Composition of nutrients, heavy metals, polycyclic aromatic hydrocarbons and microbiological quality in processed small indigenous fish species from Ghana: Implications for food security

Overview

This Norwegian-Ghanaian collaboration analyzed 24 composite samples of six processed small fish species collected from five regional markets in Ghana in November 2018. Analyses covered total protein, fat, fatty acids, vitamins, minerals, trace elements, heavy metals (Cd, total arsenic (tAs), total mercury (tHg), Pb), and polycyclic aromatic hydrocarbons (PAHs). Of the six species, five were smoked or dried marine/freshwater species; one (tilapia) was salted. Samples were analyzed as whole fish (except bigeye grunt, which was beheaded) on a wet-product-weight basis. Because most species were smoked or sun-dried, moisture content ranged from 7.9% (round sardinella) to 10.4% (African moonfish) for the smoked marine species and 28.5% for salted tilapia; these values are substantially lower than fresh fish (approximately 75-80% moisture). All reported metal concentrations are on this dried-product weight basis and are higher per kilogram than fresh-fish equivalents; after the authors normalized to 75% moisture, all species fell below EU regulatory limits for lead (Pb) (0.3 mg/kg) and cadmium (Cd) (0.05 mg/kg for most species). Methods were accredited under NS-EN ISO/IEC 17025. ICP-MS (iCapQ ICP-MS, ThermoFisher) with internal standards (Sc, Rh, Te, Ge, In or Tm) was used after microwave wet digestion for metals, minerals and trace elements. Specific LOQ and uncertainty values are given in Supplementary Reference 48 of the paper (not reprinted here). All analyses were performed in singular parallels at the IMR laboratories.

Key numbers (Table 2, mean ± SD, wet product weight)

All values in mg/kg wet weight of the processed product (see basis note above).

Converting to µg/kg (ppb) where values are present:

• Cd: tilapia <LOQ; WA pygmy herring 15 µg/kg; moonfish 65 µg/kg; grunt 116 µg/kg; sardinella 188 µg/kg; anchovy 306 µg/kg
• Pb: tilapia <LOQ; anchovy/grunt ~100 µg/kg; moonfish/sardinella ~240 µg/kg; WA pygmy herring 640 µg/kg (but high variance: SD 610 µg/kg, driven by one Accra sample at 1300 µg/kg)
• tHg: tilapia <LOQ; anchovy/sardinella 34 µg/kg; moonfish 45 µg/kg; grunt 65 µg/kg; WA pygmy herring 223 µg/kg
• tAs: tilapia 100 µg/kg; WA pygmy herring 910 µg/kg; moonfish 5700 µg/kg; grunt 4900 µg/kg; anchovy 7800 µg/kg; sardinella 9830 µg/kg

Basis note. Values are on wet weight of the processed product, which is predominantly dried/smoked whole fish (7-11% moisture) or salted tilapia (29% moisture). The same metals measured in fresh fish at 75% moisture would be approximately 3.7x lower for the smoked species and 2.9x lower for salted tilapia. After normalizing to 75% moisture, the paper states all Cd and Pb values fall below their respective EU maximum levels. Total arsenic values are very high in smoked marine species but the paper notes no EU maximum exists for arsenic in fish and that in marine fish the organic arsenobetaine form typically dominates.

Specific findings noted in the text

• WA pygmy herring from Accra showed a lead value of 1.3 mg/kg in one specific composite sample, associated with proximity to the Agbogbloshie e-waste recycling site. Lead in all other WA pygmy herring samples was not notably higher. The mean of 0.64 mg/kg with SD 0.61 reflects this outlier.
• Mercury in WA pygmy herring was the highest of all species (mean 0.223 mg/kg = 223 µg/kg), contrasting with its small size and short lifespan. The paper attributes this tentatively to contamination of freshwater catchments from illegal gold mining, but notes that freshwater species can carry higher mercury (Hg) than expected from their trophic level in contaminated watersheds.
• Salted tilapia showed the lowest or below-LOQ concentrations for all metals measured, consistent with its freshwater origin in aquaculture and lower Pb, Cd, Hg body burden.
• For methylmercury risk: if a 70 kg person consumed 500 g of WA pygmy herring (the highest Hg species), they would reach the JECFA provisional tolerable weekly intake (PTWI) of 1.6 µg/kg body weight per week, assuming all Hg is methylmercury. This is a conservative assumption given the small body size of the species.

Methods (brief)

24 composite samples (each from one species at one location), 100-500 g per batch, freeze-dried (AOAC 930.15), homogenized, then analyzed. ICP-MS with wet microwave digestion for Cd, Pb, tHg, tAs and minerals. Accredited NS-EN ISO/IEC 17025 methods. LOQ and uncertainty in Supplementary Reference 48. No specific certified reference material reported in the main text.

Evidence fitness

This is a cross-sectional market survey using composite samples (n=1 per species per location), which limits statistical inference about individual sample variability. Composite samples by design reduce apparent variability; the notable outlier in WA pygmy herring lead (Accra site) was retained in the mean. The study covers only a single sampling period (November 2018). The tilapia data (the salted species on this ingredient page) are limited to below-LOQ values for most metals, providing only upper bounds. The absence of a reported CRM in the main text is a minor quality limitation, offset by the ISO 17025 accreditation. The very high total arsenic in smoked marine species is on a dried product weight basis and represents predominantly organic arsenobetaine; the paper explicitly notes no applicable EU maximum and no PTWI for total arsenic.

Implications

• Salted-fish page: salted tilapia from Ghana shows very low contamination (Pb, Cd, tHg below LOQ) compared to Egyptian retail salted fatty fish; species and origin both matter.
• Smoked-fish context: WA pygmy herring and smoked marine species from certain Ghanaian markets show elevated lead and mercury (Hg) in one Accra sample, warranting monitoring. These are smoked species, not specifically salted.
• App: salted tilapia is a lower-contamination form; smoked small marine fish from Ghana carry elevated arsenic and potentially elevated lead/Hg near e-waste sites.

Related evidence

• Salted fish
• Fish
• Lead
• Cadmium
• Arsenic
• Mercury

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