Impact Of Habitat Moderation By Weir Installation On Invertebrate And Fish Communities In Rivers Awach–Seme (Kenya) And Nambale (Uganda

ABSTRACT

The Lake Victoria Basin (LVB) rivers and streams once rich in biodiversity have been undergoing degradation over the years resulting in decline in fish productivity. Studies have mainly focused on large rivers in Kenya and Uganda. Impacts of habitat alteration by weir installation as a river management tool are lacking. This study aimed at comparing the limnological responses of Rivers Awach Seme (Kenya) and Nambale (Uganda), after weir installation. Data was collected monthly for six months from November 2012 to April 2013 after weir installation in October 2012. River morphology, physical-chemical characteristics, riparian vegetation composition, organic matter, benthic invertebrates and fish community assemblages were assessed. The river morphology was studied by measuring stream width and depths. The physical-chemical characteristics were assessed by measuring stream velocity, water temperature, transparency, conductivity, pH, dissolved oxygen and total dissolved solutes (TDS). Fish and benthic invertebrate were caught using an electrofisher and Hess sampler respectively and their community structures were determined. Data was analysed using Statistical Package for Social Sciences (SPSS) version 16.0) where the probability value (P˂ 0.05) was used for the two tailed tests. Results showed that the rivers differed markedly in their morphology with River Nambale (Uganda) being heavily eroded while River Awach Seme (Kenya) was generally well protected. Physical-chemical characteristics of the two rivers did not differ significantly with pH averaging 7.55±0.10 for Nambale and 7.54±0.08 for River Awach Seme. Similarly, conductivity, dissolved oxygen and TDS did not differ significantly with means of 204±14.25 μScm-1, 9.2± 0.46 mgl-1 and 102±7.08 mgl-1 respectively for River Nambale and 201±18.05 μScm-1, 9.6±1.13 mgl-1, and 100±7.89 mgl-1 respectively for River Awach Seme. Riparian vegetation of the rivers consisted of similar species. More organic matter accumulated on the weir in River Nambale than at River Awach Seme with means of 965.1±64.39gm-2 and 619.5±67.23 gm-2 respectively. The lowest accumulation occurred in December 2012 while the highest in February 2013. Five orders of macroinvertebrate were sampled in River Awach Seme and they included Ephemeroptera, Trichoptera, Diptera, Coleoptera and Odonata. In addition to the five orders, order Megaloptera and class Hirudinae (Phylum Annelida) were observed in River Nambale. Fish species at the two rivers were similar. They included Barbus nyanzae Whitehead, Barbus kerstenii Peters, , Barbus jacksoni Gunther, Barbus altianalis Boulenger, Barbus cercops Whitehead, Clarias gariepinus Burchell Labeo vicrorianus Boulenger, and Gambusia affins Baird and Girard. In River Nambale, the fish community was dominated by Barbus nyanzae which constituted 40% of the total catch. Similarly, Barbus altianalis dominated the fish community of River Awach Seme constituting 46% of the total catch. Weirs impacted strongly on the river morphology and resulted in an increase in fish and macroinvertebrate abundance downstream. There is therefore need for them to be constructed as a river management tool. However, further long term surveys on invertebrate colonization succession, organic matter decomposition rates and additional niches created after weir installation are recommended to ascertain the impact of weir installation as a river management tool.