Killer whales redistribute white shark foraging pressure on seals

Abstract

Predatory behavior and top-down effects in marine ecosystems are well-described, however, intraguild interactions among co-occurring marine top predators remain less understood, but can have far reaching ecological implications. Killer whales and white sharks are prominent upper trophic level predators with highly-overlapping niches, yet their ecological interactions and subsequent effects have remained obscure. Using long-term electronic tagging and survey data we reveal rare and cryptic interactions between these predators at a shared foraging site, Southeast Farallon Island (SEFI). In multiple instances, brief visits from killer whales displaced white sharks from SEFI, disrupting shark feeding behavior for extended periods at this aggregation site. As a result, annual predations of pinnipeds by white sharks at SEFI were negatively correlated with close encounters with killer whales. Tagged white sharks relocated to other aggregation sites, creating detectable increases in white shark density at Ano Nuevo Island. This work highlights the importance of risk effects and intraguild relationships among top ocean predators and the value of long-term data sets revealing these consequential, albeit infrequent, ecological interactions.

Figure 1

Spatial and temporal overlap of two top predators, white sharks (Carcharodon carcharias), and killer whales (Orcinus orca), and their shared prey, juvenile elephant seals (Mirounga angustirostrous), in the Northeastern Pacific (see Supplement) and at Southeast Farallon Islands (SEFI). Seasonally concentrated activity of each species at SEFI (*) evident from (A) weekly M. angustirostrous counts between March and December (1987–2013), (B) daily mean number of tagged C. carcharias detected (2007–2013) with shaded standard error and, (C) monthly frequency of O. orca observed (1987–2013). Note the two predators co-occur only during the fall peak. Map was created using R software (v3.5.1; https://www.R-project.org/).

Figure 2

Predator-prey relationship between white sharks (Carcharodon carcharias) and elephant seals (Mirounga angustirostrous) altered by the presence of killer whales (Orcinus orca) at Southeast Farallon Island (SEFI). (A) Annual predation rate by C. carcharias as a function of mean fall (Sept. – Nov.) M. angustirostrous counts fit with a log-log regression line (dashed black line) showing confidence interval (dashed blue lines). Points are years where no flight response was detected, and triangles are the years in which a flight response was observed, near or before the peak of the C. carcharias season (≤November 2, inverted triangles), and near the end of the season (≥November 19, upright triangles). For comparison, an equivalent regression fit excluding flight years is shown with red dotted lines. (B) Seasonal C. carcharias kill rate as a function of the observed distance of O. orca activity to SEFI. The distribution of observed predations was reduced and truncated as a function of O. orca proximity to the common foraging ground (distance given in legend in km).

Figure 3

The flight response of white sharks (Carcharodon carcharias) triggered by the presence of killer whales (Orcinus orca) at a common foraging site, Southeast Farallon Islands (SEFI). (A) Mean daily number of acoustic tagged C. carcharias detected (2007–2013; excluding 2009; shaded standard error) at Central California receivers colored by location: Tomales Point (green), Southeast Farallon Islands (orange and orange/yellow), Año Nuevo Island (blue), and Point Reyes (purple). (B) The number of tagged C. carcharias detected per day at each site (respectively colored) during the 2009 season showing the sudden departure of all tagged individuals from SEFI in response to O. orca (Nov 2) presence. Note the subsequent influx around Año Nuevo Island where the shaded orange area represents individuals present at SEFI during killer whale interactions. (C) Detections of each tagged shark at color-coded locations are shown along the horizontal timeline illustrating the abrupt departure from SEFI by tagged C. carcharias following O. orca presence (between vertical black lines) and subsequent avoidance. Solid orange diamonds indicate the western SEFI receiver while orange with yellow centers indicate the eastern receiver. (D) Precise receiver locations are indicated by the right corner of each solid diamond and the left corner of the yellow filled diamond.