In most vertebrates (humans included), the steroid hormone cortisol is released to the blood in situations of stress and can be used as a marker for stress/immediate welfare status. In fish, small amounts of cortisol diffuse passively to the surrounding water, mostly through the gills. Waterborne cortisol (WC) is often used as stress indicator in fish research, under very controlled conditions. It has potential to become an ideal welfare indicator for fish in fish-farming facilities, as it is non-invasive and directly informative about the fish stress/welfare status. The main current disadvantage as a fish welfare indicator relates to the fact that WC analysis is time-consuming and involves sending water samples to a laboratory, making it a laboratory-based welfare indicator – LABWI, impractical to use as a routine indicator in fish facilities. However, ongoing technological efforts towards the development of online cortisol sensors could change the situation and turn WC into a good candidate for a fish-based operational welfare indicator (OWI), able to provide real time information on the physiological status of captive fish.
Even if new technology succeeds in providing tools for the continuous monitoring of WC, another problem to solve relates to the poor knowledge available about the extent of release and about the chemical fate of WC in different types of fish facilities/fish farms. First, the passive release of cortisol to the water not only depends on the concentration of cortisol in the blood of the fish, but also on the permeability of the gill to cortisol, which could be affected by different factors (species, size, biorhythms, health status, etc.). Second, there is a need for validation work to better understand how different factors in the fish production systems affect cortisol degradation/permanence in the water upon release, including water quality (temperature, salinity, organic matter, particles, microbial load, etc.) and facility components, design and operation protocols, among others. Detailed information about normal WC levels, normal variation, extent of change upon challenges and routine operations, etc., for different facility types is also lacking, and would be critical to evaluate the potential of WC as welfare indicator in fish farming. This presentation will review current knowledge about advantages and drawbacks of WC and its potential use as an operational welfare indicator for farmed fish.