Environmental and biological factors influencing mercury (Hg) and selenium (Se) levels in European perch (Perca fluviatilia) : a comparison between the lakes Djupetjern, Holmetjern and Visterflo located in Glomma catchment area
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- Master's theses (INA) 
This study is based on an investigation of Lake Djupetjern, Lake Holmetjern and Lake Visterflo, which are all situated within the Glomma catchment area. The main objectives were to see if European perch (Perca fluviatilis) from the different lakes differ in Hg and Se concentrations and to quantify the contribution from environmental and biological factors to individual variations in THg and Se concentrations. In addition were the molar ratios (Se:Hg) and management implication for recreational fishing of my THg findings assessed. This was conducted by measuring levels of Hg (mg/kg wet weight), Se (mg/kg wet weight) and stable isotopes (C and N) in the muscle tissue of perch. Further were Hg, Se and stable isotopes (15N and 13C) linked to individual characteristics of the collected fish data (length, weight, age). The result showed that 78 % of the analysed perch from Degernestjerna, and 60 % of the analysed perch from Visterflo contained valued above 0.3 mg/kg w.w.. This is an upper level from where consumption advice is given by WHO for groups at risk. The THg concentration ranged from 0.21-2.20 and 0.07-1.50 mg/kg w.w. in Degernestjerna (Djupetjern + Holmetjern) and Visterflo respectively. The one-way ANOVA analysis showed that there were not an all over difference in the THg distribution between the examined lakes, but when adjusted for individual characters (e.g. age) were there clear differences between the lakes. The multiple regression analysis showed there were two supported models (ΔAIC=0.42) fitted to predict THg concentrations in Degernestjerna and Lake Visterflo: Location*Age*∂13C and Location*Age*∂15N.They predicted slightly different outcomes in the examined lakes (1) THg in perch from Degernestjerna increased with increasing age and trophic position combined with foraging in the pelagic area (2) while THg in perch from Visterflo increased with increasing age independent on the trophic level as well as increased THg levels when feeding in the pelagic area. The Se concentrations in the three lakes was lower than detection level (<1μgSe/L), but the one-way ANOVA showed that perch from Degernestjerna had in general higher Se concentrations compared to perch from Lake Visterflo. The most supported model (ΔAIC=0) fitted to predict Se concentrations in Degernestjerna and Lake Visterflo was: Location+weight+∂13C. The model predicted similar outcomes in the examined lakes: high Se concentrations are obtained when the perch is relatively large and forage in the littoral zone. Despite the high exposure to THg, only 5 % of the analyzed perch had molar ratios Se/THg < 1. The Se/THg ratios were strongly influenced by length, weight and age. These results can be used to inform the public fishing in Østfold, so they can avoid consuming perch from the lakes with highest THg values. The groups at risk: children, pregnant and breastfeeding women should in general avoid eating perch in Djupetjern and Holmetjern.