Spatial and temporal variations of Limnothrissa miodon stocks and their stability in Lake Kivu

Limnothrissa miodon is a small pelagic clupeid that was introduced into Lake Kivu in the late 1950s to fill an empty niche. Since then, it has become the main fishery in the lake. The fish stocks were estimated by hydroacoustics between 2012 and 2018 to provide information on the fishery in the current context of changing environmental factors. The main objectives were to determine the most appropriate season for assessing stock dynamics, to characterize temporal and spatial distribution in L. miodon populations and to compare with previous surveys to help predict status of its stocks. The fish size distribution showed that the long dry season was the most appropriate season for assessing the stocks, as it provides information of recruitment for the year. The south and west basins always had higher densities (1.23 m2/ha) and biomass (21–22.7 kg/ha) than the north basins (0.62–0.77 m2/ha; 15.3–16.5 kg/ha). The stock showed declining trend from 7,000 t in 1985 to 1,000 t in 2012 and thereafter consistently increased to 4,000 t in 2018. This last value is similar to previous estimates of 1990s and 2008, showing that the stocks of L. miodon are stable. The low 2012 values could be due to particular environmental conditions in 2012–2014, when there was a shift from diatoms and cyanobacteria to green algae. There is therefore a need to combine high-quality environmental data with fishery surveys to better understand the dynamic of fish and fishery especially under the increasing influence of climate change on lake productivity processes.