Scott,
O.K. here are all the even remotely possibly causes and my comments. Sometimes mortality is caused by a combination of factors but typically hypoxia is the highest probability.
1.) Ground water influx zip in D.O. and the already low water volume (4 feet?) caused a hypoxic condition due to the shear volume of the ground water influx.
Comment: It would have to be a significant influx of low D.O. water this time of year as this cold of water typically has lots of D.O.
I think this is a good possibility. The D.O. could have been marginal in the pond due to the overcast, not windy winter that we've been having, and the recent warm spell could have melted enough of the ground to allow an influx of water into the water table, then into the pond. The pond is relatively shallow, and doesn't have much water volume "in reserve".
2) A lot of snow blew and drifted over the ice you didn't see and had melted by the time you came out? That combined with low water levels could have been a factor. We didn't get the last snowstorm that you did, we've only had a couple of inches of snow total the past month, and it's melted within a week of falling. The pond is sheltered by the trees and no drifting is possible.
3.) The fish were caught and released in cold water prior to ice cover by your client or someone trespassed and did so. Having recently been stocked the fish perhaps were already in a stressed condition. Some fish have problems with catch and release in cold water and develop water fungus problems.
The trespass is a possibility, but I was spending a lot of time on the property hunting and didn't see any signs of trespassers. My clients son fished the pond but he reports every fish caught. The last RBT caught and released was back in late November and no HSB were ever caught. Only 2 or 3 RBT were C&R. No fishing has occured in the pond that I know of since early to middle December due to the ice covering, and that was too thin to walk on.
4.) The fish went into the pond in less them optimum condition. Or the water chemistry is so different from what they are used to this along with other factors put them over the edge.
If that's the case, then why would it take almost 4 months for the dead fish to show up?
5.) The low volume water was subjected to a rapid temp change (dropped in temp quickly) along with a heavy snow fall in open water. We did have sudden cold event this fall although it obviously moderated later.
I doubt this happened due to our mild winter. Only heavy snow fall was about a month ago, and that melted within a week. That "heavy" snow fall was 4" at the pond property.
6.) A sudden influx of ground water or precip caused acid shock. (doubtful in our high alkalinity waters but it's not uncommon out east.)
7.) The ground water in his area is contaminated with something. I'm doubtful of this as you would have seen this earlier.
Plus the fish in the other ponds that are close by would have some deaths also. None are observed.
Did the dead fish show the classic appearance of environmental hypoxia? That is, the opercula flared and mouths agape in a desperate attempt to get oxygen?
YES
It may have been one of these factors or a combination of them. You may never know.
Cecil, you are freaking amazing in the deduction. Are you sure you're not an environmental scientist, with a pH also in geotechnical?
Scott, the reason why I ask you to get the oxygen metered for your pond is the same reason that Cecil described.
1. Ground water has a high level of dissolved oxygen regardless of the temperature. Soil strata create an oxygen rich environment, and when the water flows through there, oxygen content get saturated. When you pump it up, oxygen content get stirred, and oxygen remains or enriched, that is, if you're constantly introducing new ground water into the pond..or ponds. However, thermal exchange is my suspicion that shocked the crap out of the fish when new water is introduced if you're trying to fill the pond during winter. Other than that, possible pollutants, pathogens, or bacterial that targeted the fishes.
2. Little does people know, snow do bring in oxygen content due the crystallized structure creating aerated pockets. Snow falling on surface of water will mix oxygen to the surface. Ice will trap the oxygen, but as it melts, it dissolves the oxygen to the surface, thus, fish like to hang about 2 to 5 feet below the ice. Oxygen rich environment.
4. Fish tend to survive by readapting, but can only push for so long. Larger fishes have great affinity for adaptation, so, you don't normally see it until later..way later. If there are 5 conditions you must meet for them to be perfectly healthy, and fulfilled the 3, they will readjust until their bodies become exhausted. Small species will last 2 days observed in the wild. Some will last a year or two based on conditions.
7. Looks like there are signs of something that leech the oxygen content from the water if the fishes show signs of hypoxia. We need that oxygen metered from the pond. Could be a sudden CO2 shift and/or in combination to a nitrate affixiation from the primary pond that get the first wave of ground water.
