Pacific Coast of Baja California Peninsula IMMA

Size in Square Kilometres

239 132 km2

Qualifying Species and Criteria

Blue whale – Balaenoptera musculus

Criterion A; C (2, 3); D2

Humpback whale – Megaptera novaeangliae

Criterion B(2); C (3)

Grey whale – Eschrichtius robustus

Criterion A; B (2); C (1,3)

Marine Mammal Diversity 

Criterion D (2)

Balaenoptera musculus, Megaptera novaeangliae, Eschrichtius robustus

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The Pacific Coast of Baja California is one of the richest and most diverse marine mammal areas worldwide, with around 26 species of cetaceans and pinnipeds regularly present in the area. Eastern North Pacific and the endangered Western North Pacific gray whales (Eschrichtius robustus) use the areas for breeding, suggesting interbreeding between these two populations during the winter breeding season may occur. Humpback whales (Megaptera novaeangliae) and Endangered blue whales (Balaenoptera musculus) also migrate along this coast. Blue whales are known to mate, calve and nurse young in the area, and the behaviour of satellite-tracked individuals suggests they also feed in the waters of this IMMA.

Description of Qualifying Criteria

Criterion A – Species or Population Vulnerability

Two gray whale (Eschrichtius robustus) populations occur in the North Pacific. Both eastern and western populations were dramatically reduced by commercial whaling during the 19th and 20th centuries. While the Eastern North Pacific (ENP) population is now considered the Least Concern on the IUCN Red List of Threatened Species (Cooke, 2018), the Western gray whale subpopulation is Endangered (Cooke et al., 2018). Western gray whales feed in the Okhotsk Sea off Sakhalin Island, Russia, and in nearshore waters of the south-eastern Kamchatka Peninsula (southwestern Bering Sea). Little is known about the current migratory routes and wintering areas of the WNP population, but historical evidence indicates that the coastal waters of eastern Russia, the Korean Peninsula, and Japan were part of the migratory route and that areas in the South China Sea were used as wintering grounds (see review by Weller et al. 2002).However, Photo-identification records provide evidence that at least 54 gray whales migrated from the feeding grounds in Russia to the eastern Pacific and the wintering grounds encompassed by this IMMA in Baja California. These whales represent close to 20% of the known Sakhalin Island gray whale population. Cook et al. (2019) estimated the proportion of the Sakhalin feeding population that migrates to the Eastern North Pacific to be 45-80%.

The Western North Pacific population numbered 175 animals (Bayesian 95% CI 158-193) in 2016 (Cook et al. 2016). Although there is evidence that the population is slowly recovering, it remains on the edge of survival ( Abundance estimates for the Western Feeding Group in 2017 range from about 150 to 320 whales (aged 1 yr and over) depending on the assumptions used with respect to stock structure hypotheses (Cooke, 2019). Photo-identification and genetic studies have shown the complexity of mixing between the WNP and ENP.

Criterion B: Distribution and Abundance

Sub-criterion B2: Aggregations

Eastern gray whales winter in the breeding lagoons and adjacent waters of Baja California, Mexico, and then migrate north along the west coast of North America to feed in the Bering and Chukchi Seas during summer (Rice and Wolman 1971). It is well documented that individuals from the western populations aggregate in the breeding lagoons in Mexico (Weller et al 2012; Mate et al 2015).  The lagoons within this IMMA represent the only known regular breeding grounds for the species, and as such constitute significant aggregations for the species.

Criterion C: Key Life Cycle Activities

Sub-criterion C1: Reproductive Areas

Gray whales from both the Eastern and Western north Pacific spend about 3 months in the Mexican wintering areas, where they mate and give birth between December and April (Jones and Swartz, 1984; Swartz, 1986). Gray whales’ calving areas along the west coast of the Baja California Peninsula are Laguna Ojo de Liebre (Scammon’s Lagoon); Laguna Guerrero Negro (when its entrance is open); Laguna San Ignacio; and Bahía Magdalena and adjacent waters (from Estero Las Animas to Bahía Almejas). About the 70% of the mothers with calves are concentrated in Laguna Ojo de Liebre, 14% in Laguna San Ignacio, and 16% in the Bahía Magdalena-Bahia Almejas complex. The average duration of stay in a breeding lagoon for breeding adults is 10 days, although females with calves may stay a month or longer in one area (Urban et al. 2003). Whales have typically departed from the Baja lagoons by late March.  Genetic studies suggest interbreeding between Eastern and Western populations during the winter breeding season (Brüniche-Olsen et al., 2018; Lang et al., 2021).

