Common dolphins in the Salish Sea

The appropriately named common dolphin is probably the most abundant cetacean on the planet. Common dolphins occur in most of the world’s tropical and temperate waters. Historically, they have not been common in inshore waters in Puget Sound and the Salish Sea but they are becoming more frequent as water temperatures warm.

Two dolphins leaping out of the ocean water. Their streamlined grey and white bodies are clearly visible as they break through the blue, wave-patterned surface.
Two short-beaked common dolphins leap alongside a research vessel off the coast of Oregon. Photo: Thomas A. Jefferson (taken under NMFS Permit #22306).

Overview

Common dolphins are members of perhaps the most abundant dolphin species on Earth, Delphinus delphis.  They reach the northern limits of their normal range in the eastern Pacific off Washington, occasionally venturing into southern BC waters. There are two subspecies that occur here, a long-beaked subspecies (D. delphis bairdii) that is more coastal, and a short-beaked subspecies (D. delphis delphis) that occurs more frequently offshore. They can be challenging to distinguish for non-experts.  Records in inshore waters of Puget Sound and the Salish Sea are not common but are becoming more frequent as water temperatures warm.  Large schools characterize both subspecies.

Status, trends & events

Common dolphins occur in most of the world’s tropical and temperate waters, having an extensive global range and occurring in numbers not matched by another dolphin species. Their global abundance may reach over 6 million individuals (Jefferson et al. 2024). They are a familiar species to oceangoing people in many parts of the world, and although there are conservation threats in certain areas, the overall species is not in any danger of extinction. The scientific name of the common dolphin is Delphinus delphis (Latin for dolphin). It is so called because it is perhaps the most common and abundant dolphin species in the world.

 

 Line-Transect and Species Distribution Model. The x-axis shows the years from 1991 to 2018, and the y-axis shows the abundance estimate values. The graph displays the estimates for each method across the different years, allowing comparison between the two techniques.
Estimates of the abundance of the west coast stock of Eastern Pacific long-beaked common dolphins have generally been increasing since the early 1990s. From Carretta et al. (2023).

 

The US government recognizes two management stocks of common dolphins along the west coast, and these are now recognized as separate subspecies:

  • D. delphis delphis – the California/Oregon/Washington stock of short-beaked common dolphin occurs more frequently in offshore waters past the shelf edge. Its abundance is currently estimated at 1,056,308 (CV=0.207) individuals (Carretta et al. 2023).
  • D. delphis bairdii – the eastern Pacific long-beaked common dolphin has a range mostly over the continental shelf and the best estimate of abundance for the California stock is 83,379 (CV=0.216) individuals (Carretta et al. 2023).

Natural history

Tropical through warm temperate waters are home to common dolphins around the globe. However, unless there are areas of intense upwelling, common dolphins tend to occur at lower densities in equatorial waters than the more truly tropical dolphins, like pantropical spotted (Stenella attenuta) and spinner (Stenella longirostris) dolphins. Common dolphins occur in a wide range of water depths and bottom types, being found very close to shore in some areas, and yet they are also regularly seen in very deep, offshore waters. Some of this variety is related to distinct geographic forms, subspecies, and probably even species of common dolphin, which have adapted to different habitat types (Evans 1994; Murphy et al. in press).

 

The image shows a map of the Americas, with the ranges of two populations of the eastern Pacific long-beaked common dolphin. The primary range is highlighted in purple and occasional ranges in green. The range of one population extends along the United States west coast and northern Mexico, and the other off the coast of Ecuador, Peru and Chile.
The eastern Pacific long-beaked common dolphin has a disjunct range, with two main populations, one in northern Mexico and all along the US west coast, and the other off the coast of Ecuador, Peru and Chile. Long-beaks are apparently absent in the eastern tropical Pacific. From Jefferson et al. (2024).

