Test how food, predators, pollution, and temperature affect a fish population.
1Set the conditions
For a fair test, change only one condition between runs.
10 fish
Applies when you press Start over.
5 of 10
Plants, insects, and plankton.
1 predator
Large fish that hunt the small fish.
1 of 10
Toxic runoff that uses up oxygen.
25 °C
Warm water holds less oxygen.
Or load a quick-start scenario
2Run the pond
Fish
Predator
Food
Murkier water means more pollution
Results, scenarios, and the graph
3Read the results
Fish in the pond
10
Month
0
Born
0
Died
0
Eaten
0
Food supply limit
125
Dissolved oxygen
8.1 mg/L
Healthy
Press Next month to begin.
Population over time
Solid blue line: fish. Dashed orange line: food supply limit. Both lines are labeled on the graph.
Model reminder: a real pond has more interacting factors than this learning model shows.
Directions
Look at the pond. Orange fish are the population you are studying. Dark red fish are predators. Small green dots are food.
Press Next month to move time forward one month, or Run 12 months to watch a full year.
Watch three things: the number of fish, the shape of the graph, and the dissolved oxygen reading.
Change one slider only. Keep every other slider the same. That slider is your independent variable, and this is what makes it a controlled investigation.
Open the Data table to record exact numbers. You will need them as evidence.
Press Start over in the upper right to return to month 0 and test the next value.
Tip: food, predators, pollution, and temperature change the pond right away, even in the middle of a run. Starting fish only takes effect when you press Start over.
Shortcut: the scenario buttons set every slider at once and start a fresh run.
Pick one question. Run at least 24 months at each setting you test, then write a CER response.
What happens to the fish population as food supply increases?
Test food at 2, 5, and 10. Keep predators, pollution, and temperature the same.
What happens to dissolved oxygen as water temperature increases?
Record dissolved oxygen at 5, 15, 25, and 35 degrees Celsius before pressing Next month.
What happens to the fish population as pollution increases?
Test pollution at 1, 5, and 9. Watch both the oxygen reading and the number of fish.
What happens to the fish population as the number of predators increases?
Test 0, 2, and 5 predators. Does the population crash, or does it settle at a lower number?
Claim: the pattern you can read from the graph. Evidence: exact numbers from the data table. Reasoning: the mechanism, which you can find under The science.
Limiting factors and carrying capacity
A population cannot grow forever, because resources run out. The food supply limit in this model, also called carrying capacity, is the food slider multiplied by 25. As the population approaches that limit, fewer young survive and the growth curve flattens into an S shape.
Temperature and dissolved oxygen
Fish breathe oxygen dissolved in the water, and cold water holds more of it than warm water. The model estimates saturation from a standard curve:
Dissolved oxygen at saturation for four water temperatures
Water temperature
Oxygen at saturation
5 °C
about 12.7 mg/L
15 °C
about 10.0 mg/L
25 °C
about 8.1 mg/L
35 °C
about 6.6 mg/L
Below about 5 mg/L most pond fish are stressed, and below about 3 mg/L many begin to die. Temperature also controls spawning. These fish reproduce best near 24 degrees Celsius. In cold water they barely spawn, but they also burn less energy, so the population settles at a low number instead of dying out. In very hot water they spawn poorly and burn energy quickly at the same time, which is why heat is much harder on them.
Pollution
Pollution does two jobs here, just as fertilizer runoff does in a real pond. It is directly toxic, and it feeds bacteria and algae that use up oxygen. Each pollution level above 1 removes about 0.55 mg/L of dissolved oxygen.
Predators
Each predator can only catch and eat so many fish in a month, at most about 8. When fish are scarce, predators find fewer of them. That is why predators usually settle a population at a lower number rather than wiping it out.
Randomness
Births and deaths vary by about 10 percent each month, so two runs with identical settings will not give identical numbers. Real populations vary too, which is why scientists repeat trials.
The math, if you want it
Births = fish × 0.45 × (1 − fish / limit) × temperature factor × pollution factor.
Deaths = fish × (natural + oxygen stress + heat stress + pollution toxicity + crowding). Natural losses are about 4 percent per month at 24 degrees Celsius and roughly double for every 10 degree rise, because metabolism speeds up in warm water.
Eaten = predators × a saturating curve that tops out near 8 fish each.
Fish population data recorded each month
Month
Fish
Born
Died
Eaten
Food
Predators
Pollution
Temp °C
Oxygen mg/L
Copy pastes into a spreadsheet as columns. The table clears when you press Start over.
Solid blue line: fish population. Dashed orange line: the food supply limit set by the food slider. This graph updates as the simulation runs.
This simulation uses simplified math to show how limiting factors affect a population. It is a teaching model, not a prediction of any real pond.
A real pond also has:
disease and parasites
fish of different ages and sizes, not one identical group
seasons, ice cover, and day to night oxygen swings
rain, drought, and groundwater
algae blooms, and plants that add oxygen in daylight but use it at night
a full food web with many species, not one prey and one predator
sediment, shelter, and nesting habitat
fishing, stocking, and invasive species
Real biologists collect field data over many years before drawing conclusions. Use this model to explore patterns and build explanations, then compare what you find to real data from a local waterway.