Effects of Cortisol on the Nutrient Utilisation and Bioenergetics of Rainbow Trout

The welfare of farmed animals is a topic that attracts ever more attention. Welfare concerns for mammals and birds have been at public attention for a while, and the extraordinary growth of the aquaculture industry in recent decades has now increased awareness of welfare risks associated with fish production. While in the past, fish were believed to be insensitive emotionless animals, continuous research has shown that they are as affected by stress-related issues as terrestrial vertebrates. Aquaculture related issues regarding fish welfare include inter alia insufficient water quality, inappropriate stocking densities, inadequate nutrition, crowding, handling, and social stress. Some of these stressors have been shown to lead to a chronic elevation in circulating cortisol levels, the main stress biomarker of teleost fish. While the short-term effects of cortisol on fish have been well studied, less is known on the consequences of prolonged elevated cortisol levels.

The main objective of this Ph.D. project was to contribute to the understanding of the effects of prolonged cortisol elevation on the metabolism of fed and unfed rainbow trout. To this end, four studies were conducted, of which the first aimed to assess if different water temperatures had an effect on the method applied to artificially increase circulating cortisol levels. The method chosen to chronically increase plasma cortisol concentration throughout all studies was the coconut oil-based intraperitoneal hormone implantation. This method has routinely been used for this purpose in fish but an effect of water temperature on the release of cortisol from chronic coconut oil-based hormone implants has not been assessed yet. Rainbow trout were reared at 10, 15, or 20°C, and over the following three weeks, plasma cortisol levels were determined. Temperature significantly affected the circulating cortisol levels and the time-course of cortisol concentration. While at 10°C, circulating cortisol levels remained constant, at higher temperatures cortisol levels peaked and decreased more rapidly. Temperature and cortisol levels also affected growth rates and relative digestive tissue mass.

The second study intended to investigate and compare the effects of chronically elevated cortisol levels, a daily stress event, and chronic temperature elevation on growth rates and nutrient digestibility in rainbow trout. Two different cortisol dosages were applied and resulted in dosedependent declines in apparent digestibility coefficients of macronutrients. This ultimately affected the available metabolizable energy from the diets and growth rates of the fish. The differences in digestive capacity for lipids could not be explained by enzymatic activities as lipase activity in digestive organs was not affected. The intestine somatic indices of fish with high cortisol levels however showed significant reductions suggesting decreased absorption surface for nutrients to play a role in reduced digestive capacity. The observed differences in digestive capacity and hence metabolizable energy alone were not great enough to explain the severely reduced growth rates and metabolic cost of growth of cortisol-treated fish. Consequently, another study was conducted to assess if potential differences in standard metabolic rate (SMR) might be responsible for the higher cost of living.

Study 3 assessed the respiratory metabolism of rainbow trout after several weeks with prolonged elevated cortisol levels. The fish with high cortisol levels showed a significantly elevated SMR, while the lower cortisol dose did not affect SMR of the fish. This treatment however caused an increase in the maximum metabolic rate (MMR) of the fish, ultimately resulting in an elevated metabolic scope (MS). The elevations in SMR were associated with increased mass-specific oxygen consumption rates of some highly metabolically active organs whereas increased MMR of cortisol treated fish was accompanied by an increased hematocrit and cardiosomatic index suggesting alterations in the cardiovascular system. Haematological analysis also revealed increased reliance on anaerobic pathways in cortisol-treated fish.

Because of the observed atrophic effects of cortisol on digestive organs in the first and second studies, study 4 was designed to examine whether cortisol via genomic pathways is involved in the rapid downregulation of the gastrointestinal system commonly observed in starving salmonids. To test this, rainbow trout were treated either with RU486 (mifepristone) to antagonize the glucocorticoid receptor (GR), or with spironolactone to antagonize the mineralocorticoid receptor (MR), while a third group was treated with both receptor antagonists (RU + Spiro). These groups were compared against a control group that was merely implanted with the vehicle, and a cortisol-treated group. The results of this study showed that even though prolonged starvation caused significant elevations in plasma cortisol levels, the hormone was not responsible for the reduction in relative digestive tissue mass as antagonizing both corticosteroid receptors abolished starvation-induced cortisol elevation but with no effects on relative digestive tissue mass or overall weight loss during fasting for four weeks.