In Puget Sound and the greater Salish Sea, common dolphins don’t live up to their name. They are considered rare in these waters, and sightings and strandings are somewhat unusual, as they appear to reach the northern limits of their range here. However, this seems to be changing, as water temperatures rise, long-beaked common dolphins are being seen in the inshore waters more frequently (Shuster et al. 2018), and it is tempting to think that these dolphins may end up become regular inhabitants of Puget Sound. Singles or groups of a few dolphins are occasionally seen in the Sound, with some individuals looking unhealthy and even stranding (such as “Big Black Notch,” who stranded near Johnson Point in Olympia on 15 January 2020 – see link below). Short-beaked common dolphins also occur in inshore BC and Washington waters now and then, but these tend to be strandings (Ford 2005, 2014).

Common dolphins feed mostly on small schooling fish, such as anchovies, sardines, and herring, but they also consume squid and other cephalopods. Large schools of dolphins, using what appears to be cooperative feeding techniques near the surface, are the norm for long-beaked common dolphins feeding near shore. Short-beaked common dolphins in offshore waters may feed primarily relatively deep in the water column at night on small fish and squid that rise toward the surface in hours of darkness (i.e., the Deep Scattering Layer – see Evans 1994).

Biology

The appropriately named common dolphin is probably the most abundant cetacean species on the planet. Most areas of its range have not been adequately surveyed to compute reliable estimates of abundance but estimates for those areas that have been (primarily in the North Pacific, North Atlantic, and in a few small areas off southern Africa, Australia, and New Zealand) add up to over 6 million individuals. And if we assume similar densities in unsurveyed areas, the total may possibly be 10 million or more! Certainly, as a species common dolphins are in no danger of extinction, and the IUCN Red List status of the species is Least Concern. They are not listed as Endangered or Threatened under the US Endangered Species Act. However, some stocks in developing countries, especially coastal ones, may be facing more serious threats to their survival (Murphy et al. in press).

 

The eastern Pacific long-beaked common dolphin has a disjunct range, with two main populations, one in northern Mexico and all along the US west coast, and the other off the coast of Ecuador, Peru and Chile. Long-beaks are apparently absent in the eastern tropical Pacific. From Jefferson et al. (2024).
Two subspecies of common dolphins occur off the US west coast: short-beaked (D. delphis delphis – top), and long-beaked (D. delphis bairdii – bottom). Recent evidence indicates that these are actually two different species. Photos: Thomas A. Jefferson (taken under NMFS Permit #22306 - bottom).

Although they have been hunted systematically in places like the Black Sea and Mediterranean, and some are killed in small-scale fisheries in Sri Lanka, Peru, and Venezuela, common dolphins have never been subject to a direct fishery off the US/Canada west coast. The threats facing these animals today are mostly related to incidental entanglement in fishing gear (especially purse seines and offshore driftnets) and in some cases possibly habitat degradation issues (related to climate change?). None of these threats is thought to be causing any significant extinction risk to either of the US west coast stocks.

Both subspecies of common dolphins share many characteristics: a streamlined body shape, with a moderately-long, slender beak, tall falcate dorsal fin, and a lovely color pattern with an hourglass shape on the sides. The back is dark gray, the anterior flank patch is tan to ochre, and the posterior flank is brushed with light streaks. Common dolphins have dark lips and a dark stripe from the chin to the flipper. The short-beaked subspecies tends to be more robust, with a steeper forehead, shorter beak, and more bold color pattern. The long-beaked subspecies is more slender with a shallower angle between the beak and melon, longer beak and more muted color pattern. It also grow a bit longer (off California, up to 243 centimeters vs. only to about 205 cm for short-beaks – see Jefferson et al. 2024). The differences can be subtle, though, and generally require a trained eye to reliably distinguish them.

Common dolphins are not known as a migratory species, however, seasonal changes in response to changing water temperatures (probably affecting prey availability) are apparent in many parts of their range. The west coast of the US is no exception. Common dolphin numbers show a general increase when water temperatures warm, and though there is also a seasonal inshore/offshore shift, the northward shift with warmer sea surface temperatures appears to be most important.

