Eastern Pacific Warm Pool IMMA

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.