The Gulf of California: Biodiversity and Conservation

9. Marine Mammals of the Gulf of California: An Overview of Diversity and Conservation Status

CHAPTER 9 Marine Mammals of the Gulf of California An Overview of Diversity and Conservation Status jorge urbán r. Summary Interest in the study of Mexican marine mammals dates from the mid-nineteenth century, a result of their commercial exploitation. Nevertheless , until the 1970s scientific studies of the marine mammals of Mexico were infrequent and mainly limited to occasional observations by American and a few Mexican scientists. Beginning in the late 1970s, the number of reports published on individual species or groups of species in the Gulf of California began to increase substantially. The marine mammal fauna of the Gulf is surprisingly diverse, with 36 species representing 31 cetaceans in eight families (Balaenidae, Eschrichtiidae , Balaenopteridae, Physeteridae, Kogiidae, Ziphiidae, Delphinidae, Phocoenidae), four pinnipeds in two families (Phocidae, and Otariidae), and one bat in the family Vespertilionidae. The Gulf of California has 39 percent of the world’s 83 cetacean species. The Odontoceti (toothed whales) are represented by 23 species, 33 percent of the extant species, and the Mysticeti (baleen whales) by 8 species, 61 percent of the extant species. The notable diversity (i.e., species richness) and abundance of marine mammals in the Gulf can be explained by three main factors. First, the Gulf has an exceptionally high rate of primary productivity that supports complex and productive food webs. Second, the complex topography and oceanography of the Gulf presents a high diversity of habitats. And third, the warm and relatively calm waters found in the Gulf during winter and spring are exploited by several migratory species to give birth, nurse, and care for their newborn. According to the IUCN, the Gulf of California includes one species that is critically endangered (vaquita), four that are endangered (blue Marine Mammals / 189 whale, fin whale, sei whale, northern right whale), and two that are vulnerable (humpback whale, sperm whale). The Mexican Government’s Of- ficial Standard (NOM-59-ECOL-2001) includes all of the cetacean species entering Mexico’s waters. The North Pacific right whale and the vaquita are listed as “in danger of extinction” and all others as “subject to special protection.” The main human-caused mortalities of cetaceans in the Gulf of California are related to fisheries. The cetaceans most frequently involved in incidental mortality from fisheries are the short-beaked common dolphin, long-beaked common dolphin, common bottlenose dolphin, pantropical spotted dolphin, and vaquita. Yet large cetaceans, including sperm, gray, and humpback whales, also have been affected by fisheries. There is no single body of legislation enacted for the sole benefit of cetaceans in the Gulf. Instead there are several different laws relevant to their conservation and management that apply to all of Mexico. Additionally , in May 2002, Mexico established the Mexican Whale Refuge (Refugio Ballenero Mexicano), consisting of its entire Exclusive Economic Zone (about 3 million km2 ). Introduction Interest in the study of Mexican marine mammals dates from the mid-nineteenth century, mainly between 1850 and 1870, and was a direct result of their commercial exploitation. Whalers, principally American, made numerous voyages to hunt large cetaceans in the Mexican Pacific. From sailing ships and whaling dories they sought mostly gray whales, Eschrichtius robustus, but also took humpback whales, Megaptera novaeangliae (Scammon 1874/1968; Henderson 1972, 1984; Urbán R. et al. 2003b), and sperm whales, Physeter macrocephalus (Rice 1974). By the early twentieth century, modern mechanized whalers had expanded the catch to include some of the faster rorquals that were previously uncatchable by the “Yankee ” whalers: blue whales, Balaenoptera musculus, Bryde’s whales, B. edeni, and minke whales, B. acurostrata (Rice 1974). The California sea lion, Zalophus californianus, Guadalupe fur seal, Arctocephalus townsendi, and northern elephant seal, Mirounga angustirostris, were also heavily exploited on islands off the Pacific coast of Baja California (Bartholomew and Hubbs 190 / jorge urbán r. 1960; Le Boeuf et al. 1983; Zavala-Gonzalez and Mellink 2000). Before the 1970s, scientific studies of the marine mammals of Mexico were infrequent and mainly limited to occasional observations by American and a few Mexican scientists (e.g., Bartholomew and Hubbs 1952; Berdegué 1956a; Norris and McFarland 1958; Gilmore 1960; van Gelder 1960; Norris and Prescott 1961; Gilmore et al. 1967; Lluch 1969; Barham 1970; Rice 1974). Beginning in the early 1970s, an increasing number of reports began to be published on individual species or groups of species in the Gulf of California. Vidal, Findley, and Leatherwood published the first...