Rehabilitation of a tropical storm-water drain creates a novel fish assemblage

Abstract

A 2.7-km canalised section of the Kallang River, a major storm-water drain and reservoir spillway in Singapore, was rehabilitated into a 3-km naturalised, meandering river between 2009 and 2011. A combination of plants, natural materials, and civil engineering techniques were introduced to soften the edges of the waterway, to give it a more natural appearance and prevent soil erosion. Baseline data and published evidence of enhancement of aquatic biodiversity in this naturalised urban waterway are lacking, as there have not been any comprehensive biological surveys of the system to date. To determine the effect of rehabilitation, we quantitatively compared the fish assemblage and abiotic variables in the Kallang River after its rehabilitation (re-named Kallang River at Bishan-Ang Mo Kio Park or KRBAP; 2016–2018) against a downstream unrehabilitated section of the river (Kallang Canal; 2012). Secondly, we qualitatively compared fish assemblages and abiotic variables at the KRBAP and the Kallang Canal, to their source (upstream) reservoir, as well as to natural forest streams in close proximity. The KRBAP has a unique fish assemblage, which is dominated by two non-native cichlid taxa (quetzal cichlid, Vieja melanura (68%) and tilapia, Oreochromis spp. (17%)). Fish species richness (p < 0.001) and the percentage of native species (p = 0.015) was significantly higher in the KRBAP compared to the unrehabilitated canal. Moreover, the abiotic variables at the two sites are also significantly different. The fish assemblage and abiotic variables at the KRBAP resemble those of its (upstream) source reservoir, but contrasts with those of nearby natural forest streams. The unique fish assemblage in the KRBAP is shown to be stable, with similar species captured in high abundances across the three sampling years post-rehabilitation. Given the stability within the rehabilitated stream, further research and monitoring are needed to determine the established food web and predict the possible influence of future non-native species additions.