Final answer:
The decrease in fish population in the bay is due to excessive algae growth caused by fertilizer runoff, which consumes oxygen and creates dead zones.
Step-by-step explanation:
The most likely explanation for the growth of plantlike organisms in the bay and the decrease in fish population is that the plantlike organisms, such as algae, are using up all the oxygen that the fish need. This results from the excessive growth of algae caused by high levels of nutrients from fertilizer runoff. This phenomenon is known as cultural eutrophication and can lead to the formation of dead zones where dissolved oxygen levels are too low to sustain most marine life, causing asphyxiation and death.