Recent efforts have provided convincing evidence for the use of fish scale cortisol concentration in the assessment of long-term stress in fishes. However, cortisol alone is not sufficient to fully describe this state of long-term stress. Dehydroepiandrosterone (DHEA) is an androgen with actions that oppose those of cortisol. The means by which DHEA negates the effects of cortisol occurs in part via changes in the metabolism of cortisol to cortisone. The quantitation of cortisol, DHEA and cortisone could therefore provide a more comprehensive assessment of the overall status of physiological stress. As DHEA and cortisone have yet to be quantified within the fish scale, our first objective was to ensure our sample processing protocol for cortisol was applicable to cortisone and DHEA. Following this, we induced a state of long-term stress in goldfish (Carassius auratus). Some degree of elevation in all hormones was observed in the stressed fish scales. Additionally, cortisol and cortisone were significantly elevated in the stressed fish serum in comparison to controls while DHEA was undetectable in either group. Overall, these results suggest that fish scales provide an appropriate medium for the assessment of long-term stress in fishes via the quantitation of relevant steroid hormones.
Investigation of the use of fish scales as a medium for non-lethal biomonitoring has recently commenced. Fish scales have been shown to incorporate cortisol over longer periods of time than blood and thus provide a promising means of assessing long-term stress in many species of teleost fish. However, while cortisol is a major mediator of the stress response in fishes, downstream effects of chronic stress on reproduction can involve gonadal steroids such as progesterone and testosterone. The quantification of these additional hormones alongside cortisol could therefore allow for the assessment of both stress and consequential reproductive alterations. To investigate these concepts, we artificially elevated circulating cortisol, progesterone, and testosterone in rainbow trout (Oncorhynchus mykiss) using coconut oil implants for three weeks. Following this we quantified these three hormones as well as 11-ketotestosterone, a potent androgen in teleost fishes. In all cases serum samples reflected a significant increase in the injected hormone confirming the efficacy of this method; however, this did not result in significantly elevated scale concentrations of the same hormone in all cases. As the stress and reproductive axes are closely integrated, these findings are likely a result of interactions along the steroidogenic pathway indicating that a further investigation of the relationship between scale concentrations of these hormones and actual physiological processes is required. Nevertheless, the successful quantification of both stress and gonadal steroid hormones within the scale suggests that such measurements could provide a novel and informative tool in the assessment of long-term stress and the resulting effects on reproductive endocrinology in teleost fishes.
Cortisol, the primary glucocorticoid in fishes, is secreted into the bloodstream in response to stress. Circulating cortisol accumulates in scales, a durable calcified structure that can be easily sampled from many fish species. As such, the use of scale cortisol concentration (SCC) is currently being explored as a means of chronic stress biomonitoring in wild fishes. Scales serve an important role in fish physiology and thus the number of scales required for reliable cortisol analysis is a limiting factor in the non-lethal collection of such samples. To date, scale cortisol quantification has also only been performed non-lethally in captive fishes and due to differences in stress responsiveness SCCs likely differ in wild species. As such, this study aimed to (1) apply our fish scale processing and analysis protocol to wild fish species and (2) apply it to five north temperate fish species to provide information useful to future non-lethal scale sampling regimes. Cortisol was successfully measured in scales collected from wild northern pike (Esox lucius), walleye (Sander vitreus), whitefish (Coregonus clupeaformis), white sucker (Catostomus commersonii) and captive rainbow trout (Oncorhynchus mykiss). SCCs were significantly different between species and thus the sample mass required for reliable cortisol analysis differed as well. In addition to the size of the fish and the mass of their scales this is an important consideration for future scale cortisol analyses as these factors could make SCC an attainable non-lethal sample matrix in some species of fish but impractical in others.
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