Sharks, tunas, marlins and other large fish at risk from spreading "dead zones," say scientists
Oxygen in the oceans is being lost at an unprecedented rate, with the proliferation of "dead zones" and hundreds of other areas showing dangerous oxygen depletion as a result of the climate emergency and intensive agriculture, experts warned.
Sharks, tuna, marlin and other large fish species are at particular risk, the scientists said, with many vital ecosystems in danger of collapse. Dead zones, where oxygen is effectively absent, have quadrupled in extent in the last half century, and there are also at least 700 areas where oxygen is at dangerously low levels, compared to 45 found when research was conducted in the 1960s.
The International Union for Conservation of Nature presented the findings Saturday at the UN climate conference in Madrid, where governments are halfway through tense negotiations aimed at tackling the climate crisis.
Grethel Aguilar, the acting director general of IUCN, said the health of the oceans should be a key consideration for the talks. "As ocean warming causes it to lose oxygen, the delicate balance of marine life is in disarray," he said. "The potentially serious effects on fisheries and vulnerable coastal communities mean that the decisions made at the conference are even more crucial."
All fish need dissolved oxygen, but larger species are particularly vulnerable to depleted oxygen levels because they need so much more to survive. Evidence shows that depleted levels force them to move to the surface and into shallow areas of the sea, where they are more vulnerable to fishing.
Some areas of the ocean are naturally lower in oxygen than others, but these are even more susceptible to damage when their oxygen levels drop further, the report's authors said. Species that can more easily tolerate low oxygen levels, such as jellyfish, some squid, and marine microbes, can flourish at the expense of fish, upsetting the balance of ecosystems. The natural ocean cycles of phosphorus and nitrogen are also at risk.
Already the world's oceans are being overfished and assaulted by a rising tide of plastic waste, as well as other pollutants. The seas are about 26% more acidic than in pre-industrial times due to the absorption of excess carbon dioxide into the atmosphere, according to the Intergovernmental Panel on Climate Change, with damaging impacts on shellfish in particular.
Low oxygen levels are also associated with global warming, because warmer water retains less oxygen and warming causes stratification, so there is less vital mixing of oxygen-rich and oxygen-poor layers. The oceans are expected to lose about 3-4% of their oxygen by the end of this century, but the impact will be much greater at levels closest to the surface, where many species are concentrated, and at mid to high latitudes.
Intensive agriculture also plays an important role. When excess artificial fertilizer from crops, or manure from the meat industry, escapes from the land into rivers and seas, it feeds algae that bloom and then causes oxygen depletion as they decompose.
The problem of dead zones has been known for decades, but little has been done to address it. Farmers rarely bear the brunt of the damage, which mainly affects fishing fleets and coastal areas. Two years ago, the meat industry in the United States was found to be responsible for a massive dead zone measuring more than 8,000 square miles in the Gulf of Mexico.
This year's UN climate conference, known as COP25, was originally billed as the "Blue COP," with a focus on the oceans for the first time in the history of the negotiations. The approach was chosen due to the original location in Chile, a country with more than 4,000 km of coastline and a heavy dependence on the marine economy.
But the move to Madrid, forced by political unrest in Santiago, has meant that many of the planned events have been curtailed. Scientists and activists gathered on the Madrid coastline are trying to highlight the problems by showing how vital the seas are in protecting us from climate chaos, as they absorb so much excess carbon dioxide and excess heat in the atmosphere, and how much is at risk for its impacts.
Protecting marine life could help the oceans function better, absorbing more carbon and providing barriers against sea level rise and storm surges, in the form of coral reefs and mangroves.
“A healthy ocean with abundant wildlife is capable of significantly reducing the rate of decomposition of the climate,” said Dr. Monica Verbeek, executive director of the Seas at Risk group. “To date, the most profound impact on the marine environment has come from fishing. Ending overfishing is quick and deliverable action that will restore fish populations, create more resilient ocean ecosystems and decrease CO2 pollution and increase carbon sequestration, and offer more profitable fisheries and prosperous coastal communities. "
“Ending overfishing would strengthen the ocean, making it better able to withstand climate change and restore marine ecosystems, and it can be done now,” explained Rashid Sumaila, professor and director of the Fisheries Economics research unit at the University of British Columbia. “The crises in our fisheries and in our oceans and climate are not mutually exclusive problems that are tackled separately; it is imperative that we move forward with comprehensive solutions to address them ”.
A study published at COP25 by Greenpeace International showed that the restoration of marine ecosystems could play an important role in combating climate chaos.