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Marine Mammals


by
Anne Kinsinger
National Biological Service
At least 35 species of marine mammals are found along the U.S. Atlantic coast and in the Gulf of Mexico: 2 seal species, 1 manatee, and 32 species of whales, dolphins, and porpoises (see Table 1 for status of selected species). Seven of these species are listed as endangered under the Endangered Species Act (ESA). At least 50 species of marine mammals are found in U.S. Pacific waters: 11 species of seals and sea lions; walrus; polar bear; sea otter; and 36 species of whales, dolphins, and porpoises; 11 species are listed as endangered or threatened under the ESA (see Table 2 for the status of selected species).

NMFS Assessments
The National Marine Fisheries Service (NMFS), an agency within the National Oceanic and Atmospheric Administration (NOAA), conducts research and status studies on many of these marine mammals under the authorities of the Magnuson Fisheries Conservation and Management Act, the Marine Mammal Protection Act (MMPA), and the ESA. The results of the status surveys include information required by the MMPA and the ESA on abundance (population size); status (as compared with historical levels or current viability); trends (changes in abundance); and status in U.S. waters. These results, published annually by NOAA, are the basis for this summary (NOAA 1994). Table 1. Status of selected Atlantic and Gulf of Mexico coast species of marine mammals.

Species and geographic area Abundance Status Trends Official status in designated U.S. waters
Fin whale, NE U.S. 5,200 Unknown Unknown Endangered*
Humpback whale, NW Atlantic 5,100 (2,888-8,112) Possibly 65% of 1850 population Unknown Endangered*
Northern right whale, NW Atlantic 350 Probably <5% of its original number Unknown Endangered*
Pilot whales, NE U.S. Unknown Unknown Unknown  
Bottlenose dolphin        
  NE; U.S. coastal type Unknown Possibly down by 50% 1987-88 Unknown Depleted**
  NE; U.S. offshore type 10,000-13,000 Unknown    
  Gulf; of Mexico (offshore and coastal types) 35,000-45,000 Possibly down by 50% 1987-88 Unknown  
Whitesided dolphin, NE U.S. 27,600 Unknown Unknown  
Spotted dolphin, NE U.S. 200 Unknown Unknown  
Harbor porpoise, Gulf of Maine 47,200 Unknown Unknown Proposed as threatened*
Harbor seal, NE U.S. 26,000 Unknown Increasing  
Beaked whales (six species in U.S. waters) Unknown Unknown Unknown  
* Endangered Species Act.
** Marine Mammal Protection Act.

Estimates of abundance in U.S. waters are available for many, though not all, marine mammal species. Information on status and trends, however, is extremely limited because so little is known of the basic life history of many marine mammal species that scientists can determine neither status nor whether a population estimate represents a healthy, sustainable population. Moreover, long-term trends in many populations cannot be determined because historical population data are not available.
The NMFS provides assessments for 139 stocks (i.e., populations of species or groups of species that are treated together for management) of marine mammals; the status of 120 stocks is unknown, and trend data are only available for 19 stocks. The recently reauthorized MMPA requires the NMFS to conduct periodic assessments of marine mammal stocks that occur in U.S. waters. For this reason, better status and trends data are likely to become available over the next few years.
Abundance and status data for selected marine mammals are summarized in Table 1 (Atlantic species) and Table 2 (Pacific species). Trend data are mixed, but a number of conservation success stories have come from marine mammals. The bowhead and grey whales have shown significant population increases, as have California sea lions, the northern elephant seal, harbor seals in California, Oregon, Washington, and the Northeast, and the southern sea otter. These increases are largely the result of prohibition of commercial whaling by the International Whaling Commission (IWC) and by protection enacted under the MMPA and ESA. Other marine mammal populations, such as the Steller sea lion and the common dolphin in the eastern tropical Pacific, are still declining. Causes of decline in marine mammal populations include bycatch associated with commercial fishing, illegal killings, strandings, entanglement, disease, ship strikes, altered food sources, and possibly exposure to contaminants.

