Stream pollution can often be indicated by which of the following?

The presence of a zone of degradation is a key indicator of stream pollution because this term refers to an area within a water body where ecological conditions are suboptimal, typically due to the introduction of pollutants. Indicators of a degradation zone can include lower biodiversity, changes in species composition, and altered physical and chemical characteristics of the water. In such a zone, the balance of aquatic ecosystems tends to be disrupted, leading to adverse conditions for various organisms.

In contrast, increased levels of available oxygen in the water would typically indicate a healthier ecosystem. Favorable oxygen levels support diverse aquatic life, making this choice an unlikely indicator of pollution.

The presence of large numbers of crayfish and certain fish species, such as cristivomer (also known as the lake trout), may not definitively signal pollution. Depending on the specific environmental conditions, these species can thrive in both healthy and degraded ecosystems. Therefore, their abundance alone does not provide reliable evidence of pollution.

Similarly, while large numbers of small fish may suggest a thriving population, this could also occur in non-degraded environments. It is the specific context of ecological health, biodiversity, and water quality that makes the identification of a zone of degradation the most accurate reflection of stream pollution.