The Effects of El Niño on Marine Life
|by Amy L. Forrester|
Fishermen along the coasts of Peru and Ecuador have observed the phenomenon of El Niño for centuries. They noticed a warm ocean current that flowed along their coast every year during the months of December and January and during this interval fish would be less abundant. However, every few years fish in these coastal waters would virtually vanish causing a halt to their fishing businesses. The fisherman used the term El Niño, which is Spanish for the little boy, because of its tendency to arrive around Christmas time. It has since been used to describe the more severe intervals that appear every 2-7 years, with varying intensity.
Once thought to affect only a narrow strip of water off Peru, El Niño was soon recognized as a large-scale oceanic warming that affects most of the Tropical Pacific. It then became clear that El Niño was normally accompanied by a change in atmospheric circulation called the Southern Oscillation, and that the Peruvian event was just the first noticeable occurrence of an event of much wider significance. Together, the effects from El Niño-Southern Oscillation (ENSO) have an impact on fisheries and marine life, as well as climatic conditions around the globe.
The key element of the El Niño phenomenon is the interaction between the sea surface and winds (the ocean-atmosphere system). Normally, the trade winds blow east to west across the Tropical Pacific. These winds push the top layer of water along with it and pile up warm surface water in the western Pacific. This results in a sea surface that is about 0.5 m higher at Indonesia than at Ecuador and a sea surface temperature that is about 8° C higher in the west. As the surface water moves away from the coast of South America, colder, nutrient-rich water comes up from below to replace it, a phenomenon known as upwelling. As the trade winds continue to blow westward, the thermocline, the boundary between the warm surface water and deep cold water, is raised almost completely up to the surface in the east and is depressed in the west. This nutrient-rich deep water mixes with the surface water that supports the high level of primary productivity responsible for the diverse marine ecosystems and major fisheries found in the area.
During El Niño the trade winds weaken and warm water moves back east in a slow wave. This leads to a depression of the thermocline in the eastern Pacific and an elevation of the thermocline in the west. Coastal upwelling along South America is no longer able to tap into the cold, nutrient-rich water from beneath it, thus reducing the supply of chemical nutrients to the euphotic zone. As a result of this decrease in nutrients, there is a drastic decline in phytoplankton production, which in turn adversely affects all the creatures at higher levels of the marine food chain.
The weakening of the easterly trade winds during El Niño also affects the climate. As the upwelling slows, the sea surface temperature rises and warms the moist air above the ocean. Rainfall follows the warm water eastward and results in flooding in Peru and drought in Indonesia and Australia. These climatic aberrations are called teleconnections because statistical correlations have been found between these atypical weather events and El Niño. The eastward displacement of the atmospheric heat source changes the global atmospheric circulation. This in turn forces changes in weather in regions far removed from the Tropical Pacific.
The greatest biological impact of El Niño is upon the fisheries in the coastal regions of the eastern Pacific. But the effects of El Niño have been observed in a wide variety of marine life and even as far south as Antarctica, more than 6,000 km away. The decline in coastal upwelling causes a reduction in primary production, which in turn decreases the food available to the natural fish population. These combined with an increasing sea surface temperature during El Niño encourages fish located in coastal areas to migrate north and south in search of cooler waters and food. Migrating fish tend to find themselves in waters much to cold for them to survive. The fish that do not leave the region move from the surface waters deeper into the water column, becoming inaccessible to predators, such as birds and to fishing trawlers using existing fishing gear. Fish unable to migrate die from lack of food or intolerable temperature elevations.
Another factor of El Niño that alters coastal fish populations is the increase in rainfall along the South American coast. This results in an increase in turbidity and a decrease in salinity from an enhanced river discharge which brings with it large amounts of sediment and fresh water. The fish either leave their coastal habitat or die from unendurable water conditions.
It is often reported that the collapse of the Peruvian anchovy industry in the early 1970s was a direct result of the El Niño of 1972-73. During this El Niño, the anchovy population dropped from 20 million to 2 million. Not only did this collapse the industry that was based on this resource, but it also reduced the population of marine birds that fed on the anchovy and allowed sardines and jack mackerel to move into the area which created a multispecific pelagic ecosystem. The 1982-83 El Niño, which was considered the strongest El Niño thus far this century, affected all of the main pelagic resources leading the Peruvian fishery to its lowest record of catches.
