Sanctuary Integrated Monitoring Network
Monterey Bay National Marine Sanctuary
Sei Whale Photo: Protected Resources Division, Southwest Fisheries Science Center.

Special Status Species

Balaenoptera borealis - Sei Whale

Common Name: Sei Whale
Scientific Name: Balaenoptera borealis
Stock: Eastern North Pacific

Endangered Species Act (?) Status: Endangered (all stocks)
Critical Habitat: Not designated
Recovery Plan: Draft released in 19981
Five Year Status Review: None
California Endangered Species Act (?) Status: Not listed
Marine Mammal Protection Act (?) Status: Depleted; strategic stock
Stock Assessment: Updated annually2
The World Conservation Union (IUCN) (?) Status: Endangered (world-wide)
Convention on International Trade in Endangered Species (CITES) (?) Appendix I
Convention on Migratory Species (CMS) (?) Appendix I
Appendix II
Geographic Range

The distribution of this species is poorly understood. It occurs worldwide from subtropical to subpolar waters, but is most frequently found in temperate latitudes (Figure 1). The International Whaling Commission (IWC) recognizes one stock of sei whales in the North Pacific. The Marine Mammal Protection Act (MMPA) stock assessment reports recognizes a separate stock for the eastern North Pacific.2 Members of this stock are found from the northern Gulf of Alaska to southern California and occasionally off Baja California, Mexico.1 A paucity of recent data on sei whales in the North Pacific prevents a more complete determination of the genetic structure and geographic range of the stock. In addition, the accuracy of some data from past decades is questionable due to the common confusion of this species with Bryde’s whales.


This species has been sighted in offshore waters throughout the latitudinal range of the MBNMS, though usually they occur seaward of the Sanctuary’s western boundary. Sightings have become rare since the 1980s.2

Sei whale distribution map
Figure 1. The world-wide geographic range of sei whales Balaenoptera borealis.4
Download full-size figure (172 Kb PDF).

Sei whales are observed generally in deep water habitats, including along the edge of the continental shelf, over the continental slope, and in the open ocean. They do not appear to be associated with coastal features.2 Instead, the distribution of sei whales may be influenced by the location of oceanographic fronts, including major mixing zones and eddies, that enhance production and entrainment of plankton.3


Same as above. Additional sightings of this species in Sanctuary waters would be needed before preferred habitats could be identified in the Sanctuary.

Migration & Movement
General: This species is usually seen traveling alone or in small groups, although large aggregations can occur on feeding grounds. All populations of sei whales are thought to migrate from low-latitude wintering areas to high-latitude summer feeding grounds.4 They show year to year differences in arrival times and distribution on the summer feeding grounds - they may be present in high numbers in some years and completely absent in other years.3 Resident populations (populations that remain in the same area year-round) have not been identified for this species.1 Not much is known about the movement patterns of the EN Pacific stock. Two sei whales tagged off southern California were later captured off the coast of Washington and Vancouver Island, B.C..5 Analysis of whaling records also suggest that sei whales migrated past Vancouver Island, B.C. during the summer.6

Very little is known about the movement patterns of sei whales along the central California coast. This species is primarily observed off central California in the summer and early fall.5 They are less frequently encountered in the winter months and may spend the winter in offshore waters between 18-36°N latitude.7 Tagging studies suggest that individuals sighted in the MBNMS are traveling up and down the west coast of the U.S. and Canada in search of prey.5


Sei whales in the entire North Pacific may have numbered approximately 42,000 individuals prior to exploitation.8 Intense commercial whaling for this species began in 1959, after the depletion of the humpback and blue whale populations. Whaling may have reduced numbers in the North Pacific to as few as 8,600 before the sei whale was given protected status by the International Whaling Commission (IWC) in 1976.8

The abundance estimate for California, Oregon, and Washington waters out to 300 nautical miles, based on 1996 and 2001 shipboard surveys, is 56 whales.9 Although the population in the North Pacific is expected to have grown since receiving protected status in 1976, there are no data on trends in abundance for this stock.2


