Methods
What was measured, what was modeled
Measured
Catch log — species names and counts tallied manually at each session's conclusion, May 2023 – August 2026. 48 sessions total, 525 fish.
Session effort — timestamps marking the beginning and end times, totaling 269 hours.
pH — 75 total recordings of three separate points throughout each session over 25 sessions.
Air temperature and rainfall — NOAA/National Centers for Environmental Information Daily Summaries from Washington Dulles Airport Weather Station (USW00093738). Actual values were gathered roughly seven miles north of the study site.
Modeled
Water temperature — no instrument has ever been installed in this body of water to measure temperature. Estimated from the daily air temperatures via a first-order lag equation:
Twater(d) = Twater(d−1) + [ Tair(d) − Twater(d−1) ] / τ
τ represents the time constant of thermal response, which dictates how quickly the water follows the air temperatures. τ was fixed at five days, a literature value for shallow ponds. It is assumed, not fitted, because there is nothing to fit it against.
This model recreates the physical characteristics of a pond, which include lagging the air and oscillating less than the air does. Over 48 sessions, the modeled water temperature range proved to be narrower than the air temperature range, as it must be.
Finding one — the strongest result
The pond's chemistry changes over a single afternoon
Three readings per session, late morning to late afternoon. pH rose every single time.
pH across the day, all 25 sessions
MeasuredEach faint line is one session; the heavy line is the mean of all 25. Not a single session ran the other way.
What causes it
During the day, algae and aquatic plants absorb dissolved carbon dioxide from pond water through photosynthesis, lowering carbonic acid levels and raising the pH. At night, photosynthesis stops, and respiration starts, causing the pH to drop as algae and aquatic plants release carbon dioxide into the water.
The extent of that daily fluctuation is a well-recognized indicator of how biologically productive a body of water is and, thus, how nutrient-rich it is. A swing of half a unit over six daylight hours is a sign of an active and fertile pond; it is exactly what you would expect from a pond collecting lawn fertilizer from the surrounding neighborhood.
Why it matters beyond chemistry
If the pH changes that way, dissolved oxygen must change. Throughout the day, algae and aquatic plants produce oxygen, and dissolved oxygen levels climb, peaking in the late afternoon. When the sun sets, they consume dissolved oxygen through respiration, which causes its level to hit the lowest point just before dawn. Algae and aquatic plants grow quickly with high nutrients, creating enormous oxygen demand at night. During the pre-dawn hours, low oxygen stresses fish and kills them in extreme cases.
Since I have not measured dissolved oxygen, the parallel cycle is an inference based on established limnological knowledge. The next reasonable step to determine whether the nighttime drop in oxygen levels threatens fish populations is to measure dissolved oxygen at dawn.
Finding two — a null result
Total catch has no relationship with temperature
Fish per session against modeled water temperature
x-axis modeledEach point is one Saturday. The horizontal band line is the mean within each temperature band. There is no trend to describe.
Finding three
The total is flat because the species move in opposite directions
Bluegill hold the catch steady while bass fall away and bullhead climb. The aggregate hides all of it.
Mean fish per session by temperature band
Counts measuredSame sessions, split by species. Bass and bullhead run in opposite directions across the temperature range; bluegill are indifferent to it.
Every species against modeled water temperature
| Species | Spearman ρ | p | At p < 0.05 |
|---|---|---|---|
| Bluegill | -0.081 | 0.5855 | — |
| Brown bullhead | +0.397 | 0.0052 | significant |
| Largemouth bass | -0.598 | 0.0000 | significant |
| Channel catfish | +0.068 | 0.6454 | — |
| Common carp | -0.606 | 0.0000 | significant |
| Crappie | +0.338 | 0.0187 | significant |
| Golden shiner | -0.038 | 0.7999 | — |
Finding four — a trend that turned out not to exist
How an apparent bass recovery disappeared
If the figures are added together from year to year they give 7, 6, 11, and 14; this shows that the population has doubled, and it would have been a pleasant figure to have reported.
It is precisely because the sessions occurred at that time that this situation arose. At this pond, bass are basically only seen in May — there were 2.55 per session in May as against 0 in August, and in fact no bass were caught in twelve August sessions over a period of four years. Furthermore, 2026 had four May sessions while 2024 had only one: there was therefore greater activity in May and consequently more bass, even though no alterations had been made to the pond.
If we confine ourselves to the months from June through August — the same periods that were sampled in a similar way each year — then the trend vanishes.
Bass per year, all months vs. June–August only
MeasuredThe same fish, counted two ways.
Further results
Rain against dry
MeasuredMean fish per session on days NOAA recorded rainfall at Dulles, against days it did not.
Catch per hour by season
MeasuredFish per hour of recorded effort, averaged within each year. Effort-corrected, so a short season does not read as a poor one.
Species composition
525 fish, seven species
The length of the bar indicates the total number caught; following each bar is the count, together with the number of sessions during which that species appeared at all.
■ forage ■ predator ■ management concern
Open data
Download everything
All the figures on this site can be obtained from these files; the measured and the modeled values are in separate columns and the modeled value includes the method used on each row.