VICTORIA -- A new report suggests resident killer whales that live in the waters off Vancouver Island are growing smaller in length.

The new research, recently published in the peer-reviewed scientific journal, Endangered Species Research, shows that a reduction in Chinook salmon populations dating back to the 1990's may have resulted in the stunting of early growth for endangered southern resident killer whales.

The study was conducted from 2014 to 2017 and documented that some adult orcas were growing to smaller lengths in recent decades.

"We've known for a long time just how important Chinook salmon are to southern resident killers whales, but, this now gives us an idea that it's not just the Chinook salmon but the really big Chinook salmon, and those of course are fewer and farther between," said marine mammal zoologist Dr. Anna Hall.

"This gives a new focus, as far as I can see, to conservation efforts for large salmon."

The study used drones to capture images of 78 individual southern residents over the four year period. The whales were identified by their distinct "saddle patch" pigmentation. The length of each whale was estimated by measuring the dimensions from both the snout to dorsal fin and the dorsal fin to fluke. The measurements revealed adult whales under the age of 40 were slightly less than half a metre shorter than older adults.

"I think it is really most pronounced with the southern residents, but, when compared to other parts of the world we are not seeing this same trend with a size reduction of young animals, maintaining a small overall body size than what their relatives from decades ago experienced, " said Hall. "This really demonstrates the importance of having an adequate prey base that is of adequate size, this is a new idea and an extremely important one."

The growth data collected suggests the effects of nutritional stress by the limited population and size of Chinook salmon is not only deadly for the southern resident killer whales, but could also have long term consequences on the condition of the whales.

Killer whales tend to target large salmon because they deliver more calories. It is estimated that killer whales consume more than 2.5 million Chinook salmon a year. That total represents more fish than fisherman catch in the commercial, recreational and subsistence fisheries combined.

"We know full well just how closely the Chinook salmon and the southern resident killer whales tack with Chinook salmon populations," said Hall. "Now we have this new incredibly valuable piece of information."

"What can we do to help manage both the Chinook salmon and consequently the resident killer whales both northern and southern?" said Hall.

The reduced size and numbers of Chinook salmon indicates resident killer whales have to work harder to get enough to eat. The southern residents have been listed as an endangered species since 2005. The population is currently at a 30 year low with only 73 orcas remaining.