Eastern Pacific Warm Pool IMMA

Size in Square Kilometres

1 873 241 km2

Qualifying Species and Criteria

Eastern spinner dolphin – Stenella longirostris orientalis 

Criterion B (2); D (1)

Pygmy beaked whale – Mesoplodon peruvianus 

Criterion B (2); D (1)

 Offshore pantropical spotted dolphin – Stenella attenuata attenuata

Criterion B (2)

Blue whale Eastern North Pacific – Balaenoptera musculus musculus 

Criterion A; C (3)

Marine Mammal Diversity 

Criterion D (2)

Balaenoptera edeni, Physeter macrocephalus, Kogia sima, Ziphius cavirostris, Orcinus orca, Feresa attenuata, Pseudorca crassidens, Grampus griseus, Steno bredanensis, Tursiops truncatus, Stenella coeruleoalba, Delphinus delphis

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The Eastern Pacific Warm Pool is home to a diverse and abundant array of tropical cetaceans, with at least 16 species regularly recorded in the area. The Eastern Tropical Pacific (ETP) is usually defined as the waters between the tip of Baja California and northern Peru, extending westward from Central and South America toward Hawaii. It is at the eastern end of a basin-wide equatorial current system, located between the North and South Pacific subtropical gyres, and at the terminus of two eastern boundary currents: the California Current and the Peru Current. The Warm Pool is at the center of the ETP and it can be clearly defined as an area with a surface temperature >27.5 ̊C, and has been described as “an open-ocean biogeographic province with a distinct biological community”. It represents the core range of the endemic pygmy beaked whale (Mesoplodon peruvianus) and is also on the migratory corridor of eastern North Pacific blue whales travelling to and from the Central America Thermal Dome. In addition, it is home to globally important concentrations of pantropical spotted dolphin (Stenella attenuata) and spinner dolphins (Stenella longirostris); the latter comprising a locally endemic subspecies (S. l. orientalis).

Description of Qualifying Criteria

Criterion A – Species or Population Vulnerability

Although eastern North Pacific blue whales (Balaenoptera musculus musculus; Committee on Taxonomy, 2022) may be at pre-exploitation numbers and one of the healthiest blue whale populations worldwide (Sears et al., 2013, Monnahan et al. 2015), globally the species is still listed as Endangered (EN) by the IUCN Red List of Endangered Species (Cooke, 2018). Tuna purse seine fishermen in the ETP began setting their nets around dolphin schools to catch tunas in the 1960s, which resulted in the deaths of millions of dolphins, mainly offshore pantropical spotted dolphins (Stenella attenuata attenuata) and eastern spinner dolphins (Stenella longirostris orientalis). It was estimated that the fishing mortality reduced these dolphin populations to one-third and one-fifth of their historical numbers, respectively (Wade et al., 2007). Although a change in fishing gear and techniques reduced the number of dolphin deaths from an estimated high of 550,000 in 1961, to about 1000/yr now (Ballance et al., 2021), results from survey cruises through the year 2000 found that the populations were not showing signs of recovery, although the reason(s) for the lack of recovery was not clear (Wade et al., 2007). Results from additional surveys in 2003 and 2006 indicated that populations might be starting to recover (Gerrodette et al., 2008), but the results were inconclusive, and there have been no further survey cruises. Eastern spinner dolphins are still listed as Vulnerable (VU) under the IUCN Red List of Threatened Species (Hammond et al., 2012). However, the offshore pantropical spotted dolphin subspecies has not been assessed for the Red List.

Criterion B: Distribution and Abundance

Sub-criterion B2: Aggregations

The pygmy beaked whale (Mesoplodon peruvianus) is assumed to be endemic to the ETP because a large majority of the at-sea sightings have been in the vicinity of the Warm Pool (Pitman and Lynn, 2001; Hamilton et al., 2009), and all the known strandings except one (New Zealand: Baker and van Helden, 1999) have come from the eastern Pacific (Reyes and Van Waerebeek, 2018). The entire world population of eastern spinner dolphins is located in the ETP and centered on the Warm Pool area (Hamilton et al., 2009; Perrin, 2018b).

