The aim of the report is to produce improved estimates of tissue conversion factors (k) to use within environmental monitoring. The contaminant distribution across a range of marine and freshwater fish species from Northern Europe freshwater and the Baltic Sea was investigated. New tissue conversion factors were established and converted threshold limits (C-EQS or C-QS) for tissues of relevance for monitoring (liver and muscle) presented. We further explored inter-species variability in contaminant distribution, which can result in the need to create species-specific conversion factors. 

                      In this study, we use conversion factors that assumes a proportional relation between two tissue concentrations. We recommend that this method is always used when the aim is to convert between tissue concentrations for monitoring purposes. This will allow for better transparency and allow for easy comparison between studies, something that is made difficult at the moment due to the use of different alternative methods for conversion.

                      Based on the analysis we recommend four new threshold value estimates that can be used within environmental monitoring of fish. For mercury (Hg) we only looked at the kmuscle/whole fish and did not consider liver measurements. We recommend the use of a C-EQSmuscle of 24 ng g^-1 ww across all fish species. For cadmium (Cd) and lead (Pb) we calculated C-QSliver estimates based on conversion from both the QSmuscle-human health and QSwhole fish-sec pois. For Cd, we found significant differences between the two species investigated (herring and perch) as well as ten times higher C-QSliver based on the QSmuscle-human health than the QSwhole fish-sec pois. From a precautionary principle we recommend to use the herring specific k and the QS for secondary poising set for whole fish concentration to set the C-QSliver, which lead to a C-QSliver for Cd of 2.6 μg g^-1 ww. For lead (Pb), the liver:muscle dataset was considered less robust than the liver:whole fish dataset. Also, for Pb significant differences were found between k’s in herring and perch. We recommend using a C-QSliver of 0.3 μg g^-1 ww for Pb based on perch data. For PFOS we only looked at the kliver/muscle and did not consider whole fish measurements. We recommend the use of a C-EQSliver of 153 ng g^-1 ww across all fish species despite some differences in k between the six species included in the estimate. For all contaminants, more data on additional fish species and data on the same species but better distributed between marine and freshwater environments (for species living in both like perch) is needed to elucidate the need for species specific and environment specific thresholds. In addition, more observations with concentrations close to the thresholds are needed for Cd and Pb to bring down the uncertainty on the QSliver estimates, which are currently, based on extrapolation of values more than a factor ten below the thresholds.