Sub-criterion C2: Feeding Areas

North Pacific blue whales spend their summers off the west coast of the United States, where they do most of their feeding for the year, and then migrate to warmer, less productive waters, during the winter, where calving occurs. Although rorquals have generally been thought to fast while on the breeding grounds, blue whales are also known to feed on their wintering/breeding grounds in the ETP and Gulf of California (Busquets-Vass et al. 2021, De Weerdt and Ramos 2019, respectively). Data inferred from satellite-tracked blue whales show that in addition to the summer feeding areas off California and Oregon, much additional feeding apparently occurs off the west coast of Baja, in the Gulf of California, and on the Costa Rica Dome (Johnson et al. 2022).

Criterion D: Special Attributes

Sub-criterion D2: Diversity

There are 22 species of cetaceans and 4 of pinnipeds within this cIMMA (Hamilton, et al. 2009; Heckel et al., 2018; Medrano y Urbán, 2018). Among the cetacean populations, there are resident populations of bottlenose dolphins (Tursiops truncatus). Migratory whales such as blue-, humpback-  and gray whales  also use the area as a wintering ground.

Supporting Information

Ballance, L.T., R.L. Pitman, and P.C. Fiedler. 2006. ‘Oceanographic influences on seabirds and cetaceans of the eastern tropical Pacific: a review’. Progress in Oceanography 69:360-390.

Brüniche-Olsen A., Urban, J., Vertyankin V.V., Godard-Codding C. A. J., Bickham J.W. and DeWoody J.A. 2018. Genetic data reveal mixed-stock aggregations of gray whales in the North Pacific OceanBiol. Lett.142018039920180399. Available at: (Accessed 09 June, 2022).

Busquets-Vass, G., et al. 2021. ‘Isotope-based inferences of the seasonal foraging and migratory strategies of blue whales in the eastern Pacific Ocean’. Mar. Environ. Res. 163 Aviable at: (Accessed 09 June, 2022).

Cooke, J. 2019. ‘Western Gray Whale population assessment update with reference to historic range and recovery prospects’. WESTERN GRAY WHALE ADVISORY PANEL 19th meeting, WGWAP-19/22 (revised) 14-16 November 2019. International Union for Conservation of Nature. Unpublished. Available at: chrome-extension://efaidnbmnnnibpcajpcglclefindmkaj/ (Accessed 09 June, 2022).

Cooke, J. G., Taylor, B. L., Reeves, R., and Brownell, R. L. Jr. (2018). Eschrichtius robustus (western subpopulation). The IUCN Red List of Threatened Species 2018:e.T8099A50345475. doi: 0.2305/IUCN.UK.2018-2. RLTS.T8099A50345475.en

De Weerdt, J., Ramos, E. A. & Cheeseman, T. 2020. ‘Northernmost records of Southern Hemisphere humpback whales (Megaptera novaeangliae) migrating from the Antarctic Peninsula to the Pacific coast of Nicaragua’. Mar. Mamm. Sci. 36, 1015–1021.

Hamilton, T. A., Redfern, J.V., Barlow, J., Ballance, L.T., Gerrodette, T., Holt, R.S., Forney, K.A. and. Taylor, L.B. 2009. ‘Atlas of cetacean sightings from Southwest Fisheries Science Center cetacean and ecosystem surveys: 1986 – 2005’. NOAA Technical Memorandum NMFS, NOAA-TM-NMFS-SWFSC-440

Jones, M. L., and Swartz, S. L. (1984). “Demography and phenology of gray whales and evaluation of whale-watching activities in Laguna San Ignacio, Baja California Sur, Mexico,” in The Gray Whale, Eschrichtius robustus, eds M. L. Jones, S. L. Swartz, and S. Leatherwood (Cambridge: Academic Press Inc), 309–374. doi: 10.1016/b978-0-08-092372-7.50020-0

Johnson, C.M., Reisinger, R.R., Palacios, D.M., Friedlaender, A.S., Zerbini, A.N., Willson, A., Lancaster, M., Battle, Jessica., Graham, A., Cosandey-Godin, A. and Jacob T.. 2022. ‘Protecting Blue Corridors – Challenges and solutions for migratory whales navigating national and international seas’. Zenodo. Available at: (Accessed 09 June, 2022).

