Minke whales in the Salish Sea

Minke whales are the most abundant baleen whale species worldwide. In the waters of the west coast of North America, their populations have remained small, but stable over time. They are hunted by transient killer whales due to their relatively small size compared with other baleen whales. Sighting rates for minke whales in the Salish Sea are highest during the summer, however, little is known about where minke whales spend the winter months. 

A minke whale with its mouth open as it surfaces.
A minke whale (Balaenoptera acutorostrata) lunge feeding: Photo: Matt Stolmeier/Outer Island Expeditions
  • Mass: 10,000 kg (22,000 lb)
  • Length: 7-10 m (24-33 feet)
  • Lifespan: Approximately 50 years
  • Eats: Small schooling fish
  • Natural predator: Bigg’s killer whales
  • Threats: Commercial whaling, climate change, entanglement, ship strike, noise pollution
Minke whale swimming with fin above water, land in the distance
Minke whale. Photo: Northeast Pacific Minke Whale Project, taken under NOAA permit

Current status & history

Minke whales are the most abundant baleen whale species and are widely distributed throughout the world’s oceans. There are two species and three forms of minke whale: the common minke whale (Balaenoptera acutorostrata) found in the northern hemisphere; the Antarctic minke whale (Balaenoptera bonaerensis) found exclusively in the Southern Hemisphere; and the dwarf minke whale (an unnamed subspecies of the common minke), also found in the southern hemisphere.

 

Illustrations of three types of minke whale
(A) the common minke whale, (B) Antarctic minke whale, and (C) the dwarf minke whale. Illustrations: Uko Gorter

All populations of minke whale, except the West Greenland population are listed on Appendix I of CITES (the West Greenland population is listed on Appendix II of Cites). Minke whales are not listed as "endangered" under the Endangered Species Act and are not considered "depleted" under the U.S. Marine Mammal Protection Act (MMPA).

Population estimates for  minke whales inhabiting the coastal waters of California, Oregon and Washington have consistently been in the hundreds. Abundance estimates based on line transect surveys conducted from 1991 - 2014 have ranged from 407 to 1,432 (Barlow, 2016). The most recent population estimate (2018) is 915 (CV= 0.792) and was estimated using a Species Distribution Model (Becker et al., 2020). These are models generated for the region from fixed and dynamic ocean variables and have become a standard approach for estimating abundance because they provide spatially-explicit abundance estimates and can be used when survey coverage is limited (Becker et al., 2020, Caretta et al., 2023). The minimum abundance estimate for California, Oregon, and Washington is 509 whales (Carretta et al., 2023).

NOAA’s stock assessment surveys of the coastal waters of California, Oregon, and Washington suggest that the population of minke whales in coastal waters is relatively stable (Caretta et al., 2021). While these surveys do not include the inland waters of Washington State, data from long-term photo-identification studies and public sightings data suggest similar stable population trends. Photo-identification data suggests there are approximately 30-40 individuals that forage around the San Juan Islands and into Puget Sound (Hoelzel et al., 1989, Dorsey et al.,  1990, Northeast Pacific minke whale project unpublished data). Analysis of long-term opportunistic sighting data collected in the Salish Sea also suggests stable numbers but showed a notable decrease in sightings during the early 1990s (Olson, et al., 2024).  Sighting rates for minke whales in the Salish Sea have increased since 2007, however this is likely related to an increase in search effort due to both the onset of social media based sighting platforms and the establishment of shore-based whale watch educating programs (Olson et al., 2024). Photo-identification work does not suggest an increase in population size over this time period (Northeast Pacific minke whale project unpublished data), though analysis of sightings data suggest a spatio-temporal  shift in minke whale sightings towards the eastern Strait of Juan de Fuca, south of the San Juan Islands and Northern Puget Sound (Olson et al., 2024).

These recent findings from photo-id and opportunistic sightings data in the Salish Sea along with NOAA’s population estimates suggest that the abundance of minke whales inhabiting the waters of the west coast of North America has remained small, but stable over time. Additionally, the lack of commercial harvesting or subsistence hunting for minke whales in west coast waters (Scammon, 1874; Scheffer and Slipp, 1948; Carretta et al., 2021), and the absence of remains in middens (Robertson and Trites, 2018; McMillan, 2015) further suggests that minke whales are naturally rare in this region (Olson et al., 2024).

Natural history

Habitat

Along the west coast of the U.S. and British Columbia, Canada, minke whales have a coastal distribution, preferring continental shelf habitat, and appear to be resident year-round (Dorsey et al., 1990; Caretta et al., 2021). Sighting rates for minke whales in the Salish Sea are highest during the summer (Dorsey, 1983; Stern, 1990), however, little is known about where minke whales spend the winter months, nor where they breed. Stranding reports from the Washington outer coast suggest that minkes are present year-round (Norman et al., 2004), which is supported by Scammon’s (1874) suggestion that minke whales could be seen year-round in the Strait of Juan de Fuca. Low sighting rates during winter months may be an artifact of sea conditions and low search effort (Dorsey et al., 1990), but increasingly-active public sighting forums (e.g., social media and sighting networks) are resulting in more sightings during late fall to early spring.

In the Salish Sea, minke whales regularly occur around the San Juan Islands with the same individual whales returning annually to feed (Dorsey et al., 1990). Photo-identification data suggests that at least two individuals regularly returned to areas around the San Juan Islands over a 40 year time period (NEPMWP unpublished data). Their distribution within feeding areas in the San Juan Islands appears to be largely determined by the availability of their prey—juvenile bait fish (e.g. herring and Pacific sand lance; Dorsey et al., 1990). Early studies identified three core sub-regions where individual whales exhibited site fidelity, the first known evidence of non-overlapping ranges in a baleen whale species (Dorsey, 1983).

