Distribution and breeding biology of offshore pelagic cyprinids and catfish in Lake Malawi/Niassa

Lake Malawi/Niassa is a permanently stratified lake, consisting of an oxygenated layer of 230 m depth that overlies an anoxic zone that extends to the lake bottom at a maximum depth of over 700 m. Some fish are found throughout the oxygenated zone of the offshore waters, and lead a pelagic existence, although many of the species form part of the demersal community in shallower waters where the lake-bed is oxygenated. This paper reports on the distribution of cyprinids and catfish at six offshore locations, sampled with a mid-water trawl and gillnets, along the north-south axis of the lake. These two groups formed, respectively, 5% and 13% of the offshore fish biomass in the trawl catches. The offshore cyprinids comprised the small lake-spawning Engraulicypris sardella, the only species to have pelagic larvae, and the larger river-spawning Opsaridium microcephalum and O. microlepis. These cyprinids occurred mainly in the upper 100 m of the water column. The biomass of both larvae and adults of E. sardella showed considerable fluctuations over the two years of study. The spawning site of E. sardella remains a mystery, but is no longer believed to be in the offshore pelagic zone. The most abundant catfish was the mochokid Synodontis njassae, that exhibited diurnal movements from just above the oxic-anoxic boundary layer during the day to the surface layers at night. The clariid catfish of the genus Bathyclarias were caught infrequently offshore, but owing to their large size, they could form a significant part of the biomass. Their abundance was difficult to quantify, as it was believed that the majority are active in the near-surface layers and were under-sampled by the trawl and gillnets. Information on some aspects of the breeding biology is included in the paper, although our knowledge in this area is very incomplete.