Southern mullets (Chelon richardsonii) as a potential biological indicator for mercury pollution in the Walvis Bay lagoon, Namibia

Abstract

Southern mullet Chelon richardsonii is one of the important fish species found in the Walvis Bay lagoon (WBL) that is commercially exploited for local consumption. The southern mullet is, however, exposed to mercury (Hg) found in sediments, which forms part of its diet. This study evaluated the Hg concentration in muscles, stomach content, liver, and gills of C. richardsonii over two years. The Hg concentration was higher in the liver (0.005 mg kg⁻¹) and relatively lower in the gills (0.001 mg kg⁻¹). There were temporal variations in Hg concentration among the tissues; a positive correlation between mean Hg concentration and fish sizes (total length and total weight) was documented in the gills; and a negative correlation for size versus Hg concentration was recorded in the livers, stomach contents, and the muscles of southern mullets. The Hg concentrations of all tissues were below the World Health Organisation (WHO) limit for fish (0.5 mg kg⁻¹), which indicates that it does not pose a human health risk at the current levels. This study’s findings form a baseline for using southern mullets as biological indicators of Hg pollution in WBL. The study also highlights the need for regular monitoring programmes to protect and maintain the ecological characteristics of WBL as per Namibia’s mandates under the Ramsar Convention.