Common dolphins are quite vocal, like most other dolphins that occur in large schools. They produce a wide variety of sounds to communicate; these include frequency-modulated tonal sounds called whistles and pulsed sounds commonly known as burst –pulse sounds. Mean frequencies and other acoustic characteristics often vary among populations (Petrella et al., 2012). In addition, like all odontocetes (toothed whales), common dolphins produce echolocation clicks, which are used to navigate, detect obstacles, and locate prey. These sonar clicks cover a wide range of frequencies, and can go up to about 150 kHz, which is well above the human hearing range (which reaches only about 20 kHz).

The only major natural predator of eastern North Pacific common dolphins is probably Bigg’s killer whale (Orcinus orca rectipinnus). There have been several recent observations of fatal attacks along the west coast, although the numbers killed by killer whales are almost certainly low enough not to be impacting the overall common dolphin population. Large sharks, like great white and tiger sharks, are probably occasional predators, but under most circumstances, common dolphins can probably outrun them to avoid capture.

Population trends for both stocks of common dolphins that occur along the west coast of the US have not been examined in detailed, focal analyses, but nonetheless it appears that both have been increasing in abundance in US waters in the past several decades (Carretta et al., 2023). As waters warm along the west coast due to climate change-related patterns, this apparently expands the suitable habitat of both types of common dolphins, which are known to prefer warmer waters, northward. In particular, the expansion of the short-beaked common dolphin in offshore waters to the north off Oregon and Washington has become quite apparent.

There have been no dedicated ecological studies on common dolphins in the Salish Sea – they are simply too rare there for such work to be fruitful, but as indicated above, this may be changing (Shuster et al. 2018). Along the outer coast of Washington, information is largely related to estimating abundance of this species. A recent survey of this area in 2024 sighted a surprisingly large number of common dolphin schools (all short-beaked), and the evidence of a range shift into more northern waters is becoming impossible to ignore (Jefferson, pers. obs.).

Data sources & gaps

Most status information in this report comes from Carretta et al. (2023). While some monitoring and information gaps for this species exist, there is generally adequate information to determine the status and 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 short-beaked common dolphins and the California stock of long-beaked common dolphins mostly comes from vessel-based line-transect analysis, and more recently habitat-based species distribution modeling (see Carretta et al., (Jefferson et23).

References

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 Tech. Memo. NMFS-SWFSC, 704, 420 pp.

Evans, W. E. (1994). Common dolphin, white-bellied porpoise Delphinus delphis Linnaeus, 1758. In S. H. Ridgway & R. Harrison (Eds.), Handbook of Marine Mammals, Volume 5: The first book of dolphins (pp. 191-224): Academic Press.

Ford, J. K. B. (2005). First records of long-beaked common dolphins, Delphinus capensis, in Canadian waters. Canadian Field-Naturalist, 119, 110-113.

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.

Jefferson, T. A., Archer, F. I., & Roberston, K. M. (2024). The long-beaked common dolphin of the eastern Pacific Ocean: Taxonomic status and redescription of Delphinus bairdii. Marine Mammal Science, 2024, 37 pp.

Murphy, S., Jefferson, T. A. & Stockin, K. (In press). Common dolphin Delphinus delphis Linnaeus, 1758. In T. A. Jefferson (Ed.), Ridgway and Harrison’s Handbook of Marine Mammals, Vol. 2: Oceanic Dolphins (pp. ????): Academic Press.

Petrella, V., Martinez, E., Anderson, M. G., & Stockin, K. A. (2012). Whistle characteristics of common dolphins (Delphinus sp.) in the Hauraki Gulf, New Zealand. Marine Mammal Science 28(3):479-496.

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 Salish Sea Ecosystem Conference <https://cedar.wwu.edu/ssec/2018ssec/allsessions/590>.

Links

https://www.victoriawhalewatching.com/common-dolphins-spotted-salish-sea/

https://cascadiaresearch.org/project_article/long-beaked-common-dolphins...

https://cascadiaresearch.org/project_article/common-dolphin-dubbed-big-b...

https://cascadiaresearch.org/project_article/stranding-one-long-beaked-c...

https://www.cbc.ca/news/canada/british-columbia/common-dolphins-salish-s...

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.

About the Author: 
Thomas A. Jefferson is the director of Clymene Enterprises and is also an independent researcher at NOAA's Southwest Fisheries Science Center.