Mortalities
of Natural Sinarapan Population
by
Prof. Skorzeny C. de Jesus
Joseph S. Esplana
As early as 1988, the population
dynamics of Sinarapan in Lake Manapao was studied to generate biological
information needed to manage the lake as a sanctuary for the "world's
smallest commercial fish". Population parameters estimated were for
growth, mortality and exploration rate. The reproductive biology
of the fish was also studied to establish spawning periodicity and recruitment
patterns.
Electronic length-frequency
analysis was used to estimate important growth parameters such as asymptotic
length or its maximum attainable length (La) and growth constant (K).
The K value defines how fast the fish achieves La. These were fitted
to the von Bertalanffy growth formula to predict growth pattern of the
fish. Findings showed that this fish has a La of 24.6 mm and K of
2.25 per year.
Total mortality was estimated
using catch-curve analysis which was derived from length fre-
quency data collected in
ten months. Natural mortality was estimated using Pauly's empirical
formula. Fishing mortality was estimated by subtracting natural mortality
from total morta-
lity. In 1988, the
total mortality was estimated by Prof. Soliman at 9.234/yr..
With this low total mortality and since there was no fishing activity in
the area, it could be safely assumed that total and natural mortality have
close values. Subsequently a low to moderate exploita-
tion rate was calculated.
These mortalities are instantaneous coefficients which differ with mortality
rates. For instance, an instantaneous mortality value of ten (10)
means that the population of one billion is decreased to 45,000 after 12
months.
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