Fish 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 sizes of fish in the Indian Ocean were likely to shrink most, by 24 per cent, followed by a decline of 20 per cent in the Atlantic and 14 per cent in the Pacific. The Indian Ocean has most tropical waters of the three.

The scientists said fish stocks were likely to shift from the tropics towards cooler seas to the north and south.

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.