Final answer:
In a study of how dissolved oxygen affects fish populations, the control would be a water body with a constant and known supportive oxygen level, serving as a baseline for comparison.
Step-by-step explanation:
To investigate the hypothesis that the amount of dissolved oxygen affects the number of fish that can live in a body of water, and that there is an ideal dissolved oxygen concentration for fish, your control would be a body of water or experimental group with stable environmental conditions, including a constant level of dissolved oxygen that is known to support fish life. This control would serve as a baseline to compare how varying levels of dissolved oxygen impact fish populations.
It's crucial to monitor factors such as temperature, since the solubility of oxygen decreases as temperature rises. Depleted dissolved oxygen levels can lead to serious consequences for aquatic ecosystems, including large-scale fish kills. Therefore, setting a control with optimal dissolved oxygen levels is important for distinguishing the effects of changes to the oxygen levels from other variables.