Urbanization effects on growth and otolith asymmetry in Chrysichthys nigrodigitatus and Oreochromis niloticus within tropical coastal lagoon watersheds

Abstract

In this study, we investigated the growth dynamics and otolith shape asymmetry of two fish species, Chrysichthys nigrodigitatus (CN) and Oreochromis niloticus (ON), within urbanized watersheds of the southern lagoon system, Nigeria. Using the von Bertalanffy growth model (VBGM), in addition to sediment metal concentration indices such as the average shale content, index of geoaccumulation (Igeo), contamination factor (CF), pollution load index (PLI), and potential ecological risk (PER) index, contamination levels were classified, and ecological risks were assessed. Notably, a lower growth potential (t0) was observed in CN at Ikorodu than at Epe, with similar trends for ON in the Epe during the dry season. Otolith asymmetry patterns, particularly in the CN at Ikorodu and ON in the Epe during the dry season, exhibited distinct ecological variations, indicating heightened stress levels at Ikorodu. Sediment analyses revealed moderate to strong contamination (Cd, Pb, Ni, and Cr) in both Lagos Lagoon (Ikorodu) and Epe Lagoon, with Ikorodu exhibiting notably high to moderate contamination levels according to the CF index. Elevated PLI values for Cd and Pb in Ikorodu, in addition to greater PER, indicated increased risk, with Cd posing a high risk (61.42%) and Pb posing a moderate risk (49.50%). Additionally, the reduced asymptotic length in the Epe during the dry season suggests that Chrysichthys nigrodigitatus is adaptable to seasonal variations, while divergent growth patterns in both areas indicate the existence of trade-off mechanisms in response to changing conditions. Habitat-specific otolith asymmetry and metal contamination underscore species adaptability, with wider stressor variability in Lagos than in Epe. Furthermore, multidimensional scaling analysis highlights the intricate relationship between otolith shape variables and environmental factors, emphasizing the need for tailored conservation efforts in urbanized watersheds.