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Steller Sea Lion Population Assessment

(Quarterly Report for Jul-Aug-Sept 1998)

by John Sease

image of steller sea lion (20367 bytes)

Steller sea lions (Eumetopias jubatus) range from northern California through the Gulf of Alaska, the Aleutian Islands, and the Russian Far East to northern Japan and northward through the Bering Sea.  The center of abundance and distribution historically has been the Gulf of Alaska and Aleutian Islands.  Recent genetic studies identified two separate stocks within the population: an eastern stock, occurring from Southeast Alaska to California  and a western stock occuring from Prince William Sound, Alaska, westward to Russia and Japan (see “The Steller Sea Lion: Two Discrete Stocks?” Quarterly Report, January-February-March 1996).  The number of Steller sea lions declined dramatically throughout much of the species’ range, from 250,000-300,000 in the mid-1970s to as few as 116,000 animals by 1989, and fewer than 100,000 by 1994.  In 1990, a ruling under the U.S. Endangered Species Act listed Steller sea lions as “threatened” throughout their range.  In 1997, the western stock was reclassified as “endangered” in response to continuing population declines. The eastern stock remains classified as threatened.

A primary task of the National Marine Mammal Laboratory (NMML), a division of the National Marine Fisheries Service’s (NMFS) Alaska Fisheries Science Center, is to conduct assessment surveys of the two Steller sea lion populations. These surveys have been conducted since the mid-1970s and are accomplished in cooperation with the Alaska Department of Fish and Game (ADF&G), the Washington and Oregon departments of Fish and Wildlife, the NMFS Southwest Fisheries Science Center, Canada’s Department of Fisheries and Oceans, and the U.S. Fish and Wildlife Service.  For many years, the NMML has surveyed what is now classified as the western stock of Steller sea lions in the region from the Kenai Peninsula in the central Gulf of Alaska through the Aleutian Islands.

The NMFS survey areas historically include both rookeries and haulout sites, most of which are isolated islands and off-shore rocks.  Rookeries, sites where pups are born and mating takes place, usually contain hundreds and in some cases thousands of sea lions.  Haulout sites, where sea lions rest but typically don’t give birth, are more variable in the number of sea lions they have—from a few individuals to a few hundred.  Rookeries and haulout sites are well-established and traditional, thus the survey route is essentially the same each year.  Most of the rookeries and major haulouts are “trend sites” that have been surveyed consistently since the 1970s.  Comparison of counts at these trend sites allows us to estimate current trends in abundance.

Assessment of Adults and Juveniles

Population assessment of Steller sea lions typically requires two separate survey methods and efforts: aerial surveys for adults and juveniles and land surveys for pups.  We survey adult and juvenile (non-pup) sea lions most effectively during early to mid-June, the peak of the pupping season, when the greatest numbers of sea lions are likely to be on shore and visible for counting.  Aerial survey is the most practical survey method for these animals.  We take sequential, overlapping, aerial photographs at each site to guarantee complete coverage and count sea lions later from projected photographs in the laboratory.

Environmental conditions and scientific and logistic requirements of the Steller sea lion aerial survey necessitate the use of specialized aircraft.  The aircraft must be large enough to carry two pilots and a scientific party of three biologists, with windows for each biologist on the same side of the aircraft for observation.  The aircraft also must be small enough to fly at an altitude of about 500 feet and a speed of only 100 knots to obtain good aerial photographs of a site without scaring the sea lions away.  We prefer flying in a twin-engine, amphibious aircraft as we traverse many long stretches of open water, especially over the inter-island passes in the Aleutian Islands.  Only jet fuel is readily available west of Dutch Harbor, Alaska, so the aircraft also must be turbine-powered.  Radar is an essential safety feature due to the fog and inclement weather in the Aleutian Islands region where low cloud ceilings frequently require flying in the clouds but below mountain-top level.

Pup Assessment

We count sea lion pups on-site and in real-time.  The optimal time for counting pups is after all pups are born, of course, but before they are big enough to leave the rookery.  This is in early to mid-July, about 2-4 weeks after the aerial survey for non-pups.  During most pup surveys,  pups are counted by small survey teams walking directly through rookeries.  One biologist clears the beach of most adult and juvenile animals by herding them into the water, while two or three other biologists follow close behind and count all pups.  Needless to say, beach counts constitute a major disturbance at a rookery.  Ideally, we would like to count pups from aerial photographs, but for most rookeries in Alaska, this technique has not proven effective.  Pups are difficult to count reliably from aerial photographs as they can be obscured by adults or other obstacles and their dark pelage frequently blends with the substrate.  We do count pups occasionally from overlooks or from boats immediately offshore.  Typically this is done to prevent disturbance at a small rookery with very few pups and where negligible numbers of pups might be missed.

