MEPS 608:291-296 (2019)  -  DOI: https://doi.org/10.3354/meps12835

ABSTRACT: We report on an unprecedented, synchronized biennial pattern of birth and mortality in an apex predator inhabiting the eastern North Pacific Ocean—the critically endangered southern resident killer whale Orcinus orca. From 1998-2017, mortality of newborn and older whales was 3.6 times higher (61 versus 17 whales) and successful births 50% lower (16 versus 32 whales) in even years than in odd years as the population decreased from 92 to only 76 whales. Percent mortality was 3.1 times higher in even years during the recent 20 yr period of population decline than during an earlier 22 yr period (1976-1997) of population increase and relative high abundance, whereas mortality in recent odd years was 43% lower. Recognized potential mechanisms of decline (low abundance of a key prey species, Chinook salmon Oncorhynchus tshawytscha, toxic contaminants, and ship noise) cannot explain this biennial pattern. We present evidence that the causal mechanism is indirectly linked to pink salmon (O. gorbuscha), which exhibit a unique and extreme biennial pattern of abundance and interact strongly with other species in marine ecosystems in the North Pacific. Further investigation of this unique biennial pattern in southern resident killer whales is needed to inform recovery efforts for the population.

KEY WORDS: Salish Sea · Orca · Demography · Pink salmon · Chinook salmon · Population recovery · Ecosystem management