Does fish health matter? The Utility of Biomarkers in Fish for Environmental Assessment

Abstract

Two common strategies to assess exposure to environmental toxicants are to measure chemical concentrations in the environment and to examine the presence of certain species that are known to be sensitive to pollution. The two strategies can sometimes give conflict-ing results. It is, e.g., possible that there are elevated levels of contaminants but no biologi-cal effect due to low bioavailability. Furthermore, chemical measurements only give in-formation about those chemicals that are included in the analysis and abundance of species can vary due to many factors that are not linked to pollution. An alternative, or comple-menting, strategy for environmental assessment is to examine sub-organism responses (biomarkers), which includes physiological and biochemical variables. Biomarkers indi-cate exposure to contaminants and health impairment in individuals. Because the initial response to pollution is assumed to occur at low levels of biological organization, bio-markers are expected to act as early warning signals. Furthermore, biomarkers may pro-vide a mechanistic link between exposure and effects. There is, however, little evidence that link biomarker responses to effects at higher levels. In addition, confounding factors, such as migration and age, can complicate the interpretation of results. In the present thesis, which is based on six scientific papers [I-VI], the utility of bio-markers in fish for assessing the environmental status was evaluated. Fish are suitable for assessing the environment as they can be found in most aquatic environments and play a major ecological role in aquatic food webs. In [I-IV], a methodology with farmed rainbow trout (Oncorhynchus mykiss), which were reared in net cages and/or plastic tanks, was used and evaluated. This was done as it was expected that caging would increase the precision of biomarker measurements by, e.g., preventing migration and thereby achieve a standard-ized exposure. In [V-VI], biomarker responses from a 20-year data set on feral perch (Perca fluviatilis) from national reference areas on the Swedish Baltic coast were exam-ined. During this period, an increasing trend in detoxification enzyme activity (ethoxyre-sorufin-O-deethylase, EROD) in the liver and a reduction in gonad size (gonadal somatic index, GSI) have been observed, and potential explanations for these trends were evaluated in [V-VI]. The potential confounding effect of different holding conditions (net cages or plastic tanks) and differences in feeding was examined in [I-II]. The results suggested that the methodology with caged fish was robust to differences in holding conditions but that dif-ferences in feeding can affect the responses for several variables. The methodology was used to assess the exposure to pollutants in two water systems [III-IV] with, presumed, high and low anthropogenic impact, respectively. The methodology worked well in the water with high anthropogenic impact, and it was shown that rainbow trout at certain sites were exposed to increased levels of pollutants. Furthermore, this information was linked to observations of fin and skeletal damage on feral brown trout (Salmo trutta). In the water system with low impact, however, interpretation of results was more complicated and little useful information could be retrieved. A possible explanation to this is that the relative impact of confounding factors became more important when the exposure to contaminants was low. In [V], biomarker responses in perch and water temperatures were analyzed at the na-tional reference station Kvädöfjärden as well as the flow rate in a nearby small river. The results showed that the flow rate in the river correlated with EROD activity in perch liver. It is, therefore, likely that contaminants are brought to the area by runoff from land. Fur-thermore, it was found that fish that lived during years with higher EROD activity also had lower GSI, which may affect the reproductive capacity of the perch. The responses in Kvädöfjärden were further investigated in [VI] by analyzing frozen bile from perch that were collected in two years with high and low EROD levels, respectively. It was found that increasing levels of polycyclic aromatic hydrocarbons (PAHs) is a likely contributor to increasing biomarker responses in Kvädöfjärden. Biomarkers in fish can be a useful tool to evaluate environmental pollution in many, although not all, situations. The use of farmed fish can improve the precision in the meas-urements, but some ecological relevance will be lost and certain confounding factors may be introduced. The choice between farmed or feral fish depends on the situation and what questions that needs to be answered. The results from biomarker analyses are most useful when data from other methodologies can be included in the interpretation. Future work should focus on how different methods can be integrated to get an effective and more reli-able assessment of environmental conditions.