Population Trends
Whales
Fig. 1. Estimated population of grey whales, 1967-90 (NOAA 1994).
The eastern North Pacific stock of grey whale (Eschrichtius robustus) is rising (Fig. 1) and is one success story in species restoration. The NMFS estimates that the historical populations of grey whales in 1896 were around 15,000-20,000. While current population levels are below the estimated carrying capacity of 24,000, they appear higher than historical levels and represent a substantial gain. The population growth rate between 1968 and 1988 was 3.3% per year. After 3 years of review, on 15 June 1994, this species was removed from protection (delisted) under the ESA, an indication of successful management.
The bowhead whale (Balaena mysticetus) is an endangered species that has shown a significant increase since the IWC adopted new rules in 1980 regulating its harvest for subsistence purposes by Native Americans (Fig. 2). The total prewhaling (before the mid-1800's) population of the bowhead whale is believed to have been 12,000-18,000; NMFS estimates that by 1900 it was probably in the low thousands. The current population of 7,500 is about 40% of its estimated 1848 population level (Table 2), more than 3 times the population low reached in 1980. The bowhead whale population has been growing by about 3% per year since 1978. Table 2. Status of selected Pacific coast species of marine mammals.
Species and area Abundance Status Trends Official status in designated U.S. waters
Fin whale 935  Unknown Unknown Endangered*
Humpback whale, E Pacific ~1,400  Probably less than 15% of 1850 population Unknown Endangered*
Northern right whale Unknown  Unknown Unknown Endangered*
Bowhead whale, W. Arctic 7,500  About 40% of 1848population size Increasing at 3.1%/yr, 1978-88 Endangered*
Grey whale 20,869 (19,200-22,700)  Recovered to historical 1845 abundance levels Increasing at 3.3%/yr, 1968-88 Removed from ESA listing June 1994
E. tropical Pacific dolphins        
  NE; spotted 731,000  Depleted Declining  
  W;/S spotted 1,298,000  Unknown Stable  
  Coastal; spotted 30,000  Unknown Stable  
  E; spinner 631,800  Depleted, 44% of late 1950's population Stable Depleted**
  Whitebelly; spinner 1,019,000  Unknown Stable  
  N; common 476,300  Unknown Declining  
  Central; common 406,100  Unknown Stable  
  S; common 2,210,900  Unknown Stable  
  Common; (pooled) 3,093,300  Unknown Stable  
  Striped; 1,918,000  Unknown Stable  
Harbor porpoise         
  SE; Alaska 2,052       
  W; Gulf of Alaska 1,273       
  N; California 10,000       
  Central; California 3,806       
  Inland; Washington 3,298       
  Oregon;/Washington 23,701       
Hawaiian monk seal 1,550  Declined 50% since 1950's Unknown, pup counts declining to variable Endangered*
California sea lion (CA, OR, WA) 111,016  Unknown Increasing 10.2%/yr since 1983  
Harbor seal    Unknown Increasing?  
  Alaska; 63,000    Declining  
  California; 23,113    Increasing  
  Oregon;/Washington 45,713    Increasing  
Northern fur seal        
  Pribilof; Islands 982,000  < 40% of 1950's population No significant trend since 1983 on St. Paul Is. Depleted**
  San; Miguel 6,000    Increasing  
Steller sea lion 116,000  < 22% of late 1950's population Declined 73% since 1960 Threatened*
  Northern; Pacific        
* Endangered Species Act.
** Marine Mammal Protection Act.
 