Map showing ranges of minke whales
Non-overlapping ranges of minke whales identified by Dorsey, (1983). Area A was apparently abandoned by minke whales after 1984, but minkes were reported in this area in 2019.

Group composition and communication

Minke whales are generally solitary but have occasionally been observed in loose groups of two or three in the Salish Sea. Scammon (1874) also described this solitary nature in the mid-1800s. In the North Atlantic pronounced sexual segregation occurs on higher latitude feeding grounds (Risch et al., 2019). While this has not been shown in the Salish Sea, the stranding record is almost entirely of female whales (Sheffer and Slipp, 1948; Nikolich and Towers, 2018).

Minke whales use low-frequency sounds to communicate with one another and to locate prey (NOAA, 2019). In the North Pacific “boing” sounds have been attributed to minke whales (Rankin and Barlow, 2005, Risch et al., 2019), and more recently Nikolich and Towers (2018) described pulse train sounds from recordings made in the vicinity of minke whales in British Columbia. Minke whale calls have not been recorded in the Salish Sea to date.

Reproduction

Mating and calving occur during the winter with most calves born during late January and early February (NOAA, 2019) after a 10-month gestation. Calves are 2.4-3.5 meters when born and weaned within 4-6 months. The reproductive interval is estimated to be ~14 months but calving may occur annually (NOAA, 2019). It is rare to see calves in the Salish Sea as they have generally been weaned when they arrive in their feeding habitats. These juveniles are much smaller than mature adult whales at only ~4.9-5.5 meters. These younger whales are often encountered in the Salish Sea during the summer months. However, there is still little known about where minke whales in the NE Pacific breed. There is some evidence to suggest that they migrate to warmer waters because of the scarring on their bodies (Towers et al., 2013). These scars have been attributed to cookie cutter sharks -- fish with a preference for warmer waters. But there is also evidence that parasitic lamprey fish -- common around the Pacific rim and along the Pacific Northwest coast -- cause similar scarring on whales, including minke whales (Nemoto, 1977; Nichols and Tscherter, 2011).

 

A fully breaching minke whale.
A fully breaching minke whale. Photo: Tom Benson/Flickr (CC BY-NC-ND 2.0)

Predators and prey

Minke whales are hunted by transient killer whales due to their relatively small size compared with other baleen whales. A minke’s only defense is speed; unlike some other whale species minke whales do not fight back (Ford and Reeves, 2008). Transient killer whale predation attempts have been observed in the Salish Sea and several attacks on known individuals have been successful, further analysis is needed to understand if predation rates on minke whales have increased in recent years with the increased presence of transient killer whales.

Minke whales feed on small schooling fish such as juvenile Pacific herring, Pacific sand lance, and surf smelt.  In the Salish Sea they display a variety of feeding strategies depending on prey distribution, local bathymetry, and local hydrography (Hoelzel et al., 1989, Dorsey et al., 1990). In areas characterized by deep basins minke whales have been observed actively herding baitfish to the surface where they lunge up through the trapped fish. In other locations, the whales take advantage of feeding seabirds and predatory fish (Hoelzel et al., 1989). This bird-associated feeding is commonly observed in the Juan de Fuca Strait along the edges of sub-sea banks and in areas of upwelling (Hoelzel et al., 1989).

Threats

Threats to minke whales include changing ocean conditions due to a changing climate, whaling, underwater noise, entanglement in fishing gear, ship strikes and marine litter (Risch et al., 2019). In the Salish Sea threats include underwater noise, entanglement, ship strike, and climate change. As vessel traffic presence increases ship-strike risk also increases, ship strikes have been an increasing cause of death of large whales in Washington State, especially since the early 2000 (CRC, 2024). Of the 5 recorded minke whale strandings that have occurred in Washington over the last 20 yeras at two whales died as a result of ship strike, and these occured in 2022 and 2024. A known individual adult female was recovered in San Juan Channel near Lopez Island in October, 2022 (CRC 2022), and another juvenile female was recovered near Tacoma in November, 2024 (CRC 2024). Given the infrequency of minke strandings in the Salish Sea these two cases are particularly troubling because they represent just a fraction of the true number of ship strike mortalities, as these occurrences are usually not observed, and carcasses often sink and are unable to be examined (CRC, 2024). In addition to ship strike, smaller vessel collisions are also a concern. At least one other regularly encountered individual exhibited injuries that also likely were the result of a collision with a smaller boat. These instances highlight the collision risk associated with all vessel types, though the larger the vessel the more likely the interaction would be fatal.

Current research and recent news

The Northeast Pacific Minke Whale Project, led by Elanor Dorsey, Dr. Jonathan Stern and Dr. Rus Hoelzel initiated research in 1980 and continues to the present day. The project, now led by Dr. Frances Robertson is not currently conducting field work but is collaborating with local researchers, whale watch captains and naturalists, and sighting networks to track minke whale presence and distribution in the Salish Sea

References

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Cascadia Research Collective (CRC). 2022. Cascadia Research Stranding and Necropsy Report for 2022-SJ053

Cascadia Research Collective (CRC). 2024 Minke whale found dead in Tacoma was killed by ship strike. https://cascadiaresearch.org/announcement/minke-whale-found-dead-in-tacoma-was-killed-by-a-ship/

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About the Author: 
Frances Robertson is a marine wildlife biologist based in the San Juan Islands. She has studied minke whales in the Salish Sea for almost 20 years and now leads the Northeast Pacific Minke Whale Project.