect on growth and
mortality of fish was studied. using 1705 T. nflotfea
and 1109 T. zl111i caught in the months of April
1984 to April 1985.
Trammel net selectivity was found to be of the
simple type (Less or non-selective) for either of
the two species under study.
The increase in growth rates among T. nflotiea
age-groups I to “I was attri buted to the hi gher
vulnerability of age-group I of this fish to the
gear, than those of the same age of T. zfllif.
However, mortality rates calculated from catch curves
could not represent the case ,and mortality rates
estimated by the Severton and Holt (1956) formula
were more precise because the formula took into
account effect of selectivity and growth rates.