Abstract:
Although reservoirs represent lacustrine ecosystems, they are drastically different
from natural lakes for the main reason of prominence of anthropogenic perturbations,
which affect ecosystem functioning. Many reservoirs in Sri Lanka are primarily used
for irrigation of agricultural lands and are secondarily utilized for inland fisheries
production. Effects of hydrological regimes on the biological productivity in
reservoirs are therefore expected to have a significant bearing on the fish yield. Aim
of this study is to investigate the influence of hydrological regimes on fish yield. In
the present study, twelve limnological characteristics of 10 irrigation reservoirs in
the Kala Oya river basin of Sri Lanka were investigated from June 2013 to February
2016. Fish yield data in these reservoirs were obtained from the log-book records of
fisheries societies. Hydrological data were obtained from the irrigation authorities.
Relative water level fluctuation (RWLF), defined as the ratio of amplitude of mean
water level fluctuation to mean depth, showed a third order polynomial relationship
with chlorophyll-a content (Chl-a) according to, Chl-a = -2.4046 RWLF 3 + 18.732
RWLF 2 - 39.487 RWLF + 44.052 (R² = 0.4351). There was also significant third
order polynomial relationship of Chl-a, with fish yield (FY) as: FY = -0.047 Chl-a 3
+ 2.689 Chl-a 2 - 45.186 Chl-a + 328.54 (R² = 0.410). As some of the reservoirs in
the Kala Oya river basin are terminal reservoirs (e.g., Rajanganaya, Angamuwa,
Siyambalangamuwa) from which water is released mainly for irrigation while some
are, in addition to irrigation water supply, feeder reservoirs of several downstream
reservoirs (e.g., Ibbankatuwa, Kalawewa), effect of hydrological regimes on the
biological productivity were possibly represented by polynomial relationships. There
was a positive linear relationship between RWLF and FY according to the equation,
FY = 27.222 RWLF + 66.729 (R² = 0.586). This indicates that through manipulation
of hydrological regimes, reservoir fish yield can be optimized. As such, an effective
dialogue should be in place between irrigation authorities responsible for controlling
hydrological regimes and authorities responsible for inland fisheries development.
