Global warming has had more impact on fisheries relying on short life species such as shrimp or sardine, because it affects chlorophyll production which is vital for phytoplankton, the main food for both species. This is the conclusion of a research on the impact of global warming on the fishing resources in Mexican Pacific published on March 7, led by Ernesto A. Chávez Ortiz from the National Polytechnic Institute (IPN).
The research compared historical data regarding fisheries, available since 1950, from The Food and Agriculture Organization of the United Nations (FAO) to the data of weather variability and concluded a lot of the variabitlity in the fishing in the Mexican Pacific Ocean is caused by climate change. The sardine production increases in the 1970s, but decreased below average levels in the 1980s. While shrimp fishing increased above average but decreased in the 1990s. All these were affected by the climate change, possibly due to El Niño. In the case of shrimp, the production is linked to an input of water from the continent. A good raining season means an increase in the crustacean production, which is reduced by a dry season.
Even the benefits from global warming on fisheries previously thought to be true was proven wrong. New surface areas of the Arctic Ocean got opened up due to the earlier warming and later freezing. Besides providing easier access to oil and gas deposits, people thought it would increase the fish catch from the Arctic waters, which is currently 20 percent of the world’s catch, as warmth and light would bring nutrients to the newly open water. There are three types of algae. Ice algae are algae attaching to ice sheets and accounts for half of the living masses in Arctic waters. Phytoplankton (unattached algae) and zooplankton (tiny animals) are the bases of the food chain in the waters. All these three kinds are flourishing due to longer summer and more light in the water. These have created food sources north of the historic ground and are the base of the high hope for more catch.
There are three reasons such hope is bound for disappointment. The first reason requires an understanding of “pelagic” and “benthic”—the former means fish lives near the surface of the water and the latter means fish lives near the bottom. Benthic fish that can live in the bottom of Beaufort cannot survive in the central Arctic (depth difference is 200 meters vs. 4,000 meters). The second reason is the ocean acidification. Cold water absorbs carbon dioxide faster and the new opened ocean provides bigger absorbing area. The ocean turns carbon dioxide into carbonic acid, discouraging the formation of calcium carbonate which gives shells strength. This means less shelled organisms and less food for fish. The last and most important reason is the warming ocean may intensify the ocean stratification, defined as the tendency of seawater to separate into layers because fresh water is lighter than salt and cold water heavier than warm.
The Ocean Stratification poses danger to disrupt nutrients cycles critical to the output of the ocean. Most ocean creatures are pelagic and when they die, they sink to the bottom and are eaten by benthic creatures. The nutrients are fed back to the surface mainly through upwelling of water from the bottom, caused caused by the collision of cold and temperature water. The Arctic is home to two of the most important nutrient moving locations. Tropics water is nutrient-poor due to lack of such events. Stratification threatens this recycling system by suppressing the vertical movement of the water and is worsened by global warming. Some parts of the Arctic already showed bad stratification, limiting nutrient replenishment, especially at high latitudes which are going to get worse with global warning, therefore the fisheries will not get the benefits people expected.
By Tina Zhang