Stress has such a huge impact on Koi that it warrants an article on its own. Good Koi husbandry will not be possible without a decent understanding of stress and the effects on koi in a pond. To fully understand stress it is necessary to backtrack a little bit and consider the immune or defence systems of fish.
Koi share their environment with numerous organisms that causes disease. In order to survive, Koi develop comprehensive defence systems to guard against the various organisms like parasites viruses and bacteria. The defence factors can be divided into innate immune systems and acquired defence.
Externally the first line of defence is barriers of organisms and comprises mainly of skin, scales and the mucus membrane (cuticle). Internally, the new mucosa (epithelial) covering of internal cavities, are the barriers to entry. Should a pathogen penetrate the external barriers a second group of defence factors take over. Phagocystic cells are present in the blood, tissue and organs of the fish. They clean up foreign invader and damage cells by indulging them and releasing enzymes to brake down the damage or cells invaders. A number of proteins also come into play. These proteins and phagocystic cells compliments each other in the defence against invading bacteria. The by product of the attack on bacteria and the cleaning up of damage cells are release into the blood for processing by the kidney into the urine.
Acquired defences are those defences acquire through “learning” defensive measures and prepare the fish to cope with certain pathogens if it appear a second time, or where vaccination were introduced.
After this very brief description of the immune system we can move onto stress.
The response to stress is a series of physiological reactions called the General Adaptive Syndrome. This syndrome consist of the alarm reaction when stress hormones are release, a period of resistance during which adaptation occurs and if the fish cannot adapt, exhaustion followed by death.
As a fish is stressed and hormones are released it will result in an increase in cardiac output and a constriction or delusion of the blood vessels. These hormones also have an influence on the metabolic breakdown of glycogen to glucose in the muscle and liver. Although the hormone produces an instant source of energy it also increase the permeability of the gills to water in ions. This causes the fish to take on more water and to loose ions. Fish in captivity are often subjected to long periods of stress.
The following example is known to most fish keepers. You buy a fish from a show and within a short time after arriving at your home the fish get sick. Let’s consider what that new Koi has gone through before it gets to your pond. Firstly it gets netted out of a comfortable mud pond then it gets handled in the nets, put into a cramped space for sorting and after the sorting /handling and relocation into a cramped concrete pond, it’s bombarded with chemicals to keep pathogens in check. Next it is netted again and transported to retailers if it is sold locally, or if it was bred in Japan, to wholesalers where it is put into a different holding pond with different water parameters. In the case of imported Koi, it is then send to retailers overseas to go through another experience of transport etc. If your koi was lucky, you would have bought it from a dealer where it has been rested. If you bought it from a Koi show, it went through another ordeal, plus it was netted countless times by prospective buyers, curious visitors and naughty children. During this incredible process it was starved for weeks and medicated several times.
Now let’s return to the Koi that you bought home and it got sick. Your immediate reaction is to think that the dealer has sold you a sick fish. In reality, the prolonged stress that the fish went through has actually caused the defence system to weaken or become ineffective. As stated, fish exposed to chronic stress will either compensate or die. In the pond environment, there are stressors that can and will occur and if not corrected immediately, the stress may become chronic, cause disease and death.
Some of the stressors that occur in a pond are too much handling, frequent visits by predators, badly formulated or old food, temperature fluctuation, unstable pH and overcrowding. However, the most important stressor is bad water quality (see article on water quality). Ammonia, nitrite, pH, low dissolved oxygen, nitrate, dissolved gasses and unstable temperature play a vital role in the functioning of the fish’s defence systems. The above influences can actually weaken or shut the defence system down, with disastrous results. In most cases disease outbreaks or unexpected mortalities can be traces back to some chronic stressor in a pond. In other case, it can be traced back to a highly stressful situation that occurred a few weeks prior to the symptoms occurring.
The management of stress in a Koi collection should therefore never be underestimated.
Did you know?
Koi are creatures that like company and always prefer to be close to the rest of the school. In Koi health it is frequently mentioned that when inspecting a Koi collection, one must look closely at a specific fish if it isolates itself from the rest of the collection. The reasons for this phenomenon are rarely explained.
Mostly it is correctly assumed that the fish isolates itself because of some discomfort and the fish can/will not swim around endlessly. The fish isolates itself in an area where it feels most comfortable like in a corner where there is little water movement or at a specific place where it has access to the highest amount of dissolved oxygen.
The other reason is that there are many unique features in the skin of Koi. In this instance there are certain cells (Schreckstoff cells) that are found in the epidermis. These cells secrete a pheromone that is released into the water by an injured Koi to warn others of imminent danger. Koi in the vicinity respond to this chemical message by avoiding this area.
This also explains the reason why an injured/sick fish may contribute to the stress in a collection where all the fish are confined in a man-made pond.
By: Chris Neaves