Novel cichlid-dominated fish assemblages in tropical urban reservoirs

Abstract

Recent biodiversity surveys in the lentic habitats of Singapore revealed that non-native cichlids (at least 14 species from different origins) were the most abundant species in the reservoirs of Singapore. To understand the diversity and distribution patterns in six reservoirs (two inland and four coastal), we investigated the species richness and abundances of these cichlids. We also investigated foodwebs to characterise the trophic relationships of cichlids relative to the aquatic community using stable isotope and gut content analyses in each reservoir. Based on various sampling methods (electrofishing, netting and trapping), a total of 5,675 individual cichlids representing 14 species and two hybrids were caught. The three most abundant species (contributing to 66.8% of all cichlids) included the green chromide, Etroplus suratensis, the eartheater cichlid, Geophagus altifrons, and the Orinoco peacock bass, Cichla orinocensis. Based on a canonical correspondence analysis, we found that the distribution and abundances of cichlids appeared to be related to abiotic parameters such as salinity and dissolved oxygen, and that species richness and abundances of cichlids differed between coastal and inland reservoirs. We also found that the trophic positions of different cichlid species varied within and across the reservoirs surveyed, where sympatry between highly specialised as well as generalist cichlid species was commonly observed. This was reflected in both the gut contents as well as the empirical foodwebs generated. While we are unable to determine source and timing of the introductions of these cichlids, we are at least confident that many of the cichlids have already established in these six reservoirs. We postulate that the establishment success and the co-existence of this non-native group has come about through their ability to adapt to the conditions within each reservoir, and this is related to the life history strategies, feeding and behavioural patterns that these different cichlid species display.