Final answer:
Dead zones negatively affect fish and shellfish populations by reducing dissolved oxygen levels in the water due to the decay of algae, which thrives from excessive nutrients from fertilizer runoff and sewage.
Step-by-step explanation:
Dead zones have a detrimental impact on fish and shellfish populations. These dead zones occur when ecosystems experience an overload of nutrients, often from sources like fertilizer runoff and sewage, leading to rampant algae growth. When these algae die and decompose, they significantly deplete dissolved oxygen levels in the water, making it uninhabitable for aquatic life, particularly fish and shellfish which relies heavily on this oxygen to survive. This process, known as eutrophication, can create large areas devoid of normal aquatic life and is a key factor behind massive fish kills, especially during the warm summer months.