Overview

Risso’s dolphins are a common species along the west coast of United States, but they usually stay in deep waters offshore, and rarely come into the inshore waters of the Salish Sea. Records in these waters are considered extralimital, but they do occur now and then. These large dolphins feed almost exclusively on squid, and are easily recognizable by their striking light color in the adults, and the extensive set of scratches and scars on their bodies. 

Status, trends & events

The scientific name of Risso’s dolphin is Grampus griseus. ‘Grampus’ is a term used previously to refer to blunt-headed small cetaceans, and ‘griseus’ refers to the gray color that many member of the species possess. Risso’s dolphins are found in tropical to cool temperate waters around the world and occur in all three major ocean basins (Pacific, Atlantic, and Indian oceans). The species is not rare, but it may not be very familiar to many readers, due to the fact that it is a species that mostly occurs offshore, not often seen very near the coast, and it usually does not ride bow waves of vessels.

There is a single stock recognized by the National Oceanic and Atmospheric Administration (NOAA) in US west coast waters. Population structure in this area is not well understood, although it is recognized that the Californa/Oregon/Washington stock extends into Mexican (and possibly beyond), Canadian, and offshore areas beyond the stock limits of the US EEZ off the continental US. The current best estimate of the stock size is 6,336 (CV=0.32) animals (Barlow 2016), although this is likely an underestimate of the overall population in this area. Since animals move into and out of the stock boundary areas, it has been difficult to determine the overall stock size and trends in numbers. Previous estimates for this stock have been as high as 32,376 animals (Barlow 1995), but no significant trend has so far been determined (Carretta et al. 2024). However, known mortality in fisheries is apparently low, and these animals are not listed as Threatended or Endangered under the US Endangered Species Act (ESA), and are not considered a “strategic” stock for NOAA (Carretta et al. 2024).

Natural history

Although past reviews on the species have often shown their distribution to be limited to relatively nearshore waters along continental margins, Jefferson et al. (2014) showed clearly that these animals do occur in deep oceanic waters as well, and likely span across ocean basins. The range extends from about 46°S to 64°N, with only the high-latitude and polar regions devoid of Risso’s dolphins (Jefferson et al. 2014). They also occur in many enclosed seas, bays, and gulfs (e.g., Gulf of Mexico, Mediterranean Sea, North Sea, Red Sea, Sea of Japan, Okhotsk Sea, and Gulf of California), but are apparently absent in the Black Sea, and Persian Gulf, and may only occur in inshore channels and fiords of Washington, British Columba, and Alaska as rare extralimital strays. Other ‘vagrant’ records have been reported off the Kamchatka Peninsula, Russia; in the Gulf of Alaska; the Canadian east coast; Iceland; northern Norway; the Baltic Sea; and Tierra del Fuego (Hartman et al. 2024).

The primary habitat of Risso’s dolphin is relatively warm tropical to temperate waters over the outer continental shelf slope with depths ranging from 200 to 1000 m, although they are also found at lower densities in waters much shallower and much deeper (Jefferson et al. 2014; Hartman et al. 2024). The highest densities of the species occur in mid-latitude areas between 30° and 45° in both hemispheres (Jefferson et al. 2014). They seem to be especially common around some oceanic island groups, such as the Azores; Canary Islands; and the Isle of Lewis, Scotland (Hartman et al. 2024).

Risso’s dolphin records in the inshore waters of the Salish Sea are uncommon, and Ford (2014) mapped only four strandings and one sighing in Canadian waters of the Salish Sea. Gaydos and Pearson (2011) considered them rare in spring through fall, with no records at all from winter months. The specific records seem to be restricted to five different well-documented “events.” The first such event occurred in March 1975, when a single individual was live-stranded in the area of Port Discovery, inside Discovery Bay, Washington (Everitt et al. 1979; Angell & Balcomb 1982; Osborne et al. 1988). The 3.3 m female dolphin may have been evading killer whales when it stranded, and was taken to the Seattle Marine Aquarium, where it died two days later (Everitt et al. 1979). The second event was in 1978, when four sightings (6-12 individuals in each sighting) of what are thought to be part of the same group of about a dozen Risso’s dolphins were made from late August to early September in the Strait of Georgia area of ‘ Columbia (BC) (Baird et al. 1988; Baird & Stacy 1991). Then, in October of 1987, a stranding of a single Risso’s dolphin was recorded near Port Angeles, Washington (Osborne et al. 1988). The fourth event was apparently related to an extended stay of a pair of Risso’s dolphins in Puget Sound between 2011 and 2013 (see link below). These animals were sighted at least 32 times and on several occasions were photo-identified and confirmed as the same pair (Calambokidis et al. 2015; Smultea et al. 2022). Two additional sightings from aerial surveys in 2013 by Smultea et al. (2022) were likely of the same pair, although photos from the aerial platform were not of high enough resolution for the photos to confirm this. Finally, in March 2015, a single Risso’s dolphin was seen swimming in Ganges Harbor, BC, and later stranded.

