Towards understanding the presence of abundant fish in running iishana

Although the iishana (seasonal watercourses) deltaic system in northern Namibia and southern Angola is usually dry, millions of fish populate the more than 100 000 km2 area during high floods that occur irregularly about once in three years. The origin of the fish has been a topic of debate for a long time, including suggestions of refugia for breeding fish in the upper parts of the Mui and Cuvelai catchments, deep dams in both Angola and Namibia and fish arriving with flood water from the Kunene River. This paper discusses fish collections made during a major efundja (large flood with plenty of fish) in 2017 and a medium flood in 2020. The bulk of fish during major efundja comprise two species that were also collected in the flooding Cuvelai and in iishana fed from deep dams in 2020. The source of fish during medium floods is therefore ascribed to fish surviving in refugia and then breeding successfully. The fish occurring in abundance in iishana during major efundja, however, come from tributaries of the Kunene along the divide with the western iishana, where spawners and young fish cross the divide and migrate into the headwaters of the iishana. Plentiful fish during efundja relies on unhindered access into the iishana. The Cuvelai system is threatened by environmental degradation in the iishana region and inappropriate road infrastructure is a constraint. Fisheries activities should be regulated and cooperation between the Angolan and Namibian authorities is required to ensure the survival and continuation of fish resources.