Variation in scale cortisol concentrations of a wild freshwater fish: habitat quality or seasonal influences?

Abstract

A signifcant body of literature suggests that aquatic pollutants can interfere with the physiological function of the fsh hypothalamic-pituitary-interrenal (HPI) axis, and eventually impair the ability to cope with subsequent stressors. For this reason, development of accurate techniques to assess fsh stress responses have become of growing interest. Fish scales have been recently recognized as a biomaterial that accumulates cortisol, hence it can be potentially used to assess chronic stress in laboratory conditions. We, therefore, aimed to evaluate the applicability of this novel method for cortisol assessment in fsh within their natural environment. Catalan chub (Squalius laietanus) were sampled from two sites; a highly polluted and a less polluted (reference) site, in order to examine if habitat quality could potentially infuence the cortisol deposition in scales. We also evaluated the seasonal variation in scale cortisol levels by sampling fsh at three diferent time points during spring-summer 2014. In each sampling, blood was collected to complement the information provided by the scales. Our results demonstrated that blood and scale cortisol levels from individuals inhabiting the reference site were signifcantly correlated, therefore increasing the applicability of the method as a sensitive-individual measure of fsh HPI axis activity, at least in non-polluted habitats. Since diferent environmental conditions could potentially alter the usefulness of the technique, results highlight that further validation is required to better interpret hormone fuctuations in fsh scales. Scale cortisol concentrations were unafected by habitat quality although fsh from the polluted environment presented lower circulating cortisol levels. We detected a seasonal increase in scale cor- tisol values concurring with an energetically costly period for the species, supporting the idea that the analysis of cortisol in scales reveals changes in the HPI axis activity. Taken together, the present study suggests that cortisol levels in scales are more likely to be infuenced by mid-term, intense energetically demanding periods rather than by long-term stressors. Measurement of cortisol in fsh scales can open the possibility to study novel spatio- temporal contexts of interest, yet further research is required to better understand its biological relevance.