Size in Square Kilometres
518 078 km2
Qualifying Species and Criteria
Blue whale – Balaenoptera musculus
Criterion A; B (2); C (1, 2)
Striped dolphin – Stenella coeruleoalba
Criterion B (2)
Common dolphin – Delphinus delphis
Criterion B (2)
Pygmy beaked whale – Mesoplodon peruvianus
Criterion B (2)
Marine Mammal Diversity
Criterion D (2)
Stenella longirostris orientalis, Stenella attenuata graffmani, Stenella attenuata, Tursiops truncatus, Steno bredanensis, Feresa attenuata, Globicephala macrorhynchus, Kogia sima, Megaptera novaeangliae, Grampus griseus, Balaenoptera edeni, Peponocephala electra, Mesoplodon densirostris, Pseudorca crassidens, Orcinus orca, Physeter macrocephalus, Ziphius cavirostris
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The Thermal Dome region in the Central American Pacific Ocean, also referred to as the Costa Rica Thermal Dome, is characterized by pronounced and regular upwelling that leads to high levels of productivity. This provides a unique habitat for cetaceans in the Eastern Tropical Pacific. Blue whales (Balaenoptera musculus) from the North Eastern Pacific and, presumably, South Eastern Pacific blue whales to migrate, feed, and reproduce in the area. Other cetacean species using this IMMA include striped dolphins (Stenella coeruleoalba) and common dolphins (Delphinus delphis), utilising the Thermal Dome as a feeding ground. The Dome serves as both a feeding and a nursery ground for many other marine mammal species. Among the 21 cetacean species and subspecies present in the Thermal Dome region, 14 have distributions that regularly extend into the Dome, likely attracted by the high productivity and abundance of prey. Seven other species extend their range into the Dome but do not show a specific preference for this area.
Description of Qualifying Criteria
Criterion A – Species or Population Vulnerability
The Blue Whale (Balaenoptera musculus) is listed as an Endangered (EN) on the IUCN Red List of Threatened Species due to strong reductions in its populations from commercial whaling (Cooke, 2018). Blue whales off the coast of California migrate during the boreal winter-spring season to the Thermal Dome, which constitutes their primary feeding and breeding ground during that season (Balance et al., 2006, Mate et al., 2008, Fiedler et al., 2017, Leduc et al., 2017, Busquets-Vass et al., 2020). The population also breeds in the Dome region; making the site critical to life-cycle of this vulnerable species (Bailey et al., 2009; Reilly and Thayer. 2006) and a critical element in its migratory route (Bailey et al., 2009). Limited information suggests that there may be also migrants from the Southern Ocean, or even residents at the Dome year-round (Sears and Perrin, 2009).
Criterion B: Distribution and Abundance
Sub-criterion B2: Aggregations
Out of 211 blue whale sightings documented in the eastern tropical Pacific, 90% were concentrated along two areas: Baja California, and the vicinity of the Thermal Dome (Reilly and Thayer, 2006). Distribution maps demonstrate that the distribution of blue whales in the eastern tropical Pacific is closely related to the presence and fluctuations in the Dome’s occurrence (Fiedler, 2017). The distribution patterns of striped dolphins (Stenella coeruleoalba), common dolphins (Delphinus delphis) and pygmy beaked whales (Mesoplodon peruvianus) are closely-related to the Thermal Dome (Fiedler et al.,2017, Hamilton et al., 2008). All these species aggregate here probably due to the high productivity and availability of preferred prey species (Mate et al., 1999, Reilly and Thayer, 2006, Bailey et al., 2009).
Criterion C: Key Life Cycle Activities
Sub-criterion C1: Reproductive Areas
Blue whales from the northeast Pacific use the Gulf of California and the Thermal Dome in the eastern Tropical Pacific as their primary winter-spring breeding grounds (Leduc et al., 2017, Mate et al. 1999, Reilly & Thayer, 1990, Hoyt, 2009). Young calves have been observed in the Dome region during the boreal winter, attracting orcas (Orcinus orca) to the region to prey on the calf population (Pitman et al., 2007, Brower 2009).
Sub-criterion C2: Feeding Areas
Blue whales occupy the Thermal Dome year-round (Reilly and Thayer, 2006) and are mainly concentrated on the western side of the Dome (Bailey et al., 2009). It is known that biomass aggregates not at the core of the upwelling but “downstream” where sharp transitions in water density retain many nutrients, allowing phytoplankton and primary consumers to grow and aggregate (Pardo et al., 2015, White et al., 1995). The abundance of blue whales at the western side of the Dome coincides with the presence of extensive patches of krill (euphausiids), which are most abundant at depths ranging of 50-200m. Diving profiles of blue whales tagged in this region indicate regular dives of 50-300 m coinciding with krill vertical migration patterns (Stafford et al., 2005, Mate et al., 2008). The high aggregations of blue whales in winter are significantly associated with high chlorophyll concentrations and low subsurface temperatures, indicating that feeding is linked to plankton aggregations (Matteson et al., 2010).
Criterion D: Special Attributes
Sub-criterion D2: Diversity
The Thermal Dome region, due to its consistently high levels of productivity and prey availability, is a site with a high richness of marine mammal species. The distribution of three species: blue whales, common dolphins, and striped dolphins, are closely associated with the Dome region (Fiedler et al., 2017). A further 12 species have documented distributions partially overlapping with the Dome region (Fiedler et al., 2017). Of these 12, the offshore pantropical spotted dolphins (Stenella attenuata), eastern spinner dolphins (Stenella longirostris orientalis), rough-toothed dolphins (Steno bredanensis) and short-finned pilot whales (Globicephala macrorhynchus) show association with warm water regions north or south of the Dome, and are partially distributed within the Dome region itself (Fiedler et al., 2017). Common bottlenose dolphins (Tursiops truncatus), Risso’s dolphins (Grampus griseus), coastal pantropical spotted dolphins (Stenella attenuata graffmani), dwarf sperm whales (Kogia sima), pigmy killer whales (Feresa attenuata), pygmy beaked whales (Mesoplodon peruvianus), and humpback whales (Megaptera novaeangliae), though mainly coastal in their distribution, extend their range into the Dome (Hamilton et al., 2008). Six other species: false killer whales (Pseudorca crassidens), killer whales (Orcinus orca), sperm whales (Physeter macrocephalus), Cuvier’s beaked whales (Ziphius cavirostris), melon-headed whales (Peponocephala electra) and Blainville’s beaked whales (Mesoplodon densirostris), have been recorded in the Dome region but in lower densities and without a clear preference for this area (Hamilton et al., 2008, Fiedler et al., 2017).
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