Task: Question Answering
Input (English): Context: Fish species are expected to shrink in size by up to 24% because of global warming, say scientists. The scientists argue that failure to control greenhouse gas emissions will have a greater effect on marine ecosystems than previously thought. Previous research has suggested that changing ocean temperatures would affect both the distribution and the reproductive abilities of many species of fish. This new work suggests that fish size would also be heavily affected. The researchers built a model to see how fish would react to lower levels of oxygen in the water. As ocean temperatures increase, so do the body temperatures of fish. But, according to lead author, Dr William Cheung, from the University of British Columbia, the lower level of oxygen in the water is key. Warmer waters could decrease ocean oxygen levels and greatly reduce fish body weight. On the other hand, rising temperatures directly increase the metabolic rate of the fish's body function. This leads to an increase in oxygen demand for normal body activities. So the fish will run out of oxygen for growth at a smaller body size. The research team also used its model to predict fish movements as a result of warming waters. The group believes that most fish populations will move towards the Earth's poles at a rate of up to 35km per decade. "So in, say, the North Sea," says Dr Cheung," one would expect to see more smaller-body fish from tropical waters in the future." Taking these factors into consideration, the research team concludes that fish body size will shrink between 14% and 24%, with the largest decreases in the Indian and Atlantic oceans. When compared with actual observations of fish sizes, the model seems to _ what's actually happening in the seas. The researchers looked at two case studies involving North Atlantic cod and haddock. They found that recorded data on these fish showed greater decreases in body size than the models had predicted. Dr Alan Baudron, from the University of Aberdeen, UK, believes smaller-sized fish could seriously affect the ability of fish to reproduce. "Smaller fish produce fewer and smaller eggs which could affect the reproductive potential of fish stocks," he said. 
 Question: What is the key factor making the fish smaller now?
Output (English): the lower level of oxygen in the water.