New research on Southern Resident orcas highlights risks to population

Two recently published papers provide new information about the highly endangered Southern Resident orca population and threats to their survival and recovery.  This unique community of orcas lives in the Pacific Ocean off the coast of the U.S. and Canada, and is primarily threatened by prey depletion, pollution, and acoustic and physical disturbance.  Previous research has shown a correlation between the survival and reproductive success of Southern Resident orcas and the abundance of their preferred prey, Chinook salmon.

Changes in body condition

A collaborative aerial photogrammetry research program takes photos of the orcas from the air and uses the images to measure the size of individual orcas.  Comparing photos from different years or seasons can show changes in the size and body condition of orcas, which can tell researchers and recovery managers when and where the Southern Residents don’t have enough to eat.  The images can also identify pregnant females, contributing to data on reproductive success – for example, if a female Southern Resident is observed to be pregnant from the air but not later seen with a calf, the pregnancy was unsuccessful.

Females are vital to the recovery of endangered populations because of their ability to add new members.  Growing and nursing calves is a significant energy investment, and females need a reliable and abundant supply of food to support the energetic demands of pregnancy.  The photographs revealed that most of the orcas with significant changes in body size over a 5-year period were reproductive-aged females.

Small population risks

Another significant (but hard to quantify) risk to the Southern Resident population is the threat of their small population.  Fewer individuals mean there is a greater risk of inbreeding or for a catastrophic event, such as an oil spill or disease outbreak, to devastate the population. 

Previous research has established that older, larger male Southern Resident orcas are more successful at producing calves.  A new paper on paternity in the population has found that just two male Southern Residents have fathered more than half of the calves born since 1990.  The impact of this small number of successful parents, and the risk of inbreeding, is uncertain.  What may be more concerning is the observed change in birth rates in the Southern Resident population – only a handful of the reproductive-aged females are actually having calves, and those births are occurring at longer intervals than they used to, now with one calf about every 10 years.

The lead author of the paper emphasizes that the genetic results are not a reason to give up hope on the population.  There are still many unknowns in how the risk of inbreeding affects individual orcas and the overall population.