Human?fish?supplies from oceans could be at risk by 2050, according to a new study, as?weights for?fish?may fall by 14-24 percent.
Yellow jacks swim in a school of fish off Cocos Island, 300 miles off the Pacific coast.
Melanie Stetson Freeman / The Christian Science Monitor
EnlargeFish?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.
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Average maximum body weights for 600 types of marine?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?William Cheung?of the?University of British Columbia?in?Canada, told Reuters of the findings in the journal Nature?Climate?Change.
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?Canada-based scientists wrote.
They said global warming, blamed on human burning of fossil fuels, will make life harder for?fish?in the oceans largely because warmer water can hold less dissolved oxygen, vital for respiration and growth.
"As the?fish?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?fish?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?fish?live. Rising water temperatures would also add stresses to the metabolic rates of?fish.
The scientists said?fish?stocks were likely to shift from the tropics towards cooler seas to the north and south.
Average maximum sizes of?fish?in the?Indian Ocean?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?Indian Ocean?has most tropical waters of the three.
The study said a computer model projected that ranges for most?fish?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?climate?change, other human factors "such as over-fishing and pollution, are likely to further exacerbate such impacts," they wrote.
Cheung said the shrinking of?fish?would have big but unknown effects on marine food chains. Predatorfish?like cod that swallow prey whole would become less fearsome, perhaps allowing smaller species to thrive.
"Cod ... can only eat?fish?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?climate?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?climate?experts.
"The results will be quite similar," using other scenarios, Cheung said.
(Reporting By Alister Doyle; Editing by Sophie Hares)
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