Mass balance model of Lake Volta fisheries: The use of Ecopath model

Abstract

A mass balance model of trophic interactions among ten key functional producer and consumer groups in Lake Volta was constructed using the Ecopath model to study the energy flows and species interactions in the lake. The present study was based on secondary and primary data on fish catch, diet composition, phytoplankton and zooplankton biomasses, collected in 2015 and 2016. Additional information on growth parameters of major species required for balancing the Ecopath model was obtained from sampling and FishBase. The functional groups were detritus, phytoplankton, zooplankton, benthos, prey fish, Tilapia, Bagrus, Chrysichthys, Alestes and Synodontis species. Four trophic levels were identified in the Lake Volta ecosystem, with the energy flow occurring mainly within the first three trophic levels. The calculated ecotrophic efficiency value of the primary producers (phytoplankton: 0.17; detritus: 0.22) indicated they were least exploited, compared to the secondary producers, zooplankton (0.80) and benthos (0.50). All secondary consumers had ecotrophic efficiency values higher than the primary producers, indicating they are exploited in the ecosystem. The main energy flows in the lake were from phytoplankton and detritus at trophic level I, and Bagrus species, the top predator, at a level of 3.30. The network analysis, illustrating a connectance index of 0.43 and an omnivory index of 0.06, in the lake system indicated the ecosystem is unstable, somewhat immature and still in a developing stage.