Currently, sighting of sei whales during ship-board surveys along the California coast (out to 300 nmi) are rare (Figure 2).9

Sei whale sighting map
Figure 2. Sightings of sei whales based on aerial and shipboard surveys off California, Oregon and Washington, 1991-2001. Dashed line represents the U.S. Exclusive Economic Zone (EEZ); bold line indicates the outer boundary of all surveys combined. Reprinted from Carretta et al. 2005; see Appendix 2 of that report for actual transect lines surveyed.2
Download full-size figure (172 Kb PDF).

Acoustic disturbance: There is concern about the potential negative impacts to marine mammals of a variety of acoustic disturbances (e.g., noise from ships, aircraft, research boats, and military and industrial activities).10 Noise can cause direct physiological damage, mask communication, or disrupt important migration, feeding or breeding behaviors. Active-sonar, specifically low frequency (100-500 Hz) and mid-frequency (2.8-3.3 kHz) active sonar used in military activities by the U.S. and other nations, is one sound source of particular concern.10 The impact of seismic testing for geological mapping and oil and gas exploration is also unknown.

Collisions with ships: No ship strikes have been reported for this species off CA/OR/WA.2 Whales struck and killed by fast moving vessels may sink and not wash ashore.

Declining prey resources: Declining abundance of prey species could result from either natural prey population fluctuations or commercial harvest of prey species. Schooling fishes are often used for human consumption, as bait, or as feed in mariculture facilities.

Entanglement in fishing gear: No deaths of sei whales have been observed in the CA/OR offshore drift gillnet fishery since the Take Reduction Plan began in 1997.2 Because of their offshore distribution and relative scarcity, they may have a lower incidence of entanglement than other baleen whales or entangled animals may go unnoticed.

Habitat degradation (e.g., chemical pollution, oil pollution): Any increase in offshore oil and gas development would increase both the potential of oil or chemical spills and the amount of shipping traffic through sei whale habitat.

Intentional take: Either under scientific research permits or by illegal whaling. Illegal whaling is unlikely in U.S. territorial waters, but may occur on the high seas or in territorial waters of other countries.


No threats are unique to the MBNMS.

Conservation and Research

In 1976 sei whales in the North Pacific were given complete protection from whaling under the International Convention for the Regulation of Whaling. Sei whales are listed as "endangered" under the Endangered Species Act (ESA) and the eastern North Pacific stock is considered "depleted" and a "strategic stock" under the Marine Mammal Protection Act (MMPA). Under the ESA and MMPA, the National Marine Fisheries Service (NMFS) is responsible for management and recovery of sei whales in U.S. waters.

As required under the MMPA, NMFS annually updates the Stock Assessment Reports (SAR) for all strategic stocks and the most recent SARs are available on the NOAA Office of Protected Resources website. The MMPA also requires the formation of Take Reduction Plans to reduce the incidental serious injury and mortality of marine mammals from commercial fishing operations. In 1997 NMFS implemented a Take Reduction Plan for Pacific Offshore Cetaceans to address incidental takes of cetaceans in the California/Oregon swordfish drift gillnet fishery. The plan included skipper education workshops and required the use of pingers and minimum 36 feet extenders. Since implementation, overall cetacean entanglement rates in the California/Oregon swordfish drift gillnet fishery have dropped considerably.11

As required under the ESA, NMFS assembled a recovery team to write a recovery plan for this species. A joint draft recovery plan for the fin whale and sei whale was released in 1998.1 The key recommended actions were to:

  1. continue to protect from commercial whaling,

  2. coordinate state, federal, and international efforts to implement recovery efforts,

  3. establish classification criteria for the recovery status of fin and sei whale populations and develop criteria for delisting or downlisting recovering populations,

  4. determine population discreteness and stock structure,

  5. estimate population sizes and monitor trends in abundance,

  6. identify and protect critical habitats,

  7. identify causes and minimize frequency of human-caused injury and mortality,

  8. determine and minimize any detrimental effects of directed vessel and aircraft interactions, and

  9. maximize efforts to acquire scientific information from dead, stranded, and entangled animals.

