Occurrence, distribution, and tissue concentrations of organochlorine pesticides in marine sediments, bivalves, and two reef fish (Siganus sutor and Lethrinus nebulosus) from coastal waters of Unguja Island, Tanzania: baseline levels, sediment-quality screening and dietary risk

Organochlorine pesticides (OCPs) persist in tropical marine ecosystems, yet baseline data for the eastern and northern coasts of Unguja Island, Tanzania, are absent. This study quantified ten target OCPs (α-HCH, aldrin, dieldrin, endosulfan, p,p′-DDE, p,p′-DDT, isodrin, endrin, endrin ketone and heptachlor epoxide) in surface sediments, bivalves, rabbit fish (Siganus sutor) and spangled emperor (Lethrinus nebulosus) at Chwaka (east coast) and Nungwi (northern tip) by gas chromatography–mass spectrometry (three station composites per matrix per site; n = 3). Nine compounds were detected; endosulfan was not detected in any sample, and α-HCH, endrin and heptachlor epoxide were below detection in all sediments. Mean ΣOCP in sediment was 59.4 ± 23.4 ng g−1 dry weight (dw) at Chwaka and 57.5 ± 23.4 at Nungwi, dominated by endrin ketone, p,p′-DDT and p,p′-DDE. Inter-site differences were negligible to small (Hedges’ g = 0.06–0.48); at n = 3 the design resolves only differences of 72–310 ng g−1 at 80% power, so non-significant results are inconclusive rather than evidence of spatial homogeneity. Mean ΣOCP was higher in biota than in co-located sediment (bivalves 97.9; rabbit fish 121.6; spangled emperor 182.6 ng g−1 dw; matrix-to-sediment concentration quotients 1.7–3.1). Because sediment organic carbon, tissue lipid content and trophic position were not measured, these quotients are descriptive only and are explicitly not interpreted as bioaccumulation, biota–sediment accumulation factors, or trophic transfer. Sediment p,p′-DDT (9.7–29.1) and dieldrin (10.1–22.1 ng g−1 dw) exceeded CCME marine interim sediment quality guidelines and probable-effect levels, indicating a screening-level probability of adverse effects on benthic biota. Sediment DDE/(DDE+DDT) ratios of 0.29–0.40 indicate a DDT-dominated signal consistent with comparatively recent input, whereas the endrin-ketone-dominated cyclodiene profile indicates legacy input. No matrix approached US FDA action levels for fish. Deterministic dietary estimates nevertheless gave hazard indices above 1 for high and child consumers at several matrix–site combinations, and incremental lifetime cancer risks of 1.1 × 10−5 to 6.2 × 10−4 exceeding the 10−4 benchmark for high consumers driven by dieldrin and aldrin. Wet-weight confirmation, lipid and organic-carbon data, and local consumption rates are the priority next steps.