Marine and freshwater fish have high basal stress hormone levels when raised under higher temperatures.
A lot of people get more nervous on hot summer days. It may be a comfort to know that it's not just humans who react like that. A study recently published in the Journal of Fish Biology has shown that cortisol, a hormone that regulates energy metabolism and is often related to stress, is higher in fish from warmer environments, even when they are not being purposely stressed by any other factor.
"The physiological stress response is a mechanism that the body has to deal with variations in the environment. It diverts energy from a long-term investment, such as preparing your body to react to future diseases or to reproduce in adulthood, and devotes that energy to solving impending problems of the present. The problem begins when the stressing situations are chained one after the other. This is what we call chronic stress, which has a number of adverse effects."
Bastien SADOUL, co-author of the study
To reach this conclusion, the group collected data on the basal cortisol level in 126 other studies already published, as well as the temperature at which the fish were raised. All of them were kept within the tolerance limits for each species. Even so, those who were closer to the upper limit of tolerance, that is, exposed to heat, had higher baseline stress hormone levels. This practice of analyzing data derived from other studies is called meta-analysis and has allowed to obtain more reliable data on topics under dispute among the scientific community.
In addition to exploring 33 different species of fish from different regions and habitats, the research focused in more detail on a marine species, the European sea bass Dicentrarchus labrax, and a freshwater species, the nile tilapia Oreochromis niloticus. With some variations between species, the temperature was able to considerably explain the amount of cortisol in fish, both between groups and within each group. Other factors such as sex, age or environment of fish were not useful in predicting these values. It was also observed that 20% of circulating cortisol in a fish has to do with its evolution, more related fish have more similar cortisol levels.
"There are still many factors that can influence cortisol levels in fish, but to decipher this it is necessary that stress studies better describe how fish are kept before testing. We obtained data on tank temperature and fish sex, but we could have a much more complete picture by including water acidity, ambient lighting and many other factors if they were reported in the studies".
Benjamin GEFFROY (Ifremer, MARBEC), co-author of the study
To know more
Alfonso, S., Houdelet, C., Bessa, E., Geffroy, B., Sadoul, B., 2023. Water temperature explains part of the variation in basal plasma cortisol level within and between fish species. Journal of Fish Biology. https://onlinelibrary.wiley.com/doi/10.1111/jfb.15342