The aim of the report is to produce improved estimates of tissue conversion factors (k) to use withinenvironmental monitoring. The contaminant distribution across a range of marine and freshwater fishspecies from Northern Europe freshwater and the Baltic Sea was investigated. New tissue conversionfactors were established and converted threshold limits (C-EQS or C-QS) for tissues of relevance formonitoring (liver and muscle) presented. We further explored inter-species variability in contaminantdistribution, 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 tissueconcentrations. We recommend that this method is always used when the aim is to convert betweentissue concentrations for monitoring purposes. This will allow for better transparency and allow for easycomparison between studies, something that is made difficult at the moment due to the use of differentalternative methods for conversion.Based on the analysis we recommend four new threshold value estimates that can be used withinenvironmental monitoring of fish. For mercury (Hg) we only looked at the kmuscle/whole fish and did notconsider liver measurements. We recommend the use of a C-EQSmuscle of 24 ng g-1 ww across all fishspecies. For cadmium (Cd) and lead (Pb) we calculated C-QSliver estimates based on conversion fromboth the QSmuscle-human health and QSwhole fish-sec pois. For Cd, we found significant differences between the twospecies investigated (herring and perch) as well as ten times higher C-QSliver based on the QSmuscle-humanhealth than the QSwhole fish-sec pois. From a precautionary principle we recommend to use the herring specific kand the QS for secondary poising set for whole fish concentration to set the C-QSliver, which lead to a CQSliverfor Cd of 2.6 μg g-1 ww. For lead (Pb), the liver:muscle dataset was considered less robust thanthe liver:whole fish dataset. Also, for Pb significant differences were found between k’s in herring andperch. We recommend using a C-QSliver of 0.3 μg g-1 ww for Pb based on perch data. For PFOS we onlylooked at the kliver/muscle and did not consider whole fish measurements. We recommend the use of a CEQSliverof 153 ng g-1 ww across all fish species despite some differences in k between the six speciesincluded in the estimate. For all contaminants, more data on additional fish species and data on thesame species but better distributed between marine and freshwater environments (for species living inboth 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 tobring down the uncertainty on the QSliver estimates, which are currently, based on extrapolation of valuesmore than a factor ten below the thresholds.