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Compare-Contrast-Connect: Marine Mammal Decline and Conservation

NGSS Crosscutting Concepts

Worldwide, most marine mammals are threatened with extinction due to human activities (SF Fig. 6.1; SF Table 6.1). Several species have already been driven to extinction. The Steller’s sea cow was last observed in the wild in 1768, only 27 years after its discovery by western explorers. More recently, the sea mink was last observed in 1894, the Caribbean monk seal was last observed in 1952, the Baiji river dolphin was last observed in 1997, and the Japanese sea lion was last observed in 1974.


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SF Fig. 6.1.  (A) Caribbean monk seal (Neomonachus tropicalis)

Image copyright and source

Image courtesy of New York Zoological Society

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SF Fig. 6.1. (B) Baiji or Yangtze River dolphin (Lipotes vexillifer)

Image copyright and source

Image courtesy of Alessio Marrucci, Wikimedia Commons

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SF Fig. 6.1. (C) Model of Steller’s sea cow (Hydrodamalis gigas), an extinct sirenian

Image copyright and source

Image courtesy of Emőke Dénes, Wikimedia Commons


The International Union for Conservation of Nature and Natural Resources (IUCN) is a non-governmental organization best known for researching and maintaining the Red List of Threatened Species.


Currently, the IUCN considers the following marine mammals to be critically endangered: the vaquita porpoise (Phocoena sinus; SF Fig. 6.2 C), Māui’s dolphin (Cephalorhynchus hectori maui; SF Fig. 6.2 D), southern blue whale (Balaenoptera musculus intermedia), western gray whale (Eschrichtius robustus), Svalbard-Barents Sea bowhead whale (Balaena mysticetus), northeast Pacific right whale (Eubalaena japonica), Chile-Peru southern right whale (Eubalaena australis), Cook Inlet beluga whale (Delphinapterus leucas), several subpopulations of Irrawaddy dolphin (Orcaella brevirostris), Baltic Sea harbor porpoise (Phocoena phocoena), eastern Taiwan Strait Indo-pacific humpback dolphin (Sousa chinensis), and Fiordland common bottlenose dolphin (Tursiops truncatus). The vaquita porpoise is found only in the northern part of the Gulf of California. Conservation biologists expect the vaquita’s extinction in the near future, since only 30 to 60 individuals remain in the wild as of November 2016.


SF Fig. 6.2. (A) Hawaiian monk seal (Neomonachus schauinslandi) juvenile, Pearl and Hermes Atoll or Holoikauaua, Papahānaumokuākea Marine National Monument, Hawai‘i

Image courtesy of National Oceanic and Atmospheric Administration (NOAA)

SF Fig. 6.2. (B) Sei whale (Balaenoptera borealis)

Image courtesy of National Oceanic and Atmospheric Administration (NOAA)


SF Fig. 6.2. (C) Two vaquita porpoises (Phocoena sinus), Gulf of California, Pacific ocean basin

Image courtesy of Paula Olson, National Oceanic and Atmospheric Administration (NOAA)


Marine mammals considered endangered by the IUCN include the marine otter (Lontra felina), sea otter (Enhydra lutris), Hawaiian monk seal (Neomonachus schauinslandi; SF Fig. 6.2 A), Mediterranean monk seal (Monachus monachus), Galápagos fur seal (Arctocephalus galapagoensis), Caspian seal (Pusa caspica), Ungava seal (Phoca vitulina mellonae), Saimaa ringed seal (Pusa hispida saimensis), western Steller’s sea lion (Eumetopias jubatus), Galápagos sea lion (Zalophus wollebaeki), New Zealand sea lion (Phocarctos hookeri), Australian sea lion (Neophoca cinerea), blue whale (Balaenoptera musculus), sei whale (Balaenoptera borealis; SF Fig. 6.2 B), fin whale (Balaenoptera physalus), Hector’s dolphin (Cephalorhynchus hectori), South Asian river dolphin (Platanista gangetica), Okhotsk Sea bowhead whale (Balaena mysticetus), north Atlantic right whale (Eubalaena glacialis), north Pacific right whale (Eubalaena japonica), Black Sea harbor porpoise (Phocoena phocoena relicta), Oceania and Arabian sea populations of humpback whale (Megaptera novaeangliae), and Mediterranean sperm whale (Physeter macrocephalus).


The dominant human activity threatening most marine mammal species is overhunting (SF Table 6.1). Although indigenous peoples have been hunting whales for up to 4,000 years, commercial whaling for whale oil, blubber, and meat drastically reduced global whale populations beginning in the 18th century. The industry declined sharply early in the 20th century with kerosene and vegetable oils replacing whale oil. Similarly, humans hunted many species of pinnipeds and otters for the fur and meat trades. Many populations have recovered, but the IUCN-listed species remain threatened. Another threat to marine mammals is unintentional hunting when they are caught in fishing gear or traps targeting other species. Popular examples of this “bycatch” include capturing spinner dolphins inside tuna fishing purse seine nets, catching vaquita porpoises in gill nets, and Māui’s dolphin entanglement in drift nets. The polar bear and several Arctic pinniped species are also threatened by the loss of sea ice and global climate change caused by human activities.


SF Table 6.1. Threats to marine mammal conservation
Major taxon Representative species Past and current threats to conservation
Cetaceans Blue whale, gray whale Overhunting, noise pollution, boat strikes, river dams
Sirenians West Indian manatee Overhunting, boat strikes, habitat loss
Pinnipeds & otters Hawaiian monk seal, Steller's sea lion Overhunting, habitat loss, disease
Polar bear Polar bear Global climate chang (loss of sea ice)


Many of these endangered marine mammal species serve as top predators in their native habitats, where they play a crucial role in maintaining ecosystem health. Marine mammals also serve as charismatic “flagship species” or “umbrella species” that promote conservation efforts for their entire region or habitat type.


Question Set
  1. What are some examples of marine mammal extinctions? What caused these extinctions to occur?
  2. What can humans do to conserve endangered and threatened marine mammals?
  3. What is the importance of protecting a few rare marine mammal species?
Exploring Our Fluid Earth, a product of the Curriculum Research & Development Group (CRDG), College of Education. University of Hawaii, 2011. This document may be freely reproduced and distributed for non-profit educational purposes.