A surprising 60-year boom in global octopus, squid and cuttlefish numbers points to long-term changes taking place in the world's oceans, scientists say.

Key points: Cephalopods are very adaptable to changing environmental conditions

Cephalopods are very adaptable to changing environmental conditions Numbers have consistently increased since the 1950s

Numbers have consistently increased since the 1950s Increase is likely to be due to a combination of environmental changes

Research published in Current Biology today shows a steady increase in the world cephalopod population — the class of molluscs comprising octopus, squid and cuttlefish — since the 1950s, at a time of increased fishing, growing pollution and ocean warming.

The data analysis, led by Dr Zoe Doubleday from Australia's Environment Institute at the University of Adelaide, has confounded previous expectations that cephalopod populations go through cyclical booms and busts.

"Anecdotal evidence had suggested the population may experience cyclical booms and busts over time, but there is instead a very consistent increase," she said.

Dr Doubleday and her colleagues were prompted to look at patterns in cephalopod population trends when searching for the cause of a dramatic drop in giant cuttlefish numbers in South Australian waters in 2013.

They collected information on changes in cephalopod numbers from fisheries and ocean monitoring bodies around the world and were able to create a timeline of the rates at which they have been caught, or counted in sampling, dating back to 1953.

"We were surprised to see such a steady increase in the numbers over that entire period," Dr Doubleday said.

The results show the increase is consistent across six different families and across 35 species living in very different parts of the ocean, from coastal shallows to open waters.

'Weeds of the sea' adapt to change

Cephalopods such as this squid are highly responsive to environmental change. ( Nhobogood/Wikimedia Commons )

Cephalopods are sometimes called the "weeds of the sea" for their ability to adapt quickly to changing environmental conditions and fill any available ecological space.

They are able to do this thanks to a short lifespan — one to two years — rapid continuous growth, and physiology that can change according to the conditions in which the animal lives.

Earlier research has shown that octopus, squid and cuttlefish are highly responsive to environmental change and while human impact on the oceans may be detrimental for much sea life, changes such as over-fishing and ocean warming may be benefiting cephalopods.

Warmer sea temperatures have been linked to increased breeding rates and overfishing may be reducing the number of predators.

"It is unlikely to be any one factor that is causing the increase, but a combination of different changes," Dr Doubleday said.

"However, our results suggest large-scale directional processes — changes that are consistently moving in one direction — are responsible."

Figures based on 'solid data'

Associate Professor Kylie Pitt from Griffith University has done similar work looking at population change in jellyfish. She says the cephalopod figures are based on very solid data.

"The strength of this paper is that they have a lot of data sets that are independent of fishers and both the fisheries information and the fisheries-independent information have shown increasing trends," she said.

Looking at the results, she said the consistent increase was particularly surprising given they were quite a heavily fished family of animals.

"The increase might be even more extreme on the fished species if the fishing wasn't happening," Professor Pitt said.

What may happen to cephalopod populations in the future is difficult to predict, particularly if fishing pressure continues to increase.

Dr Doubleday and her team are now investigating the factors responsible for cephalopod boom.