Criterion C: Key Life Cycle Activities

Sub-criterion C3: Migration Routes

Humpback whales (Megaptera novaeangliae) and blue whales are the only two cetaceans that seasonally migrate to and from the ETP, although humpbacks tend to migrate along the coast and occupy nearshore winter habitats (Martínez-Loustalot et al., 2022). Blue whales from the eastern North Pacific spend the summer feeding in the California Current or further north and then migrate to winter feeding and nursery grounds in the Gulf of California or the Central America Thermal Dome (Reilly and Thayer, 1990; Ballance et al., 2006; Hamilton et al., 2009; Busquets-Vass et al., 2021; Johnson et al., 2022), although a few can be found year-round on these putative ‘wintering’ grounds (Busquets-Vass et al., 2021). Most migrating eastern North Pacific blue whales travel off the west coast of Baja California, Mexico, where southbound whales then either turn north, into the Gulf of California, or continue south to the Central America Thermal Dome (Johnson et al., 2022); the migration corridor of nearly all blue whales traveling to and from the Dome pass through the Warm Pool (Johnson et al., 2022).

Criterion D: Special Attributes

Sub-criterion D1: Distinctiveness

The pygmy beaked whale, with its distinctive and diagnostic adult male colour patterning, was first reported as an unidentified Mesoplodon in 1987 from multiple field observations in the eastern tropical Pacific (Pitman et al., 1987). M. peruvianus was independently described as a new species in 1991 based on 10 stranded and live-caught (fisheries) specimens from Peru (Reyes at al., 1991) but it was not until 2013 that these sightings and specimens were confirmed to be the same species, based on a genetic analysis of a stranded adult male from Mexico (Urbán et al., 2013). Since then, the only known record of this species outside of the eastern Pacific has been a stranded specimen from New Zealand (Baker and Van Helden, 1999), possibly a vagrant. Strandings in the eastern Pacific have been reported as far north as Moss Landing, California (36°47′ N), and as far south as northcentral Chile (29°17′ S), and it has been suggested that there could be separate populations in the eastern and western Pacific, as well as in the Northern and Southern hemispheres of the eastern Pacific (Reyes and Van Waerebeek, 2018). Most at-sea sightings, however, have been in the core area of the ETP (Pitman and Lynn, 2001; Hamilton et al., 2009), which has led to speculation that this species is endemic to the ETP, including the southern Gulf of California (Pitman and Lynn, 2001, Jefferson et al., 2015). Spinner dolphins occur worldwide in the tropics where they are often common. Globally, four subspecies have been identified (Perrin, 2018), including two in the eastern tropical Pacific: the Central American spinner dolphin (S. l. centroamericana), which occupies a very narrow zone along the continental shelfbreak of Central America, and the eastern spinner dolphin, which is much more abundant and widespread, but largely confined to the Warm Pool and adjacent waters (Hamilton et al., 2009).

Sub-criterion D2: Diversity

The high levels of productivity and the available prey base in the Eastern Pacific Warm Pool IMMA are instrumental in supporting a high diversity of cetacean species. In addition to the blue whales, spotted and spinner dolphins, and pygmy beaked whales that are described above, least 12 additional species are known to occur regularly within the boundaries of the IMMA (Ballance and Pitman, 1998; Hamilton et al., 2009). These include Bryde’s whales (Balaenoptera edeni), Sperm whales (Physeter macrocephalus), and ten species of small- to medium-sized odontocetes associated with deep oceanic habitats.

Supporting Information

Baker, A. N., and A. L. van Helden. 1999. New records of beaked whales, Genus Mesoplodon, from New Zealand (Cetacea: Ziphiidae). Journal of the Royal Society of New Zealand 29(3):235-244.

Ballance. L.T. and R.L. Pitman. 1998. Cetaceans of the Western Tropical Indian Ocean: Distribution, Relative Abundance, and Comparisons with Cetacean Communities of Two Other Tropical Ecosystems.  Marine Mammal Science 14(3):429-459.

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.

Ballance, L.T., T. Gerrodette, C.E. Lennert-Cody, R.L. Pitman, and D. Squires. 2021. A history of the tuna-dolphin problem: successes, failures, and lessons learned. Frontiers in Marine Science. doi: 10-3389/fmars.2021.754755

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

Committee on Taxonomy. 2022. List of marine mammal species and subspecies. Society for Marine Mammalogy. URL https://marinemammalscience.org/species-information/list-marine-mammal-species-subspecies/

Cooke, J.G., 2018. Balaenoptera musculus (errata version published in 2019). The IUCN Red List of Threatened Species 2018: e.T2477A156923585. https://dx.doi.org/10.2305/IUCN.UK.2018-2.RLTS.T2477A156923585.en

Fiedler, P.C., Talley, L.D., 2006. Hydrography of the eastern tropical Pacific: a review. Progress in Oceanography 69 (2–4), 143–180.

