Of the three spawnings I did in July of 1999, one I completely culled in the first two months due to abysmally bad backs and tails (sometimes a pairing just doesn’t work out in a really big way). But the other two spawns showed a lot of promise. So I was pretty excited to see what would happen.
But I was also apprehensive. For my previous spawns in 1997 and 1998, I had observed a significant loss of fry. What would happen is that at about 6- to 8-weeks of age, I would start losing fish. This would continue for a month or so and then stop. The first year I sustained about 50% losses. The remarkable thing is that I had no losses up to 6-weeks (in 1997, I killed one fry siphoning out poop from the tank, but that was the only fry out of 2000 that I lost prior to 6-weeks). The worst part of the problem was that, even when the losses stopped, I was left with many fish with abnormally short gill opercula and oddly distorted heads. For the heads, the fish had sort of a pug dog-like appearance: shortened snouts and large eyes in a somewhat undersized head.
For 1999, I had substantially improved my fry husbandry techniques. But the problem recurred nonetheless. I kept my losses to about 25%, but the situation is totally unsatisfactory, since much of my culling is being done by disease and not by my selection. Plus it is awfully discouraging.
I pride myself on having a scientific background and yet in dealing with this problem I have done all the usual amateur hobbyist mistakes of looking for quick fixes and trying medications without any disease diagnosis. This is particularly hypocritical since I spend so much time sagely responding to emails from other people doing the same type of thing.
Realizing I should do better, I have taken a longer-term view. I took some fish up to Shawn Prescott at Fish-Vet Inc. in Maryland (a diagnostic lab and aquaculture consultancy). He did a parasitic investigation, but found nothing. He also tried putting the symptoms I was observing into a diagnostic software product they have developed and market. This suggested several possibilities. Shawn also supplied me with Reviveä , a product they have developed combining acriflavine and formalin with a potentiating agent, to try on the fry. It is not too hard on the fish and is effective against a pretty broad set of parasites and gram-negative bacteria.
I divided the fry up into 5 different tanks (I had culled down to about 70 fish at this point). I treated 3 tanks, left two untreated, and then tried to keep precise records on losses. I did a final selection of fish I wanted to keep two weeks ago, picking 8 fish. At the same time, I went through all the fish and recorded observations on the size of each fish and its opercula (short or not). The intent is to determine if the medication had any statistically significant impact on my problem.
I have not had a chance to reach any conclusions yet. The data set is somewhat flawed, since there were variations in temperature and water quality that could be significant. Also, I just randomly distributed the fry initially, without taking any baseline data. So perhaps this is a practice run for a more sophisticated experiment in the coming months.
One issue for me has been how to "think" about this problem. How do I figure out if it is genetic, environmental, or a pathogen? If I can narrow it down to one of these categories, how do I then specifically identify the root problem? Laboratory analyses are expensive, so it is in my best interests to do as much work to eliminate possibilities as I can on my own and then use the professionals to answer specific questions.
The general answer to the above questions is that I need to run some experiments with proper techniques and controls. First off, I need to spawn some fish that are totally unrelated to my current strain; this will decide the genetic issue. Next I need to experiment a little with feeding/diet, stocking density, and temperature, to see if environment plays any significant role. And then finally I need to maintain a control population for any medicating I do. I would claim that salt, Revive, and potassium permanganate have all been somewhat helpful. But without some actual data to analyze, I can't say which is most effective or even if there is really any significant benefit from any of them.
The last item is that I need to dust off my microscope and start using it.
I hope sometime in the future I can write an article that will explain how I solved this.