Costly defense in a fluctuating environment—sensitivity of annual Nothobranchius fishes to predator kairomones
The diet of seven cichlid species of Lake Chivero, Zimbabwe, was investigated from October 2004 to April 2005 to determine the degree of dietary overlap and interspecific competition. These fish could be separated into four groups. Two microphages, Oreochromis niloticus and O. macrochir, fed mostly on blue-green algae and their diets, in all size classes, overlapped almost completely. This suggested a high degree of competition between the two species and may account for the fact that O. niloticus, introduced into the lake in the 1980s is now the dominant species. There was also a significant dietary overlap between the two macrophages Tilapia sparrmanii and T. rendalli, which were macrophages with macrophytes as a significant component of their diet. Interspecific competition between these two species may account for the increase in the abundance of T. sparrmanii and a decrease in the numbers of T. rendalli over the last two decades. The diet of two small omnivores Pharyngochromis acuticeps and Pseudocrenilabrus philander consisted of algae, zooplankton, plant material, fish and detritus and also overlapped extensively, but these species probably avoid competition by living in different habitats; P. philander is usually associated with vegetation while P. acuticeps tends to live in more open waters. The last species, Serranochromis robustus, was largely piscivorous. There were no ontogenetic variations in diet, apart from P. acuticeps where small fish fed mainly on zooplankton becoming omnivorous in larger ones, and S. robustus where the diet shifted from zooplankton in specimens <50 mm SL to fish in larger size classes. Data available from the 1960s and 1970s indicate that the diets of some species have changed with invertebrates such as chironomid larvae being previously more important in all diets, while most species now take a higher proportion of algae. This reflects changes in the lake, which is eutrophic and has experienced severe algal blooms since 1960.