| dc.description.abstract | This study describes the biology of the dory snapper, Lutjanus
fulviflamma (Forsskal, 1775) in the Kenyan inshore marine waters on
an annual cycle between October, 1991 and September, 1992. The
results are based on 891 specimens collected from the creeks
surrounding Mombasa island using traditional traps.
The population of this species showed a skewed overall sex
ratio in favour of the males (Chi-squared = 7.124, P< 0.05).
However the ratio did not depart significantly from the 1: 1 ratio, in
the months of February, March, April, October, November and
September. Size related trends in the ratio have been observed with
the males dominating the size groups less than 21.0 cm total length
and the females dominating the larger size classes. The disparity in
the population sex ratio has been attributed to spawning migrations,
while differential growth rate is thought to explain the size related
trends in the ratio.
L. fulviflamma has been found to display an isometric growth
with the length-weight relationship being symmetrical between the
sexes (F = 3.84, FO.05' (1); 852 = 5.02, p.: 0.05). Hence, male and
female data are pooled in subsequent analysis. The overall length(
weight relationship is derived as,
W = 0.0185L2.94
Although the relative condition of the dory snapper has been
observed to decline during the spawning months (suggesting the
influence of spawning), this has not been steady. The condition is
affected more by the intensity of feeding than by the strain of
spawning.
Investigations of age and growth of 1,. fulviflamma have
indicated that rings on scales do not correspond to age and so the
Petersen method was used to analyse age and growth. The growth
parameters Loo' to' and K of von Bertalanffy growth equation were
estimated by the Ford-Walford plot as 35.0 cm, -0.55 and 0.59 per
year, respectively. The growth curve for this species in the Kenyan
marine waters was fitted through the.estimated mean length at age
and expressed as,
Lt = 35 [1 - exp{1-{O.59{t+0.55)p.
The annual composition of the diet has been studied by the
gravimetric, numerical and occurrence methods. 1,. fulviflamma is a
carnivorous fish, feeding mainly on fishes and crustaceans; these
comprised 50.4 and 30.4%, respectively of the bulk of food eaten.
The diversity of food items eaten is highest during the pre-spawning
period and is dominated by fish prey during spawning season. The
size of the crab and clupeid fish preys showed strong and significant
correlations with the predator size, r = 0.89 and 0.78 for crab and
clupeid fish preys respectively. The average prey-predator size ratios
were 0.01 and 0.2 for crab and fish preys, respectively.
Based on macroscopic appearance and relative weight changes
of the gonads of this species throughout the year, 5, clearly defined
gonad maturity stages have been distinguished and used to follow the
gonad maturation cycle for this species. Both histological examination
of internal organization of the ovary and oocyte diameter distribution
counts of portion's of the same ovary preserved in Gilson's fluid, have
shown that oocyte development in the ovary of 1. fulviflamma is
asynchronous. The percentage occurrence of mature fish in the
samples together with the pattern of changes in gonad weight
indicate that the fish has an extended spawning season between
October and March, a period that falls within the north-East monsoon.
The fecundity of the dory snapper has been determined from a
count of both vacuolated and yolked oocytes. The fecundity is
variable ranging from 51,000 to 460,000 eggs for females of 17 -
30cm total length. Both pre- and post spawning atresia of the
oocytes have been observed. The degree of oocytic atresia is lowest
in stages I (2%) and II (3%) and rises sharply to 18.3% in stage III.
Sixty-two percent atresia of oocytes takes place between stages V
and lib. The present fecundity estimates have been corrected by 1%
loss due to atresia accompanying oocyte growth from stages IV to
V when spawning occurs, because fecundity was determined at stage
IV. | en |
