Upwelling is a natural oceanographic phenomenon where deep, cooler water rises towards the ocean surface, replacing warmer surface water. This process enriches surface waters with essential elements, creating productive fishing grounds globally.
Understanding Upwelling: The Process
Upwelling begins with wind interacting with the ocean’s surface. Winds push surface water away from a region, either along a coastline or in the open ocean. When moved offshore, deeper water rises to fill the void, typically at 5–10 meters per day.
Coastal upwelling, the most common type, occurs when winds blow parallel to a coastline. The Coriolis effect deflects surface water offshore, allowing colder, deeper water to ascend.
Equatorial upwelling occurs in the open ocean along the equator, where trade winds cause surface currents to diverge, pulling nutrient-rich water up from below. This mechanism results in a broad band of productivity visible even from space.
Upwelling’s Role in Marine Productivity
Upwelling brings cold, nutrient-rich water from ocean depths to the sunlit surface. These deep waters are abundant in dissolved nutrients like nitrate, phosphate, and silicic acid, originating from decomposing organic matter. Once at the surface, these nutrients become available to phytoplankton, microscopic marine plants at the base of the food web.
Phytoplankton use these nutrients, sunlight, and carbon dioxide for photosynthesis, leading to rapid growth and massive blooms. These blooms support zooplankton, microscopic animals that graze on phytoplankton and become a food source for larger organisms, including forage fish like sardines and anchovies.
The proliferation of these smaller fish attracts and sustains commercially valuable predatory fish, such as tuna, marlin, and salmon, creating fertile and productive areas. Upwelling zones, though occupying only about 1% of the ocean surface, contribute to roughly 50% of the world’s fisheries landings.
Fishermen’s Approach to Upwelling Zones
Fishermen seek upwelling zones to enhance catches due to the high concentration of marine life. One primary method involves monitoring sea surface temperature (SST) anomalies. Upwelled water is typically colder than surrounding surface water, creating distinct temperature breaks detectable using satellite data.
Fishermen also look for chlorophyll-a concentrations, indicating abundant phytoplankton blooms fueled by upwelled nutrients, which appear as greener water on satellite maps. Satellite altimetry, which measures sea surface height, also helps locate upwellings, as rising deep water can cause subtle depressions in the ocean’s surface.
Experienced fishermen observe natural signs like increased bird activity, as seabirds often congregate where baitfish are plentiful, or visible aggregations of baitfish. Understanding these indicators allows fishermen to make informed decisions about where and when to deploy gear. Commercially important fish species like tuna, sardines, anchovies, and various billfish are commonly targeted in these upwelling regions, leading to more successful fishing.