Towards Non-Invasive Methods in Measuring Fish Welfare: The Measurement of Cortisol Concentrations in Fish Skin Mucus as a Biomarker of Habitat Quality

Abstract

Simple SummaryThe analysis of circulating cortisol has been by far the most common method used as a means to assess fish stress responses and, thus, animal welfare. To avoid many of the drawbacks inherent to blood sampling, cortisol can be less-invasively detected in fish skin mucus. The measurement of cortisol in skin mucus however, has, to date, only been demonstrated as suitable for farm fish, although its application to free-ranging animals would offer many advantages. The present study was therefore designed to evaluate the applicability of skin mucus cortisol analysis as a potential tool to assess habitat quality. To that end, wild fish residing in environments of different habitat quality were sampled for blood and skin mucus. First, several physiological endpoints typically used as indicators of exposure to pollutants were accurately related to the habitat quality in the Catalan chub (Squalius laietanus). Second, cortisol levels in blood were also compared between habitats, and they were successfully correlated to skin mucus cortisol concentrations. Finally, we contrasted the patterns of response of all the endpoints assessed to skin mucus cortisol levels across the sites. The strong linkages detected in this study provide new evidence that the measurement of cortisol in skin mucus could be potentially used as a biomarker of habitat quality in freshwater fish. AbstractCortisol levels in fish skin mucus have shown to be good stress indicators in farm fish exposed to different stressors. Its applicability in free-ranging animals subject to long-term environmental stressors though remains to be explored. The present study was therefore designed to examine whether skin mucus cortisol levels from a wild freshwater fish (Catalan chub, Squalius laietanus) are affected by the habitat quality. Several well-established hematological parameters and cortisol concentrations were measured in blood and compared to variations in skin mucus cortisol values across three habitats with different pollution gradient. Fluctuations of cortisol in skin mucus varied across the streams of differing habitat quality, following a similar pattern of response to that detected by the assessment of cortisol levels in blood and the hematological parameters. Furthermore, there was a close relationship between cortisol concentrations in skin mucus and several of the erythrocytic alterations and the relative proportion of neutrophils to lymphocytes. Taken together, results of this study provide the first evidence that skin mucus cortisol levels could be influenced by habitat quality. Although results should be interpreted with caution, because a small sample size was collected in one studied habitat, the measurement of cortisol in skin mucus could be potentially used as a biomarker in freshwater fish.

Biochemical aspects of susceptibility to stressors in two small cyprinids Squalius laietanus and Barbus meridionalis from the NW Mediterranean

“…Comparisons of baseline enzymatic activities were made in fish sampled in a confirmed reference area, all considering the undeniable anthropogenic fingerprint of NW Mediterranean watercourses (Blanco et al 2019;Carbajal et al 2019;Colin et al 2016b;Maceda-Veiga et al 2013;Marqueño et al 2019;Soler et al 2020). Markers of neurotoxicity signs (AChE), protection (BuChE), energy metabolism (LDH and CS),…”

“…All procedures were conducted in accordance with the European Directive for animal experimentation (2010/63/EU). Waters, USA) as described in detail in Carbajal et al (2019). Concentrations are given in ng/g wet weight (ww).…”

Stress management in aquaculture: a review of dietary interventions

“…A reliable, sensitive and non-biased measurement of stress in fish is crucial to get its early warning for adopting better operational and management strategies to reduce or combat stress. Different stress assessment methods have been exhaustively reviewed by several authors (Sopinka et al 2016;Carbajal et al 2019;Raposo de Magalhaes et al 2020). In the present review, gists of various stress assessment methods and their limitations are briefly described.…”

Emerging indicators of fish welfare in aquaculture

“…Samples were then analysed for viscosity and metabolites (glucose, lactate, protein, cortisol) using an ELISA kit. The 3-min air exposure increased glucose and cortisol metabolite levels by 1-hr post-treatment, with elevations lasting for at least a further 5 h. 41,153,154 Absolute metabolite levels were also compared against protein levels, to control for variation in sample quality. The glucose:protein ratio increased after air exposure and bacterial challenge and decreased after fasting.…”