In its long-range effects upon fisheries, the 1982-83 El Niño is thought to have been responsible for mackerel migrating further north than usual and preying on juvenile salmon stocks off the coast of North America. It has also been linked to variations in the migration pattern of sockeye salmon in British Columbia, which resulted in increased predation upon the species. In contrast to reduced catches, Alaskan salmon fisherman in the North Pacific Ocean reported bountiful catches. In the western Pacific, increased amounts of phytoplankton in the East China Sea moved the central fishing grounds toward the coast increasing the sardine catch 10 times higher than the average.
The changes in distribution patterns of the fisheries have an outcome on other marine life. In the Weddell seal population in Antarctica, researchers noticed that the number of births decline every four to six years, coinciding with El Niño events. They believe that the seal population decline may result from changes in the fish populations caused by shifts in ocean currents. Following the 1982-83 El Niño, a dramatic reduction in the number of California sea lions occurred at Santa Catalina Island, California. Another change in cetacean population was a decline in pilot whale numbers followed by an increase in abundance of Risso's dolphins. The limited food source in the area resulted in competitive displacement, which prevented co-occurrence of these species in a restricted geographic area.
The 1982-83 El Niño was responsible for many ecological effects on other marine resources. The eastern Pacific region suffered massive coral bleaching, events and mortalities that have tragic results for the coral reef community. The increased sea surface temperatures and the rainfall-induced salinity changes in the water are believed to be the underlying causes for the incidents of coral death in the eastern Pacific. In the western Pacific, the decrease in sea level was responsible for exposing and destroying the upper layers of the coral reefs that surround many islands.
During the 1982-83 El Niño it is estimated that up to 85% of the sea bird population in Peru was killed. The causes of this mortality are difficult to determine because the information available is preliminary and limited. However, some factors which may contribute negatively to the sea bird population are: flooding of nesting sites, changing atmospheric circulation patterns, increasing sea surface temperatures, and migration of their primary food source, fish.
In spite of the destructive nature of El Niño, some marine creatures do benefit form the disturbances brought upon by the phenomenon. As a result of the 1982-83 El Niño, scallops accelerated their growth and reached enormous densities. Purple snails and octopuses became more common and the shrimp fishery reached its highest level. This is most likely the result of increased run-off from the rivers, due to increased rainfall, which provided a greater abundance of nutrients and decreased predation from a dispersed fish population.
The ability to anticipate how climate will change from one year to the next will lead to better management of fisheries, water supplies, agriculture, and other resources. Scientists are now producing numerical prediction models to indicate how the ocean-atmosphere system might evolve over the next few seasons or years. By incorporating climate predictions into management decisions, humankind is becoming better adapted to the irregular rhythms of climate. While the forecasts of El Niño are clearly of direct value to fisheries in the productive regions of Ecuador, Peru, and Chile, more accurate prediction of El Niño will also be very valuable for countries located outside the tropics, such as Japan and the United States. A reduction of the fish population reduces the amount of fishmeal produced and exported (by local industry) to other countries for feeding poultry and livestock. If the world's fishmeal supply decreases, more expensive alternative feed sources must be used, resulting in an increase in poultry and cattle prices worldwide. The foreknowledge of probabilities of anchovy catches would allow, for example, more stable pricing strategies for world supplies of cattle protein and would forgo the type of rapid rise of cattle protein prices after the 1982-83 El Niño.
El Niño forecasts in fisheries management could help coastal ecosystem managers to distinguish between changes in populations due to anthropogenic factors or changes from natural conditions. Although intense fishing pressure remains a major reason many fisheries ultimately become unproductive and uneconomical, fishing is usually neither the sole, nor necessarily even the primary force behind the fluctuations of the marine resource populations affected by El Niño.
Regardless, population collapse and the inability of the fishing industry to recover would certainly be accelerated by further exploitation. More accurate predictions of El Niño could help ecosystem managers to mitigate the cyclic event and prevent increased ecological disturbances. To respond to the impact of an unexpected El Niño by using measures such as closing the fishing season after the population has declined, will continue to be ineffectual and costly.
Recent predictions report that the 1997-98 El Niño is shaping up to be one of the biggest in 50 years. It will provide scientists with a natural laboratory to increase our understanding of El Niño and its aftermath. With widespread distribution of current forecast results and unprecedented levels of mainstream media attention, it is safe to say that the 1997-98 El Niño will be the most closely watched event by scientists and decision-makers alike.