A standardised hydroacoustic time series reveals increasing fish biomass and southward redistribution of pelagic fish in Lake Turkana

Lake Turkana, the fourth largest of the African Great Lakes by volume and the largest permanent desert lake in the world, remains among the least monitored, with comprehensive lakewide assessments scarce since the 1972–1975 Lake Turkana Project. Building on an initial two-survey baseline, a third lakewide hydroacoustic survey was conducted in March 2026, extending the record to three surveys (May/June 2024, November 2024, and March 2026) and enabling, for the first time, a standardised time series of standing fish biomass and its spatial distribution. Two key findings emerge regarding the trajectory of the resource. First, standing biomass increased substantially across the series, from an estimated 74,723 tonnes in May/June 2024 to 124,601 tonnes in November 2024 and 182,439 tonnes in March 2026, an increase driven predominantly by the mid-water small pelagic layer dominated by the endemic Alestes. Second, this increase coincided with a marked southward redistribution of pelagic biomass, with the acoustic centre of gravity shifting approximately 28 km south and the spatial distribution widening from 37 to 61 km, a departure from historical accounts identifying the northern basin as the lake's principal centre of production. Three independent diagnostic checks confirm that neither finding is an artefact of the change in survey frequency between 2024 and 2026. Community size structure remained stable throughout, with small-bodied fish continuing to dominate total biomass. These trends provide the first robust evidence base for tracking how Lake Turkana's fishery resource is changing under a fluctuating and increasingly regulated hydrology. The study recommends (1) a dedicated local cross-frequency calibration to strengthen absolute biomass estimates, (2) continuation of the standardised survey series at regular intervals to build a statistically robust long-term trend, and (3) integration of biological sampling and night-time transects to improve species attribution and account for diel vertical migration. Keywords: hydroacoustics; biomass trends; spatial redistribution; Lake Turkana; small pelagic fish