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

Abstract

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.

Traditional summer habitat use by Southern Resident killer whales in the Salish Sea is linked to Fraser River Chinook salmon returns

“…While the abundance of the SRKW population also declined from 2004 to 2020, the relative magnitude of that decline is far smaller than the decline in core summer habitat occupancy both overall and at the pod level (Table 2), suggesting that the decline in occupancy is not driven by patterns in SRKW abundance. Our findings corroborate a recent study showing that the phenology of SRKW visitation to the San Juan Islands (within the traditional core summer habitat area) is linked to the timing of Fraser River Chinook returns, and has shifted later in the summer season as spring returns have declined dramatically (Ettinger et al, 2022). The potential impact of declining prey availability on SRKW foraging strategies was highlighted in 2019 and 2020, when despite a modest increase in mean Fraser River Chinook CPUE, SRKW presence in the core summer habitat remained near its minimum level for the study period.…”

“…It is possible that our selection of a 10-day lead period was inadequate to capture the migratory period of a salmon between the SRKW core summer habitat and the Albion test fishery, although we did explore a range of leads from 0 to 21 days, all of which had similarly nonsignificant relationships between SRKW attendance and daily Albion CPUE. We suggest that a more likely explanation is that SRKW are not responding to prey availability at such fine (i.e., daily) timescales, but are instead responding to seasonal patterns in Chinook returns, as suggested by Ettinger et al (2022). Further, if daily Chinook returns are generally high but have periods of low abundance between pulses, this overall prey availability may be sufficient for SRKW to remain within the core summer habitat but may obscure estimated relationships with daily returns and be better reflected in average returns, as indicated by our model results.…”

“…for 2004-2020 (Figure4a-c) and the count-based GAM for 2014-2020 (Figure6a-c) mirror the phenological shifts described inEttinger et al (2022). J pod peak attendance shifted slightly later in the summer in the 2014Relationships between the number of Southern Resident killer whales observed in the traditional core summer habitat and mean annual Albion test fishery Chinook CPUE (count model).…”

“…We then estimated missing values using a simple linear interpolation (Figure 2, Table S1). Finally, we moved the observed and interpolated CPUE values forward by 10 days to account for the approximate time it would take a Chinook salmon to migrate from the central San Juan Islands to the mouth of the Fraser River (Ayres et al, 2012; Ettinger et al, 2022). In addition to daily CPUE values, we calculated the average daily CPUE in the Albion test fishery from April 1 to October 31 for each year from 2003 to 2020, excluding interpolated values.…”

“…The summer and fall diets of SRKW, in particular, appear to be composed largely of Chinook salmon returning to tributaries in the Salish Sea, including the Fraser River and Puget Sound (Hanson et al, 2021). Similarly, analyses of body condition (Stewart et al, 2021), demographic rates (Vélez‐Espino et al, 2014), and the timing of SRKW visitation to the Salish Sea (Ettinger et al, 2022) all suggest that SRKW health, population dynamics, and movement patterns may be influenced by Fraser River and Puget Sound Chinook returns. Traditionally, all three matrilineal family units have been found regularly in the central Salish Sea between April and October (Olson et al, 2018), in what has been designated as their core summer habitat (National Marine Fisheries Service, 2008; Figure 1).…”