Fish migrations in two seasonal streams in Zimbabwe
Fish migrations into the streams of seasonally-flooded depressions (or dambos) that cover the central plateaus of Africa are a well-known phenomenon to scientists and lay people alike. However, the factors causing seasonal migrations are poorly understood and most scientific references to them are anecdotal, lacking rigorous description. Fish migrations in two dambos in Zimbabwe were investigated during the 1999/2000 rainy season, by electrofishing and using traps. The fish fell into two categories: those that used the dambo, and those that remained in the main river. At the beginning of the rainy season Barbus paludinosus, B. lineomaculatus, Clarias gariepinus and Tilapia sparrmanii moved from the perennial river into the small dambo streams and spawned there, then moved back to the perennial river, leaving their young in the dambo, which acted as a nursery area. Barbus spp. were the most numerous migrants and they moved mainly during daylight hours. Other fish species present in the system included Opsaridium zambezense, Leptoglanis rotundiceps, Chiloglanis neumanni, Labeobarbus marequensis, Labeo cylindricus, Oreochromis mossambicus, Tilapia rendalli, Marcusenius macrolepidotus and Micropterus salmoides. These did not enter the dambo streams but remained in the perennial river. Later in the season, larger juveniles of all the migratory species that had entered the dambo moved downstream into the perennial river, but the smaller juveniles remained in the dambo until the next rainy season, demonstrating that downstream movement is ontogenetically cued.