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Inter. J. of Marine Science 2012, Vol.2, No.4, 24-30
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29
as ovarian development is more reliable for detailing
spawning periodicity (West, 1990).
Fish were sampled for length (TL (from snout to tip of
longest lobe of caudal fin), nearest mm) and wet
weight (nearest 0.1 g), and gonads removed and
weighed (nearest 0.01 g). To determine lunar periodicity
in spawning, a sub-set of samples collected weekly
over one complete lunar cycle were evaluated
microscopically with gonad maturity stage assessed
using a developmental scale ranging from immature to
spent (Table 1). To document lunar periodicity and
confirm seasonal periodicity, gonado-somatic index
(GSI) for females was estimated using the following
formula:
GSI = weight of ovary (g)/weight of fish (g)
－
weight
of ovary (g)×100.
Sampled gonads were preserved in a 10% formalin
solution, buffered with acetic acid and calcium
chloride (FAACC) for histological preparations.
Microscopic staging was based on standard techniques
(Ntiba and Jaccarini, 1990; West, 1990; Samoilys and
Roelofs, 2000). An automatic tissue processor (Leica
TP 1020, Wetzlar, Germany) was used to process
haematoxylin/eosin-stained tissue samples which were
then sectioned at 5~7 µm. Reading of slides was done
by two observers using Table 1 criteria.
The minimum size at first maturity, L
50
, the size at
which 50% of females that were reproductively active
during the main spawning periods, was estimated by
fitting a logistic curve, using the least square method,
to the proportion of macroscopically mature individuals
by size class (see Kolding and Skaalevik, 2010). This
method, termed as
effective maturity
recognises that,
despite maturity, not all females are reproductively
active in the spawning season (Pears et al., 2006), and
also minimises errors in assigning immature fish as
inactive females (Samoilys and Roelofs, 2000).
Gonads that were undetermined in terms of maturity
stage were excluded from this assessment.
Authors Contributions
SA completed data analysis, essayed and provided techniques
for data analysis, designed the research, guided the process,
prepared and modified this paper.
Acknowledgements
The author is grateful to the Director Kenya Marine &
Fisheries Research Institute (KMFRI) for laboratory space and
to Boaz Orembo and James Gonda who assisted in data
collection and histological laboratory analyses at KMFRI. Dr.
Melita Samoilys is thanked for providing guidance and advice
both in the field and in the laboratory. Thanks to Jan Robinson
for providing good suggestions during preparation of this
chapter. Much thanks to Dr. Kevin Rhodes for useful comments
when he reviewed this chapter. The author is grateful to the
anonymous reviewer for useful comments. This project was
funded by Western Indian Ocean Marine Science Association
(WIOMSA) through Marine and Coastal Science for
Management (MASMA) programme.
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