Comparative ecology of Serranochromis species (Teleostei: Cichlidae) in the Upper Zambezi River floodplain

Abstract

The Upper Zambezi River drainage of Zambia contains six Serrunochromis species belonging to the largemouth, piscivorous subgenus Serranochromis and three smallmouth, invertebrate‐feeding species of the subgenus Sargochromis. A survey of the Barotse floodplain and associateddrainages revealed that two species, S. (S.) longimanus (Boulenger) and S. (S.) thumbergi (Castelnau), were uncommon on the floodplain. S. thumbergi appeared to be more abundant in the Kafue, Chobe, and Kabompo rivers than the Upper Zambezi proper. Adults of the three largest species (i.e. S. (S) robustus (Günther), S. (S.) altus (Winemiller & Kelso‐Winemiller), and S. (Sar.) giurdi (Pellegrin)) inhabit the main channel of the Zambezi River during low water, whereas most adults of the other four species (i.e., S. (S.) macrocephalus (Boulenger), S. (S.)ungusticeps (Boulenger), S. (Sar.) codringtoni (Boulenger), and S. (Sar.) carlottae (Boulenger)) were collected from lagoons, canals and backwaters. Comparisons of population size structure and gonad condition during the falling water (May‐August) and low water (September‐ December) seasons indicated that the seven common Serranochromis initiate spawning near the end of the low water period.Examination of gonad condition during the pre‐spawning period indicated that Serrunochromis attain maturity at the following sizes: S. robustus, males (275‐300 mm s.l.), females (250‐275 mm); S. macrocephalus, both sexes (150‐200 mm); S. altus, males (275‐325 mm), females (250‐300 mm);S. angusticeps, males (250‐275 mm), females (175‐225 mm); S. giardi, males (275‐300 mm), females (225‐250 mm); S. codringtoni, males (150 mm), females (125‐150 mm); S. carlottue, males (150‐175 mm), females (100‐125 mm). Age was estimated from scale annuli comprised of closely spaced circuli that corresponded to the period of rising water (December‐ March). S. robustus, S. altus, S. angusticeps, and S. giurdi matured at approximately 3 years, and S. macrocephalus, S. codringtoni, and S. curlottae matured at approximately 2 years in the Barotse floodplain. The average numbers of mature oocytes per female were as follows: S. robustus‐ 1165; S. macrocephalus‐810; S. altus‐920; S. angusticeps‐627; S. codringtoni‐580; S. carlottue‐362. Fecundity exhibited a weak positive relationship with female size. Analysis of stomach contents showed that interspecific diet overlap was low among both immature and mature size classes of Serrunochromis. Adult size classes of S. robustus fed primarily on small Synodontis catfishes, S. mucrocephalus fed mostly on mormyrid and cichlid fishes, S. altus specialized on mormyrids, and S. angusticeps fed primarily on small cyprinid and characid fishes. The three members of the subgenus Sargochromis consumed mostly invertebrates. Molluscs were the most important item in the diet of S. giardi, and aquatic insects and fish scales were predominant in the diet of S. carlottae. Larger size classes of S. codringtoni consumed large amounts of seeds (Graminaceae). If habitat and diet are considered together, niche separation among Upper Zambezi Serrunochromis species appears to be nearly complete.