NMFS is responsible for implementing the actions recommended in the recovery plan. On-going research projects by NMFS scientists include:

Shipboard Cetacean Surveys (Lead Scientist: Jay Barlow, Coastal Marine Mammal Program, Southwest Fisheries Science Center (SWFSC)). The abundance of cetaceans along the U.S. west coast (out to a distance of approximately 300 nautical miles) is periodically estimated from shipboard surveys. Most recently, surveys occurred in 1993, 1996, 2001 and 2005. These surveys are anticipated to continue every 4-5 years.12 The multi-year (2004-2006) SPLASH (Structure of Population, Levels of Abundance, and Status of Humpbacks research program, though targeting humpback whales, is recording the distribution and abundance of other cetaceans as time allows.

Marine Mammal Health and Stranding Response Program Network (Southwest Regional Stranding Coordinator: Joe Cordaro, SWFSC). The network consists of volunteer groups that respond to marine mammal strandings in different parts of the southwest region. Samples from stranded animals provide information on biological parameters, including age, length, reproductive condition, contaminant loads, stock discreteness, types of parasites or diseases, and cause of death. In addition to collecting data from stranded animals, this program assesses health trends, correlates health with available data on physical, chemical, environmental, and biological parameters, and coordinates effective responses to unusual mortality events.

The Pacific Fishery Management Council (PFMC) is one of eight regional fishery management councils established by the Magnuson Fishery Conservation and Management Act of 1976 for the purpose of managing fisheries 3-200 miles offshore of the United States of America coastline. The Pacific Council is responsible for fisheries off the coasts of California, Oregon, and Washington. In March 2006, the PFMC adopted Amendment 12 to the Coastal Pelagic Species Fishery Management Plan (CPSFMP). This amendment adds krill to the species managed under the (CPSFMP), and prohibits harvesting krill in the Economic Exclusive Zone off the west coast of the U.S. The amendment makes no provision for future or experimental fisheries. The ban on krill fishing protects sei whales from competition with commercial fisheries for krill resources in federal waters.


CSCAPE - Collaborative Survey of Cetacean Abundance and the Pelagic Ecosystem (Principal Investigator: Karin Forney, NMFS-SWFSC). The 2005 shipboard cetacean survey was part of a collaboration with the National Marine Sanctuary Program called CSCAPE. The primary objective of CSCAPE was to combine the typical marine mammal assessment survey with fine-scale surveys within the boundaries of the five west coast National Marine Sanctuaries. A secondary objective was to characterize the pelagic ecosystem within the study area, through the collection of underway and station-based biological and oceanographic data, seabird studies, and acoustic sampling. A final objective was to conduct biopsy sampling and photo-identification studies of marine mammal species of special interest.

Beach COMBERS - Coastal Ocean Mammal and Bird Education and Research Surveys (Project Leader: Hannah Nevins, Moss Landing Marine Laboratories). In 1997 the MBNMS began a beach survey program using trained volunteers to survey beached marine birds and mammals monthly at selected sections of beaches throughout the Monterey Bay area. Currently, the program monitors 45 km of beaches in the MBNMS. The program is a collaborative project between MLML, MBNMS, and other state and research institutions, with the specific goal of using deposition of beach cast carcasses as an index of the health of the sanctuary. The Marine Mammal Health and Stranding Response Program Network is notified of all stranded or dead cetaceans so that data can be collected and the cause of the stranding event determined.


The California Department of Fish and Game (CDFG) is not required to have research and monitoring programs for sei whales because this species is not listed as threatened or endangered under the California Endangered Species Act. However, under the federal ESA, CDFG is required to decrease or eliminate negative impacts of state-managed fisheries on sei whales. Currently, no state-managed fisheries are known to have a negative impact on this species and, given the offshore distribution of this species, interaction with state-managed fisheries is not likely in the future.


