The Nile perch population explosion has been accompanied by:

a decrease in the dissolved oxygen in the shallowest portions of Lake Victoria.
a decrease in the dissolved oxygen in the deepest portions of Lake Victoria.
an increase in the dissolved oxygen in the deepest portions of Lake Victoria.
an increase in the dissolved oxygen in the shallowest portions of Lake Victoria.

To determine the correct answer, we need to understand the effects of the Nile perch population explosion on the dissolved oxygen levels in Lake Victoria.

The Nile perch is a predatory fish that was introduced to Lake Victoria in the 1950s. Since then, it has experienced a population explosion, mainly due to its predatory nature and lack of natural predators in the lake.

One of the consequences of this explosion is the alteration of the lake's ecosystem, including changes in the dissolved oxygen levels. As a large and voracious predator, the increased numbers of Nile perch have led to a significant decrease in the population of other fish species, such as the indigenous cichlids.

Cichlids are known for their importance in maintaining the ecological balance of the lake, as they help control algae growth and increase dissolved oxygen levels. With their decline, the balance of the ecosystem has been disrupted.

As a result, the decrease in cichlids and other fish species has contributed to a decrease in the dissolved oxygen levels in the lake, particularly in the shallowest portions. This is because with fewer fish to consume algae, there is a proliferation of algae blooms. When these blooms die and decompose, the process consumes oxygen and leads to oxygen depletion in the water.

Therefore, the correct answer to your question is: a decrease in the dissolved oxygen in the shallowest portions of Lake Victoria.