Lang, A.R., Weller, D.W., Burdin, A.M., Robertson, K., Sychenko,O., Urbán, J., Martínez-Aguilar, S., Pease, V.L., LeDuc, R.G., Litovka, D.I., Burkanov, V.N and Brownell, R.L. 2021.’Population structure of North Pacific gray whales in light of trans-Pacific movements’. Marine Mammal Science, 1-36. DOI: 10.1111/mms.12875.

Lluch-Belda, D., Lluch-Cota D.B., and Lluch-Cota, S.E. 2003. ‘Baja California’s Biological Transition Zones: Refuges for the California Sardine’. Journal of Oceanography, Vol. 59, pp. 503 to 513.

Mate, B. R., Yu Ilyashenko, V. Y., Bradford, A. L., Vertyankin, V. V., Tsidulko, G. A., Rozhnov, V. V., et al. (2015). Critically endangered western gray whales migrate to the eastern North Pacific. Biol. Lett. 11:20150071. doi:10.1098/rsbl.2015.0071

Medrano G., L. y J. Urbán R. 2019. ‘Mamíferos marinos: identidad, diversidad y conservación’. Ciencia 70(3): 8-17.

Monnahan CC., Branch T.A., Stafford K.M., Ivashchenko Y.V., Oleson EM. 2014. ‘Estimating Historical Eastern North Pacific Blue Whale Catches Using Spatial Calling Patterns’. PLOS ONE 9(6): e98974.

Monnahan CC., Branch T.A. 2015. Sensitivity analyses for the eastern North Pacific blue whale assessment. Internatinal Whaling Commission, Scientific Committee SC/66A/IA/15.

Reilly SB, Bannister JL, Best PB, Brown M and others (2008) Eschrichtius robustus (western subpopulation). In: IUCN Red List of Threatened Species. Version 2011.1. Available at

Reilly, S.B., Thayer, V.G., 1990. ‘Blue whale (Balaenoptera musculus) distribution in the eastern tropical Pacific’. Marine Mammal Science 6(4):265–277.

Rice, D. W., and Wolman, A. A. (1971). The Life History and Ecology of the Gray Whale (Eschrichtius robustus). ASM, Special Publication 3. Lawrence: American Society Mammalogist, 142.

Rugh, D. J., Shelden, K. E. W., and Schulman-Janiger, A. (2001). Timing of the gray whale southbound migration. J. Cetacean Res. Manag. 3, 31–39.

Sears, R., Ramp, C., Douglas, A., Calambokidis, J., 2013. ‘Reproductive parameters of eastern North Pacific blue whales Balaenoptera musculus’. Endanger. Species Res. 22, 23–31.

Stewart, J.D. and Weller D.W.2021. ‘Abundance of eastern North Pacific gray whales 2019/2020’. U.S. Department of Commerce, NOAA Technical Memorandum NMFS-SWFSC-639. Available at: (Accessed 09 June, 2022).

Swartz, S. L. (1986). Gray whale migratory, social and breeding behavior. Rep. Int. Whaling Comm. 8, 207–229.

Urbán, R. J., Rojas-Bracho, L., Pérez-Cortéz, H., Gómez-Gallardo, A. U., Swartz, S., Ludwing, S., et al. (2003). A review of gray whales on their wintering grounds in Mexican waters. J. Cetacean Res. Manag. 5, 281–295.

Urbán R J, Jiménez-López E, Guzmán HM and Viloria-Gómora L (2021) Migratory Behavior of an

Eastern North Pacific Gray Whale From Baja California Sur to Chirikov Basin, Alaska. Front. Mar. Sci. 8:619290. doi: 10.3389/fmars.2021.619290

Weller, D.W., Klimek, A., Bradford, A.L., Calambokidis, J., Lang, A.R., Gisborne, B., Burdin, A.M., Szaniszlo, W., Urbán, J., Gomez-Gallardo Unzueta, A., Swartz, S. and Brownell, R.L., Jr. 2012. ‘Movements of gray whales between the western and eastern North Pacific’. Endangered Species Research 18:193-199

Weller, D. W., Klimek, A., Bradford, A. L., Calambokidis, J., Lang, A. R., Gisborne, B., et al. (2012). Movements of gray whales between western and eastern North Pacific. Endanger. Species Res. 18, 193–199. doi: 10.3354/ esr00447


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