Oxygen-starved fish to shrink significantly due to global warming
| OSLO
Fish聽are likely to get smaller on average by 2050 because global warming will cut the amount of oxygen in the oceans in a shift that may also mean dwindling catches, according to a study on Sunday.
Average maximum body weights for 600 types of marine聽fish, such as cod, plaice, halibut and flounder, would contract by 14-24 percent by 2050 from 2000 under a scenario of a quick rise in greenhouse gas emissions, it said.
"The reductions in body size will affect whole ecosystems," lead author聽William Cheung聽of the聽University of British Columbia听颈苍听Canada, told Reuters of the findings in the journal Nature听颁濒颈尘补迟别听颁丑补苍驳别.
His team of scientists said a trend towards smaller sizes was "expected to have large implications" for ocean food webs and for human "fisheries and global protein supply."
"The consequences of failing to curtail greenhouse gas emissions on marine ecosystems are likely to be larger than previously indicated," the U.S. and聽Canada-based scientists wrote.
They said global warming, blamed on human burning of fossil fuels, will make life harder for聽fish聽in the oceans largely because warmer water can hold less dissolved oxygen, vital for respiration and growth.
"As the聽fish聽grow bigger and bigger it will be difficult to get enough oxygen for growth. There is more demand for oxygen as the body grows. At some point the聽fish聽will stop growing," Cheung said of the study, based on computer models.
GASPING
As water gets warmer, it also gets lighter, limiting the mixing of oxygen from the surface layers towards the colder, denser layers where many聽fish聽live. Rising water temperatures would also add stresses to the metabolic rates of聽fish.
The scientists said聽fish聽stocks were likely to shift from the tropics towards cooler seas to the north and south.
Average maximum sizes of聽fish聽in the聽Indian Ocean聽were likely to shrink most, by 24 percent, followed by a decline of 20 percent in the Atlantic and 14 percent in the Pacific. The聽Indian Ocean聽has most tropical waters of the three.
The study said a computer model projected that ranges for most聽fish聽populations would shift towards the poles at a median rate of 27.5 km to 36.4 km (17.1-22.6 miles) a decade from 2000 to 2050.
Adding to聽climate聽change, other human factors "such as over-fishing and pollution, are likely to further exacerbate such impacts," they wrote.
Cheung said the shrinking of聽fish聽would have big but unknown effects on marine food chains. Predatorfish聽like cod that swallow prey whole would become less fearsome, perhaps allowing smaller species to thrive.
"Cod ... can only eat聽fish聽that can fit into their mouth. They are not like lions or tigers" that can attack animals that are larger than they are, he said.
The聽climate聽scenario used in the study would mean an increase in world temperatures of between 2 and 5.4 degrees Celsius (3.6 to 9.7 Fahrenheit) by 2100, the second biggest gain of six scenarios used by the U.N. panel of聽climate聽experts.
"The results will be quite similar," using other scenarios, Cheung said.
(Reporting By Alister Doyle; Editing by Sophie Hares)
