Distribution and breeding biology of offshore cichlids in Lake Malawi/Niassa

Lake Malawi/Niassa is the second largest rift valley lake in Africa, with an area of 28 800 km2, and an average and maximum depth of 292 m and >700 m, respectively. The lake is well known for the great diversity of fish occurring in the inshore zone. However, the offshore fish community is poorly documented. To rectify this, regular sampling was undertaken over two years, using trawl and gillnets at six offshore locations. This paper reports on the species composition, spatial distribution and breeding biology of the dominant cichlids species from the offshore pelagic zone. Cichlids formed approximately 88% of the offshore fish biomass. Most abundant were two species of zooplanktivores in the genus Diploturodon that made up 71% of the offshore fish biomass. An undescribed species, given the cheironym D. 'bigeye', was mainly found at a depth of 220 m during the day, but moved into near surface waters at night when the moon was full. This species was absent from the shallow regions of the lake. The most abundant offshore species was D. limnothrissa, which was distributed evenly throughout the lake to depths of 220 m. A less common offshore zooplanktivore was Copadichromis quadrimaculatus that formed 5%, of the biomass and was confined to the upper l00m of the water column. The main piscivores were in the genus Rhamphochromis and formed approximately 10% of the offshore fish biomass. The two dominant taxa were R. longiceps and the 'large Rhamphochromis' group, and both were more common in the southern half of the lake. The former occurred mainly in the upper 100 m of the water column and the latter mainly at depths of 100-150 m. The length at maturity and fecundity for the dominant offshore species were estimated and seasonal breeding cycles determined from gonad activity and gonado-somatic indices.