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 (https://iwc.int/management-and-conservation/conservation-management-plans/western-gray-whale-cmp). 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.
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