Geography
How Continental Shelves Influence Marine Biodiversity and Fishing Economies
Quick fact
Although continental shelves make up only about 8% of the world's ocean area, they host an estimated 90% of the world's commercial fish catches and are among the most biologically productive marine ecosystems.
Why this is interesting
Have you ever wondered why most of the world's commercial fish are caught near coastlines, not in the open ocean? The answer lies in the underwater extension of the continents themselves.
Read the full explanation
Understanding How Continental Shelves Influence Marine Biodiversity and Fishing Economies
Imagine walking out from a beach. The water gradually gets deeper, and the seafloor slopes gently. That's the continental shelf. It is the submerged edge of a continent, extending miles offshore until it reaches a steeper drop called the shelf break. Because the shelf is shallow, sunlight penetrates to the seafloor, allowing phytoplankton--microscopic plants--to photosynthesize. These plants are the base of the marine food web. They attract zooplankton, which feed small fish, which in turn attract larger fish. The shallow, sunlit waters also provide nutrient-rich environments due to mixing by wind and tides, and often from nutrients brought in by rivers. This combination of light and nutrients makes the shelf a hotspot of productivity, or the rate at which organic matter is created.
A deeper explanation
The mechanism behind the shelf's productivity and biodiversity is a combination of shallow depth and nutrient availability. Sunlight is essential for photosynthesis, so phytoplankton are limited to the euphotic zone--the top layer of the ocean where light penetrates. On the shelf, this zone extends all the way to the seafloor, meaning that the entire water column can support photosynthesis. Additionally, nutrient concentrations are higher on the shelf due to river discharge, erosion, and the stirring of sediment by wave action and currents. These nutrients fuel the growth of phytoplankton, which are consumed by zooplankton, providing energy to the entire food chain. The shallow waters also offer a safe refuge for juvenile fish and shellfish, which can hide from predators in seafloor habitats like seagrass beds or rocky reefs. Many commercial fish species, such as cod, flounder, and shellfish, rely on these shelf habitats for spawning and nursery grounds. As a result, fisheries concentrate on shelves, bringing economic benefit to coastal nations. Some countries claim jurisdiction over extended shelf areas to control fishing rights, which is a major economic and political issue. The overfishing of shelf species can deplete fish stocks and collapse local economies, illustrating the delicate balance between biological productivity and human extraction.