Known to be fine regulators of gene expression, microRNAs are small molecules that remain particularly stable in biofluids—blood, urine, etc. In humans, these molecules have revealed great potential as prognostic or diagnostic tools for many diseases such as various cancers, strokes, and, particularly, heart attacks. However, few studies have been carried out on their potential as non-invasive biomarkers in animals, either for disease control or as a tool to assess their physiological status. This lack of information is particularly noticeable in cold-blooded aquatic species, whose physiological status is directly affected by environmental fluctuations, both in the wild and on farms. For this reason, specialists in fish reproduction and nutrition at INRAE, in collaboration with an American team from the University of Oregon, decided to conduct a study to evaluate the potential of circulating microRNAs as biomarkers of the metabolic or reproductive state of fish.