Abstract:
The impact of fish stock supplementation on the energy flow in
Hemavathy reservoir ecosystem in Karnataka, South India was assessed
using mass balance modeling. The mass balanced model structured
around thirteen ecological groups which included the nonliving group,
detritus. Hemavathy Reservoir with an area of about 9162 ha was
constructed in 1981, on the river Hemavathy for irrigation purpose in
Karnataka. The fish landings data of Hemavathy reservoir was collected
and studied for three years from 1983 by earlier workers. During this
period there was no systematic stocking of fish in this reservoir. Central
Inland Fisheries Research Institute investigated this reservoir in 2002 to
2004 when regular stocking and harvesting regime had become an
established management practice. Hence the present study was conducted
to analyse the impact of fish stocking on the energy flow and ecosystem
maturity of a reservoir ecosystem by comparing two scenarios; pre- and
post- fish stocking regimes. The mass-balance models of the reservoir
ecosystem were constructed for two periods, 1984 and 2003 using
Ecopath with Ecosim software to compare the fluctuations that occurred
within each of these years before systematic stocking and post-stocking
period. Flow at the trophic level II largely involves the zooplankton (the
dominant herbivore), bottom biota and the two groups of fishes the minor
carps and the exotic carps. Though aquatic birds occupy the highest
trophic levels in both phases, among fish groups Eels in the pre-stocking
phase and murrels in post-stocking phase occupy top trophic levels. The
pattern of energy flow through the network is described in terms of the
number of trophic interactions of each group and the mean length of
pathways (MLP) through the network. For top predators, the number of
connections was high (78–233) and with a high number of steps (MLP =
3.69). In the middle of the trophic chain, the exotic carps and major carps were (MLP = 2.87 &2.27) and the groups at the bottom of the pyramid
like zooplankton and bottom biota are characterized by low numbers of
trophic interactions (2 & 3 respectively) and path lengths (2.0). The
ecotrophic efficiency of the carps were low in post-stocking phase
showing these groups were not heavily exploited by the fishermen but the
increase in biomass due to stocking are evident. The resilience of
reservoir ecosystem before stocking the reservoir and the impact on
ecosystem after fish stocking were compared using various indices of
ecosystem maturity. A decrease of the flow of the system from pre-stock
to post-stock is evident (22.28%). The system overhead was higher by
0.29% in the post-stock phase indicating maturity. The ecosystem indices
tested indicate that the reservoir during post-stock phase was in a more
resilient state compared to the pre-stock phase. The maturity of the
ecosystem showed an improvement which indicates a positive impact of
stocking.
