Assessment of general condition of fish inhabiting a moderately contaminated aquatic environment

The assessment of general condition of fish in the moderately contaminated aquatic environment was performed on the European chub (Squalius cephalus) caught in September 2009 in the Sutla River in Croatia. Although increases of the contaminants in this river (trace and macro elements, bacteria), as well as physico-chemical changes (decreased oxygen saturation, increased conductivity), were still within the environmentally acceptable limits, their concurrent presence in the river water possibly could have induced stress in aquatic organisms. Several biometric parameters, metallothionein (MT) and total cytosolic protein concentrations in chub liver and gills were determined as indicators of chub condition. Microbiological and parasitological analyses were performed with the aim to evaluate chub predisposition for bacterial bioconcentration and parasitic infections. At upstream river sections with decreased oxygen saturation (~50%), decreased Fulton condition indices were observed (FCI: 0.94 g cm-3), whereas gonadosomatic (GSI: 2.4%), hepatosomatic (HSI: 1.31%) and gill indices (1.3%) were increased compared to oxygen rich downstream river sections (dissolved oxygen ~90% ; FCI: 1.02 g cm-3 ; GSI: 0.6% ; HSI: ~1.08% ; gill index: 1.0%). Slight increase of MT concentrations in both organs at upstream (gills: 1.67 mg g-1 ; liver: 1.63 mg g-1) compared to downstream sites (gills: 1.56 mg g-1 ; liver: 1.23 mg g-1), could not be explained by induction caused by increased metal levels in the river water, but presumably by physiological changes caused by general stress due to low oxygen saturation. In addition, at the sampling site characterized by inorganic and faecal contamination, increased incidence of bacterial bioconcentration in internal organs (liver, spleen, kidney) was observed, as well as decrease of intestinal parasitic infections, which is a common finding for metal contaminated waters. Based on our results, it could be concluded that even moderate contamination of river water by multiple contaminants could result in unfavourable living conditions and cause detectable stress for aquatic organisms