Spring koi death

I cannot understand why I lose fish at Spring time. My pond is approximately 1,500-gallons and has a very good waterfall and a venturi which are both switched off during the winter. My filter is always working and consists of a 30W UVS. The water goes to a tank containing brushes and into the next with rollers, then to a further two tanks containing Canterbury Spa, according to my test kits the water is perfect.

I lost some koi last spring. There was one in particular that was fine up until a week previously when she started to swim alone and got in the way of the male fish who kept bumping into her. After she died I opened her up and she was full of eggs and a lot of blood . I wondered if she’d had an internal haemorrhage or had become egg bound.

I’m so worried that this is going to happen again, and with the Spring rapidly approaching some advice would be very much appreciated.

Mystery or apparently unexplainable losses such as you have described can be very disheartening, if not soul destroying, especially if you have become particularly attached to the koi concerned. It can be difficult to discover the causes, or find an explanation to what has happened, especially when it appears that you have done everything possible to ensure the health and well-being of your koi.

As you describe in your letter, your water quality parameters are perfect and you take measures overwinter to protect your water from extreme cold. I would expect the water parameters in a pond in early spring, (just as koi or becoming active again to be very low in pollutants) as in your pond, with the weeks after the commencement of feeding proving the riskiest for experiencing a peak in ammonia or nitrite. You do not mention that you had started feeding at this stage, but I will assume that you would have not.

The other disconcerting details are that the majority of your koi have overwintered well and are showing typical healthy behaviour, swimming inquisitively about the pond. If a wholesale environmental problem had been the cause of your mortalities, then you would expect all of your koi to be showing similar listless behaviour. As this is not the case, we must be looking for factors that have affected a specific minority of your koi.

You mention that you lost some koi, but do not list how many or whether they were a mix of both male and female. The details you have given about the female that you lost last spring are interesting but you should be careful not to jump to any conclusions.

Your koi had been spawn bound, then you would have noticed an abnormally excessive swelling of the abdomen from July onwards. As you haven’t mentioned such an obvious symptom, then it suggests that this koi was not spawn bound. Furthermore, if you have been losing male koi as well, (which I assume to be the case) then the cause of the loss of your koi in spring is not likely to be gender specific.

Having opened up the koi, you found both eggs and ‘a lot of blood’ in the abdomen. All female koi that are sexually mature actually develop eggs around July and August, immediately after that years spawn. They are then carried through the winter months and continue to develop into mature eggs through the spring and summer months of the following year. So every healthy, fertile and mature female koi will be carrying eggs within their ovaries at this time of year. If their ovaries are empty, then they will not have the means to spawn that year. What you discovered it does not necessarily mean that your female was egg bound (especially if she wasn’t bloated) but shows that she was an intact, fertile female that would have spawned under the correct conditions that summer.

The quantity of blood within the body cavity may be significant, but may also have resulted from the act of cutting through the tissue. The abdominal area is well supplied by blood vessels, some of which are particularly delicate and prone to being easily damaged during a dissection.

If your koi have been dying intact with no apparent external signs of disease, then you still need to establish the cause of death, which has not been instantaneous but preceded by a period of ill-health, as evidenced by the week-long episode of abnormal behaviour with your latest female to die.

If you lose another koi this spring under similar circumstances, given the approximate one week’s notice before death, I would approach a vet experienced in koi health to euthanase the koi and take internal swabs for bacteriological analysis. This would give an indication as to whether an internal bacterial problem with associated septicaemia was the cause. If conclusive, you could then track back and try to identify any factors that could have made that (and your previous koi) susceptible to attack from bacteria during the spring rise in water temperature.

This is the tack I would take now, even though you do not have categorical evidence that disease has caused the death of these koi. Has there been a trend in the types of koi that you have lost? Are they smaller or older koi or are some more recent acquisitions? Have the more complex varieties (Go Sanke) been affected more than the more robust single coloured varieties (Ogon, Chagoi etc)? If so, this may give you a clue as to what is the cause behind these selective mortalities. Also, even though these mortalities are occurring in spring, the stressors are likely to be occurring months before, with the real consequences of a period of stress only manifesting itself as the pond temperature and the koi themselves rise from their period of inactivity and ‘hibernation’.

Winter protection.

Healthy and robust koi can overwinter satisfactorily in a suitable koi pond. However, that pond must provide koi with what their wild ancestors also require to over winter safely – and that in particular is a deep pond that offers them a stable temperature, sheltered from the extremes of air temperature. You mentioned that your pond is 1500 gallons in volume that is still turned over in winter. This could be the cause of your problems, and will require that you look into it further.

Pond Depth. For large koi to overwinter satisfactorily, a depth of 4 ft is considered by most koi keepers to be the minimum that offers reliable protection. (Some koi keepers I know have had few problems in ponds with a depth of 3 ft). If your pond is not this deep, then your koi may be experiencing variations in water temperature that for some may be proving too stressful.

Water circulation. Many koi pond owners choose not to recirculate their pond water through a filter over winter to prevent the chilling action that may occur between the filter water and cold air, returning chilled water back to the pond. You very wisely have turned off your waterfall and Venturi to reduce this cooling impact, but if the recirculation of chilled water is adding to the pressures on koi that are in a pond whose depth is only just adequate for protection against extremes in air temperature, then you may be causing your koi to experience periods of unnecessary stress during the coldest intervals. This will not cause an immediate decline in koi health, only manifesting itself when water temperatures start to rise, enabling micro-organisms to multiply and cause infection in your koi.

I am sorry that I cannot offer a more precise remedy. Nevertheless, I feel that you should satisfy yourself that your pond protects even your weakest koi from the temperature extremes of winter; and I hope you enjoy better success this coming spring.

Kill blanketweed and string algae.