In a report, WWF International said that hotter water means less food, less offspring, less oxygen and more disease for marine and freshwater fish, and that other species, including humans, will feel the impact.

"The balance is set to tip, as climate change continues the pressure on fish populations already strained by overfishing, pollution and habitat loss," said Katherine Short of the WWF's Global Marine Programme.

"We must act to protect fish," she said. "They are one of our most valuable biological, nutritional and economic assets."

The report said that warmer temperatures are expected to stunt the growth of some fish, resulting in fewer offspring.

Fish metabolisms normally speed up in line with temperature increases, accelerating competition for food -- and insufficient food supplies can slow growth and reproduction rates, the WWF study said.

In addition, some temperate fish such as salmon, catfish and sturgeon cannot spawn at all if winter temperatures do not drop below a certain level, the study said.

In lakes in particular, warmer temperatures hamper the circulation of the oxygen that fish need to breathe. Fish filter oxygen from water, but the amount of oxygen dissolved in water decreases as temperatures rise.

The impact on other species is another cause for concern, the WWF said.

Warming could cause fish to move to deeper, cooler waters as they seek to maintain the temperature normal for their habitat, and this can leave other species in dire straits.

In the Gulf of Alaska in 1993, as fish moved into cooler waters, around 120,000 sea birds starved to death as they were unable to dive deep enough to reach their relocated prey, the WWF noted.

Worldwide, marine and freshwater fisheries generate more than 130 billion dollars annually, employ at least 200 million people, and feed billions reliant on fish as an important source of protein.