Feeding

Risso’s dolphins appear to feed almost exclusively on cephalopds (squid, octopus, and cuttlefish). Fish are extremely rare in their stomach contents, but at least 82 species of cephalopods make up their known prey (Hartman et al. 2024). Both small and larger species are taken, but there is evidence that feeding habits differ somewhat between males and females. They may also use different foraging areas, but this does not appear to be the case in all parts of their range (Hartman et al. 2024).

Biology

The global abundance of Risso’s dolphin is not known, but abundance has been estimated for a number of portions of the species’ range, mostly in the Northern Hemisphere. Adding these estimates together gives a total estimate of about 425,000-433,000 dolphins, but since many areas of the range have not been surveyed, this most likely represents only a minimum estimate of global numbers (Hartman et al. 2024). The overall trend in species’ numbers is not known, but they appear to be decreasing in some areas (e.g., northern Gulf of Mexico, Mediterranean Sea, possibly the US west coast, Sri Lanka) and increasing in others (e.g., around Catalina Island, off California) – see Hartman et al. 2024 for a summary.

Scheffer and Slipp (1948) recorded no Risso’s dolphins at all in their review of Washington cetacean records. The first record for BC, and also the first in inshore Pacific Northwest waters, was a stranding in May 1964, from Stuart Island, in the inshore waters of central BC, but north of the Salish Sea (Guiget and Pike 1965). On the outer coasts of Oregon, Washington, and British Columba, Risso’s dolphins were once considered rare, but now there are many records from these areas, indicating that the species regularly occurs there, at least in the warmer periods of the year (Ford 2014). Typical group sizes are in the range of 5-30, but Risso’s dolphins sometimes gather into huge schools of hundreds or even thousands, especially when they school together with Pacific white-sided (Lagenorhynchus obliquidens) and northern right whale (Lissodelphis borealis) dolphins. Then most often move slowly, but at times they can be quite active, and much aerial behavior can be seen.

Data sources & gaps

Most status information in this report comes from Carretta et al. (2024). While some monitoring and information gaps for this species exist, there is generally adequate information to determine the abundance (but not trends) of this species along the west coast. Information specific to the Salish Sea or Puget Sound, however, is largely lacking.

Methods & statistics

Methods of determining the population status of the California/Oregon/Washington stock of Risso’s dolphins mostly comes from vessel-based line-transect analysis, and habitat-based species distribution modeling (see Carretta et al., 2024).

References

Marine Birds and Mammals of Puget Sound: Puget Sound Books.

Baird, R. W., Langlelier, K. M., & Stacey, P. J. (1988). Stranded whale and dolphin program of B.C. - 1987 report. British Columiba Veterinary Medical Assocation Wildlife Veterinary Report, 1(1), 9-12.

Baird, R. W., & Stacey, P. J. (1991). Status of Risso's dolphin, Grampus griseus, in Canada. Canadian Field-Naturalist, 105<, 233-242.

Barlow, J., Brownell, R. L., DeMaster, D. P., Forney, K. A., Lowry, M. S., Osmeck, S., . . . Small, R. J. (1995). U.S. Pacific marine mammal stock assessments: 1995. NOAA Technical Memorandum NMFS-SWFSC, 219, 162 pp.&

Barlow, J. (2016). Cetacean abundance in the California Current estimated from ship-based line-transect surveys, 1991-2014. Southwest Fisheries Center Administrative Report, LJ-2016-01.

