On July 28, 2015, a heavy rains poured down on the fields, bluffs and valley surrounding the South Branch of the Whitewater River in southeastern Minnesota, one of the state's most heralded trout streamsLater that day or the next, between 9,000 and 10,000 fish in a 6.5- mile stretch of the river in Olmsted County were killed suddenly. The event nearly wiped out the wild brown trout population in that stretch However, the question of what caused the fish kill remains a mysteryWhich environmental condition is LEAST likely to have played a role in the fish kill? A The extensive rains lead to a change in the of the river that killed off water-dwelling insects, minnows and crayfish leaving the trout with insufficient food sources. B Agricultural practices led to the wash of fertilizer into the river, causing an algal bloom that led to a hypoxic that decreased oxygen concentrationskilling the sensitive trout population Herbicidespesticides and fungicides, some that are lethal to aquatic life, were sprayed on crops by helicopter in the days leading up to the kill were washed into the river in concentrations to high to sustain life for the vulnerable trout D Manure from local dairy farms, especially manure containing copper sulfate has been spread as fertilizer near the river in the days leading up to the fish kill, washing potentially toxic levels of the manure into the river with the extensive rains.
C. Herbicides, pesticides, and fungicides that were sprayed on crops by helicopter in the days leading up to the kill were washed into the river in concentrations too high to sustain life for the vulnerable trout.
This is the least likely environmental condition to have played a role in the fish kill because herbicides, pesticides, and fungicides are not typically known to directly cause mass fish kills in rivers. While these chemicals can have negative effects on aquatic life and water quality, they are not as likely to directly cause the sudden death of thousands of fish in a short period of time.