Larsen, M.M., 2019 Omregning af indhold af miljøfarlige stoffer i forskellige organer i fisk. Med særlig fokus på kviksølv. Aarhus Universitet, DCE – Nationalt Center for Miljø og Energi, 44 s. - Teknisk rapport fra DCE - Nationalt Center for Miljø og Energi nr. 144. http://dce2.au.dk/pub/TR144.pdf
The Water Framework Directive (WFD; EU, 2008) sets an Environmental Quality Standard (EQS) for mercury based on top-predator safety. In most long running monitoring programs (e.g. OSPAR CEMP; OSPAR, 2016), the preferred organ for analysis of metals in general are liver, and for mercury the preferred organ is muscle. This leads to a discrepancy between the monitored organs and the EQS values based on whole organism analysis, which lead to an investigation of conversion factors between organs and whole fish mercury level. As other EQS for biota are also available, the study also looked at supplemental conversion factors between organs and whole fish for other substances from literature, but mercury is the main topic for this study.
The ratio of mercury levels in liver and muscle relative to the whole fish was thus measured in five Danish fish species (Danish, latin name in parenthesis): Perch (aborre, Perca fluviatilis), Plaice (rødspætte, Pleuronectes pletessa), Flounder (skrubbe, Platichthys flesus), Eelpout (ålekvabbe, Zoarces viviparus) and Round goby (sortmundet kutling, Neogobius melanostomus). For each specie, at least five individual samples from one or two stations were dissected and the liver and a piece of muscle below the dorsal fin was taken for analysis. The rest of the fish was homogenized by an Ultraturex homogenizer and analysed. The concentration of mercury in each compartment was analysed, and finally used to calculate the ‘whole fish’ level, taking into account the weight of the individual organs. As the Danish NOVANA program is based on pooling of at least five (freshwater) or ten (marinewater) fish, in some cases a pooled sample of the livers and subsample of muscle was analysed together with the remainder of the fish, to estimate the ratio in pooled samples also. For most species, the results are thus based on 10 individuals and some pooled samples. Results are compared with similar investigations in other countries to verify the calculated conversion factor, and known confounding factors like age and fish size was investigated.
The mercury level was generally highest in the muscle, followed by liver and ‘whole fish’. For some species (Round goby and Eelpout), the liver concentration was lower than ‘whole fish’. In general, the liver concentrations showed the highest variation (as relative standard deviation) between individual samples from the same station.
The calculated ratio of mercury concentrations in muscle relative to the ‘whole fish’ for dry weight varied between 0.63 and 0.68 for flatfish (Flounder respectively Plaice) and 0.83 (Perch) and was on average 0.76 ± 0.29. The variation of the ratio of muscle relative to ‘whole fish’ within each specie (expressed as relative standard deviation, RSD) typically varied between 15 and 46% individual stations. For wet weight calculations, the ratio varied between 0.81 (Perch) and 1.2 (Eelpout), on average 0.93± 0.35, with an RSD between 17 and 46% for individual stations. In both wet and dry weight cases, one station of Round goby stood out with an RSD of ~59% for both wet and dry weight measurements in one of the two Round goby stations.
The calculated ratio of mercury concentration in liver relative to ‘whole fish’ for dry weight varied significantly more than for muscle. The ratio ranged from 0.5 (Plaice) to 4.4 (Round goby), with an average of 1.33 ± 1.14 for all species. The variation (RSD) of the ratio of liver relative to ‘whole fish’ varied between 17 and 46% for individual stations. For wet weight calculations, the ratio varied between 0,55 (Plaice) and 2,1 (Round goby) with an RSD between 14 and 47% for individual stations. In both cases, one station of Round goby stood out with an RSD of ~90% for both wet and dry weight measurements in one of the two Round goby stations, interestingly the other than for the muscle ratio RSD.
The length versus mercury concentration relationship was also tested, but only showed proportion of variance that explained more than half of the variance (r2>0,5) for Plaice and Round gobies. This could be due to the low number of results, small span of fish lengths and mix of different stations with different contamination levels, particularly for the Eelpouts.
Summary of results for mercury
In conclusion, due to the larger variation in liver to ‘whole fish’ ratios observed in the five Danish fish species, muscle to ‘whole fish’ ratios are suggested as more reliable. Although species-specific ratios were calculated, an average ratio of 0.73 for dry weight and 0,96 for wet weight mercury in muscle can be used as general conversion factor, at least for the five studied species of fish. For the wet weight muscle measurements, the effect of the conversion is thus -4% ±50%, so no conversion seems necessary within the uncertainty of the factor between species. The use of conversion factors should be done before applying any correction for trophic level according to EU Guidance Document No. 32 on biota Monitoring under the WFD (EU, 2014). For a more precise evaluation, the calculated species-specific ratios can be used, but the relative low variation in the ratio between species (<30%) is probably much lower than the expected uncertainty from the trophic level adjustments, which are usually measured in factors of 2-10, not percentage.
The factors determined agrees with results from literature values from Sweden (Danielsson et al 2018 and Faxneld et al 2015) and the Netherlands (Foekema et al 2016).
