Physical Stress This includes all the environmental conditions that we control for our fish, the most important of which is temperature. All fish have a temperature range within which they will thrive, and a wider range within which they will survive. Fish are cold-blooded animals and do not have the ability to expend energy to maintain a constant internal body temperature like we do. They are completely at the mercy of the temperature of their surrounding water. If that water temperature goes outside of their optimal, or ‘thriving’, range they will eat less, or stop eating all together, and they become more susceptible to disease. That said, this is sometimes carefully employed as a technique called ‘cold banking’ in order to slow down their growth rate.. Cold banking is especially effective with fingerlings, when you are trying to stagger your fish production.

Another form of physical stress is sudden exposure to light and vibration. Fish are alarmed when we flip on a light switch and shock their world instantly from night to day. They will often start banging against the walls of the tank to escape the light. However, just like with cold banking, this sensitivity to light can be used to the aquaculturalist’s advantage by employing a technique called ‘phase shifting’ whereby you trick the fish into thinking that it is spawning season (or not) by timing the amount of light they get during the day to mimic the season in which they normally spawn (or not).

And because they ‘hear’ vibrations with their entire bodies, rapping against the wall of a tank feels like ‘yelling’ to them and will also cause them undue stress.

Interestingly, another form of physical stress can be water velocity. Fish originating from still lake waters, like tilapia and perch, do not like much movement in their tank water. However, river fish, like trout, find it stressful not to have a current present in their tank.

Chemical Stress This is mostly centered on maintaining the quality of the water. We all know that escalating ammonia and nitrite levels stress our fish. Nitrate levels, however, can go as high as 500 – 700 ppm without harming the fish. Maintaining a very low pH can also be stressful. And insufficient filtration of the solid waste and not enough dissolved oxygen are, not surprisingly, other forms of chemical stress.

Biological Stress This last category refers to viruses, bacteria, fungi, and parasites. Just like in our world, most of these pathogens are often present but only fully express themselves when the right conditions occur. For aquaponic fish, this likely means that some of the stress factors listed above must also be in place for biological threats to have an impact.

In aquaponics we have adopted the technique of ‘salting’ fish or adding salt (sodium chloride) to the water to help them ward off disease, but this practice can be harmful to our plants because they may be sensitive to sodium. It is actually the chlorine that helps the fish, and not the sodium. You can get the same effect with a more ‘plant-friendly’ treatment such as potassium chloride or magnesium chloride.