At selected rookeries we conduct other pup-related research in association with the pup counts.   Typically this includes weighing and measuring up to 50 pups and collecting tissue samples.  We catch each pup by hand, determine if it is a male or female, and place it in a large net bag, and suspend it from a weight scale. Most of the pups are about 1 month old and weigh from 25 to 50 kg (55-110 pounds).  We remove the pup from the bag to measure its length, its girth immediately behind its fore- flippers, and length of a foreflipper.  Before releasing each pup, we apply colored, numbered tags to the trailing edge of both foreflippers. The tags are ear tags designed for use on cattle but have proven well-suited for use on sea lions.  From a subsample of these pups we also obtain samples of blood and skin for physiological and genetic analyses.  The whole process requires only a few minutes for each pup.

These research efforts are part of the NMML’s ongoing evaluation of the physical condition of Steller sea lions and the investigation of possible causes of the Steller sea lion’s population decline in Alaska.  Analysis of blood samples may detect physiological conditions that may not be apparent from measurements and the overall physical appearance of the pups. Genetic samples are used for a variety of research investigations. Similar samples were instrumental in identifying and describing the separate eastern and western stocks within the Steller sea lion population.  Future work may reveal rookery-specific traits that could be useful for a variety of investigations such as describing seasonal movements of sea lions.

Scat Collection

Fecal material (scats) for food habits analysis is collected whenever we go ashore at a sea lion rookery or haulout.  We collect scats individually in Ziploc bags, freeze them in air-tight barrels, and ship them to Seattle.  In the laboratory, we thaw and wash the scats and recover hard prey remains:  bones and otoliths from fishes and beaks from cephalopods.  These prey remains are then compared to a reference collection for identification.  A promising new line of research focuses on genetic material contained in scats.  DNA in epithelial cells sloughed from the digestive tract of the sea lion may allow us to identify its gender and the rookery where it was born.  DNA from prey species may allow precise identification of food items that are otherwise unidentifiable.

The 1998 Aerial Survey for Non-pups

The 1998 NMFS aerial survey for non-pup Steller sea lions was conducted from 10 to 16 June including sites from Outer Island, off the Kenai Peninsula in the central Gulf of Alaska to Chirikof Point, Attu Island, the western-most tip of the Aleutian Islands, with a total of 49 hours of flight time. The aircraft was a chartered McKinnon (Grumman) turbine Goose, a twin-engine, amphibious airplane, from Aero Air, Hillsboro, Oregon, piloted by Dan Vollum and Alan Williams .  The same pilots and aircraft flew Steller sea lion surveys in the same region in 1993 and 1996.  One biologist photographed sea lions with an auto-focus 35 mm camera with a motor drive and 70-210 mm zoom lens and moderately fast, color transparency film.  Another biologist photographed each site with a video camera, which provided a more continuous record of the survey than the still photographs and also served as an emergency back-up in case the first camera malfunctioned.  We also experimented with medium-format and digital cameras during the 1998 survey.  Aerial surveys in Alaska are very weather-dependent.  It is not unusual to be grounded for several days by unflyable conditions.  We were extremely fortunate this year;  favorable weather prevailed and we were able to fly every day.

Results of the 1998 aerial  survey indicate that the number of non-pups in the region surveyed by the NMML, which comprises the greatest part of the western stock animals in Alaska, continued to decline in 1998.  The NMML survey counted a total of 27,185 Steller sea lions at 249 sites.  The 73 trend sites in this region included 18,228 non-pups in 1998.

Table 1. Counts of non-pup Steller sea lions from aerial surveys at trend rookeries and haulouts in Alaska,  1994-98.

