Formaldehyde in locally landed and imported frozen fish from a Ghanaian fishing harbor: occurrence, compliance with regulatory limit, and dietary risk

Abstract Formaldehyde, an IARC Group 1 human carcinogen, forms naturally in fish muscle through post-mortem demethylation of trimethylamine N -oxide (TMAO) but is also applied illegally as formalin to extend the apparent freshness of fish. No quantitative data were available for fish traded on the Sekondi–Takoradi coast of Ghana. Three individual fish each of locally landed tuna and herring and of imported frozen salmon (market names; n = 9) were purchased from separate vendors at the principal fishing harbour, extracted with 6% (w/v) trichloroacetic acid and analysed by 3-methyl-2-benzothiazolinone hydrazone (MBTH) spectrophotometry at 628 nm. Calibration was linear over 0.5–2.5 mg L − 1 ( R 2 = 0.9992), with limits of detection and quantification of 0.17 and 0.50 mg kg − 1 (tissue basis). Formaldehyde was quantified in every sample, at 0.77–11.01 mg kg − 1 wet weight (overall mean 4.21 ± 2.93 mg kg − 1 ). Species means were 2.12 ± 1.18 (tuna), 7.12 ± 3.52 (salmon) and 3.39 ± 0.66 mg kg − 1 (herring). Two salmon samples (6.18 and 11.01 mg kg − 1 ) exceeded the 5 mg kg − 1 reference limit, whereas all tuna and herring complied under the conventional extraction-volume calculation. Because both salmon extracts exceeded the absorbance of the highest calibration standard, these exceedances are robust even though their magnitudes were extrapolated. Interspecific differences were not significant (one-way ANOVA, F 2,6 = 4.29, p = 0.070) despite a large effect size (η 2 = 0.59), reflecting low statistical power. The magnitude of the salmon values, the low-to-moderate TMAO status of all three species and their identical post-purchase storage history are more consistent with external adulteration than with natural formation, most plausibly within the imported frozen supply chain, although confirmation by a formaldehyde-selective method is required. Estimated daily intakes (0.76–10.86 × 10 − 3 mg kg − 1 bw d − 1 ) yielded hazard quotients of 0.004–0.072, indicating negligible non-carcinogenic risk at average Ghanaian consumption (74 g d − 1 ). The findings support targeted surveillance of imported frozen salmon and the establishment of a national maximum limit for formaldehyde in fish.