The last snapshot of natural pelagic fish assemblage in Lake Turkana, Kenya: A hydroacoustic study

Abstract

A hydroacoustic survey and supplementary gillnet investigation were carried out in the open water of the central part of Lake Turkana in September 2009. Overall acoustic fish density and biomass were assessed as 1381 ind./ha and 30 kg/ha, respectively. The fish density estimate was lower than the results from two previous investigations in the 1970s and 1980s (long-term average 3739 ind./ha), but the biomass remained relatively unchanged (long-term average 25.4 kg/ha). A decreasing gradient in pelagic fish density from the western to eastern shore of the lake was observed. Fish were distributed unevenly within the water column. During the day, a majority of fish aggregated in the mid-water layers (10–12 m below the water surface), creating on echograms the so-called Midwater Scattering Layer. This feature dissipated completely during dusk and the majority of fish occurred in the surface layers at night. These diel vertical fish migrations influenced day and night hydroacoustic estimates of the total fish abundance. Synodontis spp., Lates spp. and Schilbe uranoscopus dominated the catch of both mid-water and bottom gillnets installed in open water areas. Hydrocynus forskalii and Brycinus spp. contributed significantly to the catch of mid-water gillnets while Bagrus bayad and the endemic Haplochromis macconneli occurred only in the catch of the bottom gillnets