Heavy metal contamination in European conger (Conger conger, Linnaeus 1758) along the coastline of Morocco

Background:

In Morocco, fish is an important protein source especially, even not exclusively, for coastal communities and marketed fresh all along the coastline. One of the main targets of coastal artisanal fisheries is the European conger (Conger conger, Linnaeus 1758), a widely distributed benthic predatory species of a maximum weight of up to 50 kg. However, information on heavy metal contamination of conger is scarce. Therefore, concentrations of mercury, lead and cadmium were analysed in 108 European conger specimens from nine locations along the along the Atlantic and Mediterranean coasts of Morocco to describe the spatial distribution of heavy metal contamination.

Results:

The average heavy metal concentration in all conger samples under investigation was 246.90 ± 216.83 μg mercury/kg wet mass, 74.14 ± 87.02 μg lead/kg wet mass and 255.12 ± 287.15 μg cadmium/kg wet mass respectively. Mercury and cadmium showed a clear site‑specific bioaccumulation in European conger but lead does not. Hence, the effect of fish length bias on contamination was corrected through a generalized linear model (GLM) prior to any spatial comparison.

Conclusions:

Different regional hotspots for the three analysed metals are identified and local sources are discussed. Mercury levels in big specimens of European conger exceeded the European threshold level for human consumption at some of the locations under investigation.

Method:

Hg determination
Briefly, 2 g of muscle tissue samples were freeze dried using a lyophilizer (LD 1-2, Christ, Osterrode, Germany) and subsequently homogenized using an agate mortar or an ultra turrax tube drive dispenser (IKA, Staufen, Germany) to obtain a dry and homogenous sample powder. Total Hg was determined by atomic absorption spectrometry using a Direct Mercury Analyzer (DMA-80, MLS, Leutkirchen, Germany). 20–30 mg of each sample was weighted into the boat containers of the DMA-80. Direct analysis for total Hg content was performed using a 10-level calibration. The limit of detection (LD) and the limit of quantification (LQ) were calculated from a standard curve according to DIN 32645 [12] with a confidence level of 99%. Considering the sample preparation, an LD of 0.08 μg/kg wet mass (WM) and a LQ of 0.23 μg/kg WM were determined for Hg. No values below these limits were found in any sample under investigation. Precision of the method was 8.2% and recovery was 100.3%.

Hg determination

Briefly, 2 g of muscle tissue samples were freeze dried using a lyophilizer (LD 1-2, Christ, Osterrode, Germany) and subsequently homogenized using an agate mortar or an ultra turrax tube drive dispenser (IKA, Staufen, Germany) to obtain a dry and homogenous sample powder. Total Hg was determined by atomic absorption spectrometry using a Direct Mercury Analyzer (DMA-80, MLS, Leutkirchen, Germany). 20–30 mg of each sample was weighted into the boat containers of the DMA-80. Direct analysis for total Hg content was performed using a 10-level calibration. The limit of detection (LD) and the limit of quantification (LQ) were calculated from a standard curve according to DIN 32645 [12] with a confidence level of 99%. Considering the sample preparation, an LD of 0.08 μg/kg wet mass (WM) and a LQ of 0.23 μg/kg WM were determined for Hg. No values below these limits were found in any sample under investigation. Precision of the method was 8.2% and recovery was 100.3%.

Cd and Pb determination

Quantification of Cd and Pb was performed in liver samples, defatted according to Smedes which have been subsequently freeze-dried and homogenized as described above. Dry and fat-free sample powder (1–5 mg) was weighted and placed into the graphite furnace atomic absorption spectrometer (GFAAS) ContrAA 600 (Analytik Jena, Germany) with a continuous source. The GFAAS was calibrated using a 5-level calibration diluted from certified standard solutions in 0.1 M nitric acid. Measurements were performed after adding 5 μg of palladium matrix modifier (Pd(NO3)2 in nitric acid) to each sample. The LD and the limit of quantification (LQ) were calculated from a standard curve according to DIN 32645 with a confidence level of 99%. Considering the sample preparation, a LD of 0.25 (2.61) μg/kg WM and an LQ of 0.74 (7.82) μg/kg WM were determined for Cd (Pb). No values below these limits were found in any sample under investigation. Precision of the method was 15.4 (6.0%) and recovery was 84.1 (96.1) % for Cd (Pb).