Gerrodette, T., Watters, G., Perryman, W., and Ballance, L. (2008). Estimates of 2006 dolphin abundance in the Eastern Tropical Pacific, with revised estimates from 1986-2003. NOAA Technical Memorandum NOAA-TM-NMFS-SWFSC-422. Available online at: https://swfsc.noaa.gov/publications/PubBIN#/search/

Hamilton, T. A., et al. 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.

Hammond, P.S., Bearzi, G., Bjørge, A., Forney, K.A., Karkzmarski, L., Kasuya, T., Perrin, W.F., Scott, M.D., Wang, J.Y. , Wells, R.S. & Wilson, B. 2012. Stenella longirostris ssp. orientalis. The IUCN Red List of Threatened Species 2012: e.T133712A17838296. dx.doi.org/10.2305/IUCN.UK.2012.RLTS.T133712A17838296.en. Accessed on 17 October 2022.

Jefferson, T.A., M. A. Webber, and R. L. Pitman. 2015. Marine Mammals of the World, 2nd edition. Academic Press, San Diego, CA. 610 pp.

Johnson, C.M. et al. 2022. Protecting Blue Corridors – Challenges and solutions for migratory whales navigating national and international seas. Zenodo. https://doi.org/10.5281/zenodo.6196131

M.F. Lavín, P.C. Fiedler, J.A. Amador, L.T. Ballance, J. Färber-Lorda, and A.M. Mestas-Nuñez. 2006. A review of eastern tropical Pacific oceanography: Summary. Progress in Oceanography 69 (2006) 391–398.

Martínez-Loustalot, P., Audley, K., Cheeseman, T., De Weerdt, J., Frisch-Jordán, A., Guzón, O., Olio, M., Ortega-Ortiz, C. D., Ransome, N., Villegas-Zurita, F., & Urbán R., J. 2022. Towards the definition of the humpback whale population units along the Mexican and Central American coasts in the Pacific Ocean. Marine Mammal Science, 1– 16. https://doi.org/10.1111/mms.12980

Monnahan, C. C., T. A. Branch and A. E. Punt. 2014. Do ship strikes threaten the recovery of endangered eastern North Pacific blue whales? Marine Mammal Science 31:279–297.

Perrin, W. F. 2018a. Pantropical spotted dolphin Stenella attenuata. In B. Würsig, J. G. M. Thewissen, & K. Kovacs (Eds.), Encyclopedia of marine mammals (3rd ed., pp. 676-678). San Diego, CA: Academic Press.

Perrin, W. F. 2018b. Spinner dolphin Stenella longirostris. In B. Würsig, J. G. M. Thewissen, & K. Kovacs (Eds.), Encyclopedia of marine mammals (3rd ed., pp. 925-928). San Diego, CA: Academic Press.

Pitman, R.L., Anelio Aguayo-L., and Jorge Urban-R. 1987.  Observations of an unidentified beaked whale (Mesoplodon sp.) in the eastern tropical Pacific. Marine Mammal Science 3:345-352.

Pitman, R.L. and M. S. Lynn. 2001.  Biological observations of an unidentified mesoplodont whale in the eastern tropical Pacific and probable identity: Mesoplodon peruvianus.  Marine Mammal Science 17(3):648-657.

Reyes, J. C., J. G. Mead, and K. Van Waerebeek. 1991. A new species of beaked whale Mesoplodon peruvianus sp. n. (Cetacea: Ziphiidae) from Peru. Marine Mammal Science 7:1-24.

Reyes, J. C., and K.  Van Waerebeek. 2018. The lesser beaked whale Mesoplodon peruvianus Reyes, Mead & Van Waerebeek 1991 revisited, with biological observations on new specimens from Peru. J Mar Biol Oceanogr 7:4.

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. https://doi.org/10.3354/esr00532

Thomas, P. O., R. R. Reeves, and R. L. Brownell, Jr. 2016. Status of the world’s baleen whales. Marine Mammal Science.

Urbán J, Viloria L, Valdovinos LA. 2013. Molecular determination of the identity of Mesoplodon Sp. A. Document SC/65a/SM28, IWC Scientific Committee

Wade, P. R., G. M. Watters, T. Gerrodette, and S. B. Reilly. 2007. Depletion of northeastern offshore spotted and eastern spinner dolphins in the eastern tropical Pacific and hypotheses for their lack of recovery. Marine Ecology Progress Series 343:1-14.


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