Heavy metal contamination and otolith sr:ca signatures among anguillids from the Athi-Galana-Sabaki and Ramisi rivers, Kenya

Abstract

The utilisation of different habitats by catadromous anguillids increasingly exposes them to threats such as pollution in freshwater and marine environments. The yellow eel is especially vulnerable to pollutant bioaccumulation as a result of its benthic predatory behaviour and high- fat reserves. However, further investigation is needed to understand whether contamination levels depend on the species of anguillids and stage-specific habitat use. This study examined heavy metal contamination and habitat use using otolith Sr:Ca signatures among anguillids from Rivers Athi-Galana-Sabaki and Ramisi, Kenya, with varying contaminant sources and anthropogenic disturbance levels. Heavy metal (As, Cd, and Pb) and trace metal (Cr, Cu, Zn, Fe, Co, Ni, and Mn) concentrations in eel tissues, sediment, and water, along with otolith strontiumzcalcium (Sr:Ca) ratios, were measured using an Inductively Coupled Plasma Optical Emission Spectroscopy. Results showed that arsenic (4.45:l:2.88 mg/kg) and cadmium (0.88i0.6l mg/kg) concentrations were significantly higher in silver eel tissues (p§0.05) than in yellow eels, elvers, and glass eels. This suggests accumulation of heavy metals during the feeding stage (yellow eel), which involves long residency in rivers that are often polluted. Additionally, arsenic (As), lead (Pb), and cadmium (Cd) concentrations in all eel life stages exceeded the FAO/WHO maximum permissible limits in freshwater fishes. Significant differences were observed in muscle and liver heavy and trace metal concentrations among the four anguillid species (p§0.05). Significant differences were also observed in the mean concentrations of heavy and trace metals in water and sediment between the Athi-Galana- Sabaki and Ramisi rivers (pS0.05). Results revealed the presence of As (0.l84i0.006 mg/L) and Cd (0.035:l:0.002 mg/L) in water, as well as Pb (l4.032:l:5.662 mg/kg), As (4.342:l:O.241 mg/kg), and Cd (0.687i0.039 mg/kg) in sediment from River Athi-Galana-Sabaki. As (0.l92i0.009 mg/L), Cd (0.033i0.002 mg/L), and Pb (0.003i0.003 mg/L) were present in water from River Ramisi. The mean As, Pb, and Cd concentrations in sediment from River Ramisi were 4.6410173, 3.0412095, and O.809i0.045 mg/kg, respectively. Notably, As, Cd, Ni, and Mn levels in water from both rivers were above the WHO-recommended limits for drinking water. Comparison among species indicated that otolith Sr:Ca signatures ranged from 0.023 in A. marmorata to 0.l82i0.044 in A. bicolor, with no significant difference (p20.05). There was no significant association between heavy metal loading in muscle tissues and otolith Sr:Ca signatures among silver and yellow eels (pZ0.05). Results indicate the presence of As, Cd, and Pb in eels above FAO/WHO pennissible limits in fish, which raise concerns for public health and aquatic ecosystems.