Large marine animals are more likely to become extinct than smaller creatures, according to a new study that blames human behaviours, such as fishing, for this unprecedented pattern of extinction in oceans.

"We have found that extinction threat in the modern oceans is very strongly associated with larger body size," said Jonathan Payne, from the Stanford University in the US.

"This is most likely due to people targeting larger species for consumption first," said Payne.

Researchers examined the association between extinction threat level and ecological traits such as body size for two major groups of marine animals – mollusks and vertebrates – over the past 500 years and compared it with the ancient past, stretching as far back as 445 million years ago and with a particular emphasis on the most recent 66 million years.

"We used the fossil record to show, in a concrete, convincing way, that what is happening in the modern oceans is really different from what has happened in the past," said Noel Heim, a postdoctoral researcher in Payne's lab.

Researchers found that the modern era is unique in the extent to which creatures with larger body sizes are being preferentially targeted for extinction.

"What our analysis shows is that for every factor of 10 increase in body mass, the odds of being threatened by extinction go up by a factor of 13 or so. The bigger you are, the more likely you are to be facing extinction," Payne said.

The selective extinction of large-bodied animals could have serious consequences for the health of marine ecosystems, scientists said, because they tend to be at the top of food webs and their movements through the water column and the seafloor help cycle nutrients through the oceans.

While researchers did not directly examine why large modern marine animals are at higher risk of extinction, their findings are consistent with a growing body of scientific literature that point to humans as the main culprits.

It is a pattern that scientists have seen before. On land, for example, there is evidence that ancient humans were responsible for the massacre of mammoths and other megafauna across the globe.

"We see this over and over again. Humans enter into a new ecosystem, and the largest animals are killed off first," Heim said.

"Marine systems have been spared up to now, because until relatively recently, humans were restricted to coastal areas and did not have the technology to fish in the deep ocean on an industrial scale," he said.

The study appears in the journal Science.