Fig 1.
Study area and submarine canyon system of Monterey Bay, California.
Base map Reprinted from NCEI under a CC BY license, with permission from NOAA, original copyright 2022.
Fig 2.
Geographic locations of recorded observations of transient killer whales in Monterey Bay, California.
(A) Distribution of recorded observations (sightings and surveys) where each white circle represents an independent observation of the presence of one or more whales (n = 261 recorded observations; 2006–2021); and the inserted map indicates the location of the main study area off the central coast of California. (B) Relative spatial densities of transient killer whales based on recorded observations (panel A) showing hotspots over the Monterey Submarine Canyon System. (C) Locations where predation by transient killer whales occurred on 9 species of marine mammals (n = 87; 2006–2021). (D) Tracks of the research vessel as it followed groups of transient killer whales (n = 100 focal follows; 2006–2018) that were typically <50 m from the research vessel. Data are visualized using kernel density against NOAA 10-meter resolution bathymetry. Base maps reprinted from NCEI under a CC BY license, with permission from NOAA, original copyright 2022.
Fig 3.
Group sizes of transient killer whales in Monterey Bay, California from 2014–2021.
Group sizes ranged from 1–28 individuals, with the most frequently observed group size being 6 whales (mode = 6, ± SE = 9.71 ± 0.38, n = 323 occurrences).
Fig 4.
Seasonal presence of transient killer whales relative to the availability of prey species.
(A) Number of occurrences of transient killer whale by month in Monterey Bay, California, 2014–2021 (n = 323 sightings); and (B), mean group sizes with standard error bars relative to (C) seasonal presence (grey shading) of California sea lions, grey whale calves, and northern elephant seals. Each occurrence is a specific matrilineal transient family, and prey phenologies are adapted from [16–18].
Table 1.
Activity budget of transient killer whales based on 270 hours of behavioural observations.
Fig 5.
Mean monthly numbers of transient killer whales and grey whale calves.
Numbers (± SE) of individually identified transient killer whales were observed in Monterey Bay (solid black line) and mean numbers of grey whale calves were counted at the Piedras Blancas field station off the central coast of California (dashed grey line) from 2014–2021. Grey whale calf census data from [18–20].
Fig 6.
Detailed movements of transient killer whale groups during focal follow surveys, 2006–2018.
These examples show n = 4 of 100 focal follows in Monterey Bay, California that is typical of (A) travelling, (B) feeding, (C) open water foraging, and (D) shelf break foraging. Groups of whales were usually followed within 50 m while recording whale behaviours and georeferenced data. Each red circle represents a georeferenced location of the research vessel when an accompanying transient killer whale group surfaced, and arrows indicated direction of movement. Note scale differences between panels. Base map reprinted from NCEI under a CC BY license, with permission from NOAA, original copyright 2022.
Fig 7.
Frequency with which 10 species of prey were killed by transient killer whales during 87 predation events observed during focal follow surveys and reports by whale watchers in Monterey Bay, California 2006–2021.
California sea lions and grey whale calves accounted for >60% of the kills. Seabird species are combined.
Table 2.
Prey species targeted by transient killer whales and a description of predatory behaviour, number of attacks, and foraging group size.
Fig 8.
Predation events by transient killer whales on marine mammals in Monterey Bay, California.
Prey include, (A) California sea lion being pursued, (B) juvenile northern elephant seal being hit by the tail of a transient, (C) Pacific white-sided dolphin being thrown through the air, (D) Adult male transient killer whale OCT004 grasping the rostrum of a minke whale. Photo credits: Chelsea Mathieson (A), Peggy West-Stap (B-D).
Fig 9.
Predation of a grey whale calf by transient killer whales.
Key moments include (A) the grey whale mother being separated from her calf, (B) the calf being rammed by an adult male transient, (C) before being submerged and drowned and (D) transient killer whales feeding on the grey whale calf lower jaw and tongue. Photo credits: Peggy West-Stap (A), Eric Austin-Yee (B), Stephanie Marcos (C), Peggy West-Stap (D).
Fig 10.
Deceased grey whale calf found stranded near Moss Landing, California on May 12, 2011.
This calf was observed being attacked by a group of 10 transient killer whales the previous day in Monterey Bay. Note the missing lower jaw (A, C) and parallel cuts (B) made by the teeth of killer whales. Photo credit Peggy West-Stap.
Table 3.
Description of the two main sources of data collection that outline key sampling parameters used for the seasonal, geospatial, and behavioural analyses in this study.
Table 4.
The number of consecutive days that photo-identified transient killer whales were observed in Monterey Bay, California (2014–2021).
Table 5.
Categories used to quantify the behaviours of transient killer whales.