A species of ocean plankton vital to the food chain is facing extinction with rising sea temperatures and may take fisheries with it, Australian scientists say.

Warming oceans are a risk to Calanoid copepods, a plankton species that is a significant food source for fish larvae and therefore important for all commercial fisheries, they said.

Scientists at Deakin University, with colleagues from Britain, said a cold-water plankton species in the North Atlantic, a critical food source for fish such as cod and hake, is in decline as the oceans warm with climate change.

"There is overwhelming evidence that the oceans are warming and it will be the response of animals and plants to this warming that will shape how the oceans look in future years and the nature of global fisheries," Deakin marine science professor Graeme Hays said.

The researchers analyzed 50 years of data from the North Atlantic on the distribution and abundance of two very common but contrasting species of ocean plankton, Calanus helgolandicus that lives in warmer water and Calanus finmarchicus that lives in cold water.

Both are vital food for fish and underpin many commercial fisheries in the North Atlantic region.

"We know that warm water species are expanding their ranges as warming occurs, and vice versa. What is not known is whether species are able to adapt to new temperatures," Hays said. "Will, for example, cold water species gradually adapt so they can withstand warming seas and not continually contract their ranges?

"From the results of our study, it is looking like the answer is no."

The cold water C. finmarchicus has experienced an ongoing contraction of its range over 50 years of warming, the researchers found.

"In other words, even over 50 generations [each plankton lives for one year or less] there is no evidence of adaptation to the warmer water," Hays said.

The study suggests cold-water plankton will continue to become scarcer as their ranges contract to the poles, and ultimately disappear, the researchers said.