Fig. 2. Actual counts of bowhead whales, 1978-90 (NOAA 1994).
The endangered western North Atlantic population of right whales (Eubalaena glacialis) is considered by NMFS to be the only northern hemisphere right whale population with a significant number of individuals, about 300-350 animals (Table 1). Other stocks are considered virtually extinct: only five to seven sightings have been made in the last 25 years. Estimates of the pre-18th century population are as high as 10,000. NMFS believes that human influences such as ship strikes and net entanglements are affecting about 60% of the population. The agency notes that the annual loss of even a single right whale has measurable effect on the population, by greatly inhibiting recovery of the species.
Dolphins and Porpoises
The coastal migratory stock of Atlantic bottlenose dolphin (Tursiops truncatus) is listed as depleted under the MMPA (Table 1). This coastal stock incurred a loss of up to 50% during a 1987-88 die-off. Long-term trends are unknown, but the stock may require as many as 50 years to recover.
Harbor porpoises (Phocoena phocoena) occur on both U.S. coasts and are faring relatively well. The northwestern Atlantic harbor porpoise is found from Newfoundland, Canada, to Florida. The NMFS 1991-92 population estimate of the Gulf of Maine population is 47,200 (Table 1), but estimates of abundance for other populations do not exist. NMFS has found that harbor porpoise mortality from sink gill-net fisheries along the east coast of North America from Canada to North Carolina appears large compared with the species' natural reproduction rates. Management actions are being taken to address this issue, but long-term trends are unknown. On the west coast, NMFS's combined population estimate for northern California, Oregon, and Washington coastal stocks is 45,713.
The NMFS assesses 10 stocks of eastern tropical Pacific dolphins. Although population trends for most populations cannot be detected, the northeastern stocks of offshore spotted dolphin and the common dolphin may be declining (Table 2). These two stocks, as well as the eastern spinner and the striped dolphin, are incidentally taken in the international fishery for yellowfin tuna in the tropical Pacific waters off Mexico and Central America. Although mortality has been reduced in recent years, populations are still declining, or at best not increasing.
Seals and Sea Lions
According to the NMFS, harbor seal (Phoca vitulina) populations have increased recently throughout much of their range because of protection by the MMPA. Recent NMFS surveys estimate that at least 26,000 harbor seals inhabit the Gulf of Maine (Table 1). Populations of California harbor seals are also increasing; a recent survey resulted in a count of about 23,000 harbor seals residing in the Channel Islands and along the California mainland (Table 2), an increase from about 12,000 in 1983. The population of harbor seals in Oregon and Washington has been estimated at 45,700, and is also increasing. Harbor seal counts in the Central Gulf of Alaska, however, have declined significantly in the past two decades; numbers are currently estimated by NOAA at 63,000 seals.
The northern fur seal (Callorhinus ursinus) is considered depleted under the MMPA. Production on one of its major breeding areas, Alaska's Pribilof Islands, dropped more than 60% between 1955 and 1980, but has since stabilized. The current population is less than 40% of the mid-1950's level; no significant trend in the Pribilof Islands population has been noted since 1983 (Table 2).
The northern sea lion or Steller sea lion (Eumetopias jubatus) is listed as threatened under the ESA. Species numbers have declined sharply throughout its range in the last 34 years (Table 2). The number of adults and juveniles in U.S. waters dropped from 154,000 in 1960 to 40,000 in 1992, a reduction of 73%. Most of this decline occurred in Alaska waters, and is believed due to a combination of factors, including incidental kills, illegal shooting, changes in prey availability and biomass, and perhaps other unidentified factors.
The U.S. population of California sea lions (Zalophus californianus) is increasing at a rate of about 10% annually. In 1990, NMFS estimated that the U.S. population was 111,000 individuals (Table 2). A number of human-related interactions, such as incidental take during fishing, entanglement, illegal killing, and pollutants, result in sea lion deaths.
For further information:
Michael Payne
National Marine Fisheries Service
Office of Protected Resources
F/PR2
1335 East-West Highway
Silver Spring, MD 20910
References
NOAA. 1994. Our living ocean: report on the status of U.S. living marine resources, 1993. NOAA Tech. Memorandum NMFS-F/SPO-15. National Oceanic and Atmospheric Administration, National Marine Fisheries Service, Silver Spring, MD. 136 pp.


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