Not a very pleasant headline, but all the same a very realistic issue that in the experiences of a pond owner or koi keeper, will at some stage, probably become a reality.
The background to such a fatality may be along the lines of the following. The water is as clear as it has ever been, the fish are eating food like it’s going out of fashion and they have become so tame that they will eagerly take food from your hand. It all points to a successful, trouble-free koi keeping season.
The following morning, out of the blue, during feeding time, your eye is drawn to a silver shaft of reflected light, shooting up from the depths. A lifeless koi lies stiff and on its side on the pond bottom, and yet only yesterday, it was feeding like an aquatic tiger.
What is the sensible and practical course of action to take if such an incident arises in a pond?
1. Remove the fish and carry out a simple external inspection, looking for obvious signs to cause of death.
Look for any external damage or any areas where the skin and scales are not intact. Examine the fish in all dimensions, even looking into its mouth, for signs of ulceration or damage. Quite a common phenomenon in ‘overnight mortalities’ are the presence of ulcers on the ventral surface (underneath) of a koi going unnoticed until it has developed into a significant lesion. If this is the case, as may be with fish that have been sitting and rocking on the pond bottom over winter, then it would be wise to net and check all of your fish to look for similar damage. If it has happened to one koi, it may be common to others, with time to treat and repair the damaged tissue.
If the koi is completely clean and intact then, unless the fish is very fresh, the issue of a post-mortem is probably out of the question and is not likely to shed any new light onto the causes of a sudden death.
The danger when considering the cause of a koi’s death is to adopt a simplistic view of koi health. For example, koi are like any other fish (and animal) in that a wide range of internal complaints can develop rapidly, like a dormant volcano, becoming active with lethal consequences for no apparent reason. It is useful to remember that the majority of complaints and health problems that can develop in koi are not represented by the number of remedies and pond treatments that are available over the counter. A lot of health problems are inherited, internal and physiological, not being caused by a disease-causing organism. I.e. there is nothing to see, diagnose or treat.
2. Test water
If upon removing the koi, it is intact, then just to be safe, it would be wise to carry out a number of key water tests. This will establish that the incident is isolated and not caused by environmental problems. It will also put your mind a rest that other fatalities are not likely to follow.
Test for the usual ammonia, nitrite and nitrate reading, only accepting readings of 0, 0 and <50 as satisfactory. Also check that the pH is alkaline and between 7 and 9 and that the pond is well aerated. Nevertheless, it may still be advisable, if a freak fatality occurs, to wash out any settlement in a filter and carry out a 30% water change to freshen up the pond.
3. Retrace your steps
The final course of action is to retrace the last couple of weeks or so to try to identify whether you have done anything differently with the pond, water or koi. Was the koi that died a new introduction? Was that the fish that jumped out last week and apparently made a full recovery? Any other wholesale changes such as medicating the pond or carrying out a water change would affect all the fish rather than a single casualty, so unless this particular fish was predisposed to a problem brought on by such an action, it is unlikely that this would have caused a problem.
However, in such a case, it is useful, where possible, to keep a notebook or diary in the shed with the medication and pond equipment, just to note down your routine (and non-routine) maintenance and interaction with the pond. This will make it much easier to retrace your steps and spot trends and perhaps identify any trends in koi health that may follow work on the pond. This will allow you to look out for the occurrence of problems in the future, having carried out work that in the past may have lead to a mystery fatality.