The World Conservation Union (IUCN) lists the sei whale as “Endangered” worldwide. The sei whale is listed under Appendix I of the Convention on International Trade in Endangered Species (CITES), which includes species threatened with extinction and prevents trade of Appendix I species except in exceptional circumstances. In addition, this species is listed under Appendix I and Appendix II of the North American Convention on the Conservation of Migratory Species (CMS), which includes migratory species that have been categorized as being in danger of extinction throughout all or a significant portion of their range and would benefit significantly from co-operation between Canada, Mexico and the U.S.

The International Whaling Commission (IWC) was set up under the International Convention for the Regulation of Whaling which was signed in Washington D.C. on December 2, 1946. The purpose of the Convention is to provide for the conservation of whale stocks and the development of the whaling industry. The IWC has prohibited the taking of sei whales in the North Pacific since 1986. Currently, the IWC has 57 member nations and all members have agreed to uphold the prohibition on take of sei whales. However, Article VIII of the 1946 Convention gives member states the right to issue scientific permits that allow take for research purposes. In addition, Article VIII requires that the animals be utilized once the scientific data have been collected. This requirement allows whale products to be sold in the commercial markets of the permitting country, but export of whale products is prohibited in most cases under CITES (the Convention on International Trade in Endangered Species). In 2001, Japan started issuing scientific collection permits for sei whales in the pelagic waters of the North Pacific.18 Between 2001 and 2005, 191 sei whales were collected under scientific permits.

The "Witness for the Whales" DNA surveillance program has a website, hosted by the University of Auckland, that can be used to identify illegal products from protected species of cetaceans. The website contains DNA sequences from a comprehensive and representative range of cetaceans against which a researcher can compare a DNA sequence acquired from "unknown" whale meat or other body parts. This information can be used to identify individuals or countries that are taking protected species. Between 1993 and 1999, analysis of cetacean products purchased in commercial markets in Japan revealed that 10% came from protected species, including 8 samples from sei whales.14 The source of these samples is unclear; they were collected before Japan began taking sei whales legally under scientific permit. These results have raised some concerns about illegal whaling or illegal import/export of whale products collected under scientific permit.

Research Gaps

The low abundance of this species along the west coast of the U.S. makes research difficult and expensive. If areas of predictable occurrence can be identified then research dedicated to this species could be done effectively. Researchers studying other marine mammals and birds should be coordinated to collect data (e.g., distribution, abundance, photographs, behavior, genetic samples, telemetry, etc.) from any sei whale sighted along the west coast. In addition, the following general cetacean research programs could provide data on this stock:

  • Conduct systematic, MBNMS-wide aerial surveys to determine distribution and abundance of large whales in Sanctuary waters. Although such surveys would not be targeting sei whales specifically, they may aid in documenting additional rare sighting.
  • Deploy multiple remote acoustical recording packages to detect the presence of different species of cetaceans in the Sanctuary and determine seasonality of movement through the area.
  • Determine the impacts of various types of acoustic disturbance that occur in the MBNMS, including noise from ships, boats, aircraft, and research, military and industrial activities.15,16
Recommended Actions
  • Support a continued international ban on commercial hunting and other directed lethal take. Support efforts to detect and prevent illegal whaling and to minimize the number of animals collected for scientific research.
  • Reduce the threat of entanglement in and ingestion of marine debris, particularly fishing gear. Efforts should include education outreach to fishing industry, abandoned gear recovery, and entanglement/stranding response teams.17
  • If certain acoustical disturbances are found to negatively impact sei whales, work to minimize those activities in the MBNMS.
Cited References
1. Reeves RR, Silber GK, Payne PM (1998) Draft Recovery Plan for the Fin Whale Balaenoptera physalus and Sei Whale Balaenoptera borealis. Draft Report prepared for the Office of Protected Resources, National Marine Fisheries Service, National Oceanic and Atmospheric Administration, Silver Spring, Maryland.

