Shifting phenology of an endangered apex predator mirrors changes in its favored prey

The timing, or phenology, of predator activity in relation to their prey is critical for survival and fitness, yet rarely quantified for marine species, even those of conservation concern. We use a large database of professional and crowd-sourced observations analyzed with hierarchical spline occupancy models to quantify seasonal variation in occurrence of an endangered apex predator, the southern resident killer whale (SRKW) Orcinus orca, in inland waters of the northeast Pacific Ocean. We find that timing of SRKW occurrence has shifted in their summer core habitat within the central Salish Sea: the day of year of peak occurrence probability shifted later at rates of 1-5 d yr-1 over 2001-2017 (resulting in shifts of 17-85 d across this 17 yr time period). These shifts are consistent with shifts in their preferred prey, Fraser River Chinook salmon Oncorhynchus tshawytscha, as the relative number of fish returning to spawn in the spring has declined compared to numbers returning in summer and fall. The shift in timing of fall/winter SRKW occurrence outside the summer core habitat, however, is not consistent with shifts in other prey populations (Chinook, coho Oncorhynchus kisutch, chum Oncorhynchus keta salmon) returning to nearby rivers. Our findings demonstrate the complexity of consumer phenological responses and highlight gaps in our understanding of links between management actions that affect resource phenology and consequences for organisms relying on those resources.