Calambokidis, J., & Baird, R. W. (1994). Status of marine mammals in the Strait of Georgia, Puget Sound and the Juan de Fuca Strait and potential human impacts. In R. C. H. Wilson, R. J. Beamish, F. Aitkens, & J. Bell (Eds.), Review of the marine environment and biota of Strait of Georgia, Puget Sound and Juan de Fuca Strait (Vol. 1948, pp. 282-303): Canadian Technical Report of Fisheries and Aquatic Sciences.

Calambokidis, J., Hanson, B., Huggins, J., Anderson, D., Lambourn, D., Jeffries, S., Evenson, J., Diehl, B., & Oliver, J. (2015). Status of Harbor Porpoise in Washington State and recent return to Puget Sound. [Power- Point presentation] Puget Sound Ecosystem Monitoring Program 2015 Meeting of the Marine Mammals Workgroup.

Carretta, J. V., Oleson, E. M., Forney, K. A., Bradford, A. L., Yano, K., Weller, D. W., . . . Brownell Jr., R. L. (2024). U.S. Pacific marine mammal stock assessments: 2023. NOAA Technical Memorandum NMFS-SWFSC, 704, 420 pp.

Everitt, R. D., Fiscus, C. H., & DeLong, R. L. (1979). Marine mammals of northern Puget Sound and the Straits of Juan de Fuca. NOAA Technical Memorandum ERL-MESA, 41, 141 pp.

Ford, J. K. B. (2014). Marine mammals of British Columbia. Royal British Columbia Museum, Victoria, British Columbia, Canada. 460pp. ISBN 978-0-7726-6734-2. 2014.

Gaydos, J. K., & Pearson, S. F. (2011). Birds and mammals that depend on the Salish Sea: A compilation. Northwestern Naturalist, 92, 79-94.

Guiget, C. J., & Pike, G. C. (1965). First specimen record of the gray grampus or Risso dolphin, Grampus griseus (Cuvier) from British Columbia. Murrelet, 46, 16.

Hartman, K. L., Chen, I., Hodgins, N., Ames, A. E., & van der Harst, P. A. (2024). Risso's dolphin Grampus griseus (G. Cuvier, 1812). In T. A. Jefferson (Ed.), Ridgway and Harrison’s Handbook of Marine Mammals, Vol. 1: Coastal Dolphins and Porpoises (pp. 267-312): Academic Press.

Jefferson, T. A., Weir, C. R., Anderson, R. C., Ballance, L. T., Kenney, R. D., & Kiszka, J. J. (2014). Global distribution of Risso’s dolphin (Grampus griseus): A review and critical evaluation. Mammal Review, 44, 56-68.

Jefferson, T. A., Webber, M. A., & Pitman, R. L. (2015). Marine Mammals of the World: A Comprehensive Guide to Their Identification: Academic Press/Elsevier.

Osborne, R., Calambokidis, J., & Dorsey, E. (1988). A Guide to Marine Mammals of Greater Puget Sound: Island Publishers.

Scheffer, V. B., & Slipp, J. W. (1948). The whales and dolphins of Washington State, with a key to the cetaceans of the west coast of North America. American Midland Naturalist, 39, 257-337.

Shuster, L., & D. Anderson, J. L. H., A. B. Douglas, N. Harrison, J. Calambokidis & S. Berta. (2018). Dolphins in the Salish Sea: Are warmer water species expanding into our region? Paper presented at 2018 Salish Sea Ecosystem Conference (Seattle, Wash.) – see link below.

Smultea, M. A., Jefferson, T. A. & Lane, R. S. (2022). Marine mammal occurrence, distribution, and behavior in the inland waters of Washington from aerial surveys, 2013-2016. Northwestern Naturalist 103(2):118-135. 2022.

Links

NOAA Marine Mammal Stock Assessment Reports by Species/Stock

Cascadia Research Sightings of Risso’s dolphins in southern Puget Sound – 30 December 2011

2018 Salish Sea Ecosystem Conference Dolphins in the Salish Sea: Are warmer water species expanding into our region?

Acknowledgements

The author thanks the University of Washington Puget Sound Institute for funding, and also the Puget Sound Ecosystem monitoring Program's Marine Mammals Working Group for helping to facilitate the production of this chapter.

Licensing & attribution

Data and products from the PSEMP Marine Mammal Work Group are governed by a Creative Commons BY-NC-SA license. Attribution should be to: “PSEMP Marine Mammal Work Group” with a link back to https://psemp.net/mmwg.