2. Carretta JV, Forney KA, Muto MM, Barlow J, Baker J, Hanson B, Lowry M (2005) U.S. Pacific Marine Mammal Stock Assessments: 2004. NOAA Technical Memorandum NMFS-SWFSC-375, U.S. Department of Commerce. PDF download

3. COSEWIC (2003) COSEWIC assessment and status report on the sei whale Balaenoptera borealis in Canada. Committee on the Status of Endangered Wildlife in Canada, Ottawa. PDF download
4. Perry SL, DeMaster DP, Silber GK (1999) The Great Whales: History and Status of Six Species Listed as Endangered Under the U.S. Endangered Species Act of 1973. Marine Fisheries Review 61:1-74.
5. Rice D (1974) Whales and whale research in the eastern North Pacific. In: Schevill WE (ed) The whale problem. Harvard University Press, Cambridge, MA, p 170-195
6. Gregr EJ, Nichol L, Ford JKB, Ellis G, Trites AW (2000) Migration and population structure of northeastern Pacific whales off coastal British Columbia: An analysis of commercial whaling records from 1908-1967. Marine Mammal Science 16:699-727.
7. Rice DW (1977) Synopsis of biological data on the sei whale and Bryde's whale in the eastern North Pacific. In: Fay FH, Schults LM, Dieterich RS, Melteff BR (eds) Report of the Special Meeting of the Scientific Committee on Sei and Bryde's Whales. International Whaling Commission, La Jolla, CA, p 92-97
8. Tillman MF (1977) Estimates of population size for the North Pacific sei whale. International Whaling Commission Report of the Commission 1:98-106
9. Barlow J (2003) Preliminary estimates of the abundance of cetaceans along the U.S. west coast: 1991-2001., Southwest Fisheries Science Center Administrative Report LJ-03-03, La Jolla, CA.
10. National Research Council (2005) Marine Mammal Populations and Ocean Noise: Determining When Noise Causes Biologically Significant Effects. Committee on Characterizing Biologically Significant Marine Mammal Behavior. National Academies Press, Washington, DC. 142 pages.
11. Barlow J, Cameron GA (2003) Field experiments show that acoustic pingers reduce marine mammal bycatch in the California drift gill net fishery. Marine Mammal Science 19:265-283.
12. Calambokidis J, Steiger GH, Cubbage JC, Balcomb KC, Ewald C, Kruse S, Wells R, Sears R (1990). Sightings and movements of blue whales off central California 1986-88 from photo-identification of individuals. Rep. Int. Whal. Commn, Special Issue 12:343-348.
13. Braham HW (1984) The status of endangered whales. Marine Fisheries Review 46.
14. Baker CS, Lento GM, Cipriano F, Dalebout ML, Palumbi SR (2000) Scientific Whaling: Source of Illegal Products for Market? Science 290:1695-1696.
15. Addressed in part by JMPR Wildlife Disturbance Issues - Marine Mammal, Seabird and Turtle Disturbance Action Plan: Low Flying Aircraft Disturbance Strategy. Joint Management Plan Review (JMPR). Proposed Action Plans. Draft report. Monterey Bay National Marine Sanctuary.
16. Addressed in part by JMPR Wildlife Disturbance Issues - Marine Mammal, Seabird and Turtle Disturbance Action Plan: Acoustic Disturbance Strategy. Joint Management Plan Review (JMPR). Proposed Action Plans. Draft report. Monterey Bay National Marine Sanctuary.
17. Addressed in part by JMPR Wildlife Disturbance Issues - Marine Mammal, Seabird and Turtle Disturbance Action Plan: Marine Debris Strategy. Joint Management Plan Review (JMPR). Proposed Action Plans. Draft report. Monterey Bay National Marine Sanctuary.
18. IWC website
Acknowledgement of Reviewers

Thank you to Karin Forney and Jay Barlow for reviewing this report and providing helpful comments and corrections.

Content Last Modified: 12/2005
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