There’s little debate that an insufficient lack of salmon is harming the Southern Resident killer whale population, evinced by proposals to ramp up hatchery production currently at historic lows. However, the lack of food may not be the sole cause for the increased mortality rate among orca calves during certain years. A new study by fisheries scientists has found a possible connection between higher calf deaths and the migration patterns of pink salmon.
However, the authors stress that further research is needed before any firm conclusions can be made. It’s a possibility that stakeholders may want to explore as part of a statewide holistic approach to helping the killer whales recover.
The plight of the endangered species’ newborns was vividly demonstrated last year when an orca whale carried her dead calf for 17 days, garnering national attention.
In their paper published by the Marine Ecology Progress Series, authors Gregory T. Ruggerone, Alan M. Springer, Leon D. Shaul and Gus B. van Vliet examined the mortality rates of newborn and older orca between 1998-2017. They found that the death rate for older whales was 3.6 times higher in the even years than odd years, while the successful births were 50 percent lower during the odd years.
While other factors such as ship noise and low salmon supply can contribute to killer whale mortality, the authors concluded that these “cannot explain this biennial pattern.”
To help explain the phenomenon, the authors “suspect” that the higher death rates are “indirectly linked to the extreme biennial pattern in abundance of adult pink salmon…in the Salish Sea.”
Their argument: annual returns of pink salmon are disproportionately high in odd years. “This is the only biennial pattern in the physical or biological environment of the northeastern Pacific Ocean of which we are aware that could potentially drive the demographic pattern observed in SRKWs.”
The paper notes that the connection is indirect, because killer whales rarely consume pink salmon. However, pink salmon’s migration patterns had a “significant” effect on the survival rates of juvenile Chinook salmon, which composes most of killer whale diet. One possibility is that the overabundance of pink salmon during the odd years make it hard for orcas to eat an already low supply of migrating summer and early fall Chinook salmon during a critical two-month period. In the Salish Sea, there are 50 times more pink salmon than co-migrating Chinook.
A previous study coauthored by Ruggerone, a Natural Resources fisheries scientist, found that there are more Pacific salmon now than ever recorded, and pink salmon make up a disproportionate percentage of them. The study further concluded that the ocean has reached the maximum amount of salmon it can support.
“The finding that lower nutritional status of SRKWs is linked to higher rates of miscarriages and mortalities of newborns…supports our observation of higher newborn mortality in even years,” the study stated.
Another possibility is that pink salmon augment the killer whales’ diet and as a result lead to a lower mortality rate in odd years.
The authors also noted that despite “compelling evidence” that “climate variability, at various scales, is correlated with variability in salmon production and abundance dynamics…we cannot envision how climate would initiate and maintain such an extreme, synchronized biennial signal in SRKWs.”
“Increased understanding of SRKW demographics in relation to this biennial pattern will provide greater opportunities for advancing their recovery,” the authors conclude.