Comparative mass-balanced trophic models to assess the impact of environmental management measures in a tropical reservoir ecosystem

Comparative estimates of trophic status and energy flow were carried out to assess the impact of environmental management measures taken to conserve the fish stocks in tropical reservoir ecosystem in India. Mass-balanced models of reservoir ecosystem was constructed for two periods, for 1995-1996 and 2002-2003 using Ecopath software showing the impacts on different groups before and after introduction of fisheries management measures. The models were structured around seventeen functional groups including commercially important fish groups and prawns. The ecotrophic efficiencies (EE) of Tilapia, other cichlids, murrels and prawns were high during the post-ban phase. Barbs, catfishes, major carps and gobids had low EE values. The primary producers are heavily exploited (EE > 0.88) in the system during the pre-ban phase. mixed trophic impact (MTI) analyses indicate that phytoplankton and detritus have positive impact on most other groups. On the other hand, zooplankton had negative impact on phytoplankton and murrels had a negative impact on major carps. The resilience of reservoir ecosystem during unlimited fishing access period and the impact on ecosystem after the introduction of fishing ban were compared using indices of ecosystem maturity. A decrease of the flows of the system from pre-ban to post-ban is evident (−82.7%). TB/TST was higher by 63.6%, net system production was lower by −82% and ascendancy was lower by −0.14% in the post-ban phase indicating maturity. Detritus accumulation decreased by 4% in post-ban phase. The ecosystem indices tested indicate that the reservoir during post-ban phase was in a more resilient state compared to the pre-ban phase. The tropical reservoir possesses a great amount of reserve energy showing that it is a system that is resilient to perturbations. The successful development of similar ecosystem models will provide powerful new tools to evaluate the impact of human and environmental factors on reservoir ecosystems.