Non-pup counts at
Trend Sites   

Percent change







Western Aleutian Islands 2,037 2,190 1,913 - 6.1 - 12.6
Central Aleutian Islands 5,790 5,528 5,761 - 1.5  + 3.2
Eastern Aleutian Islands 4,421 4,716 3,847 - 13.0 - 18.4
Western Gulf of Alaska 3,982 3,741 3,361 - 15.6 -10.2
Central Gulf of Alaska 4,520 3,915 3,346 - 26.0 - 14.5
Totals/average 20,750 20,090 18,228 - 12.2 - 9.3

This represents declines of 9% from 1996 (20,090 animals), 12% from 1994 (20,750), and 28% from 1989 (25,256) (Table 1 above), an average decline of about 5% each year from 1989 to 1998 after a decline of 76% from the late 1970s to 1989.  The central Aleutian Islands area (Islands of Four Mountains to Kiska Island) showed a marginal increase (+3.2%) in the number of non-pups from 1996 to 1998.  However, the number of non-pups in the western Gulf of Alaska and eastern Aleutian Islands (Shumagin Islands to the Islands of Four Mountains), a region that had experienced a slow increase in numbers of non-pups since 1989, declined by about 15% from 1996 to 1998 (from 8,457 to 7,208).

Pup Counts at Rookeries

The 1998 pup counts were conducted range-wide this year.  The NMML and ADF&G performed counts in Alaska.  The NMML’s pup-count work was entirely ship-supported in 1998. The Tila, a 121-foot research vessel operated by the U.S. Fish and Wildlife Service, Alaska Maritime National Wildlife Refuge, carried one scientific party to most of the rookeries in the Aleutian Islands.  The Big Valley a 92-foot fishing vessel that frequently charters as a scientific research platform, supported a second team in the Gulf of Alaska and in the Unimak Pass region.

The NMML counted pups at 33 rookeries from the central Gulf of Alaska through the Aleutian Islands during June and July 1998. Colleagues at ADF&G counted pups at eight more rookeries in the eastern Gulf of Alaska and in Southeast Alaska.  The combined  count of 13,607 pups was the most comprehensive count ever in Alaska and included virtually all rookeries.  The last state-wide pup count in Alaska was held in 1994.  For the 31 rookeries from the western stock that were counted in both surveys (from Prince William Sound to Kiska Island), the number of pups declined by 19% (from 10,402 pups to 8,423) over 4 years (Table 2 below).

Table 2. Counts of Steller sea lion pups at rookeries in Alaska,

No. of

              Pup count

   Percent change

Region rookeries 1994 1997 1998 '94-'98 '97-'98
Western Aleutian Islands   4 0 979 803 0 -18.0
Central Aleutian Islands 15 3,166 2,849 -9.5
Eastern Aleutian Islands   6 1,870 0 1,516 -18.9 0
Western Gulf of Alaska   4 1,662 1,493 -10.2
Central Gulf of Alaska   5 2,831 0 1,876 -33.7 0
Eastern Gulf of Alaska   2 903 610 689 -23.7 +13.0
Western Stock subtotal (excluding the western
Aleutian Islands)
32 10,402 00 8,423 -19.2 00
Southeast Alaska   3 3,770 4,160 4,234 +12.3 +1.8

In general, pup numbers increased slightly in parts of the central Aleutian Islands (eight rookeries from Seguam Island to the Delarof Islands) but declined elsewhere.  The NMML counted pups at all rookeries in the western Aleutian Islands (on Attu, Agattu, and Buldir islands) for the first time in 1997.  Pup numbers at these four rookeries declined by 18.0% in one year (from 979 pups to 803).  The trend is very different for the eastern stock.  At the three rookeries in Southeast Alaska numbers of pups are 12% greater than in 1994, but essentially unchanged from 1997 to 1998.

The NMML’s pup studies in the Gulf of Alaska and Aleutian Islands during 1998 indicate pups appeared to be of good size and generally healthy.  We captured, measured, and weighed a total of 334 pups at eight rookeries.  Mean weights for each rookery ranged from 28.8 to 31.2 kg for females and from 30.4 to 35.2 kg for males, which is within normal limits.  Lengths, girths, and calculations of mass:length and mass:volume ratios also were within normal ranges.  We collected blood samples from 51 pups at six rookeries in the Aleutian Islands.  Analyses of these samples have not been completed, but preliminary results do not suggest any abnormalities.  Processing and analysis of scats is a protraced process that will continue throughout the winter.


Results from the 1998 aerial and ship-based surveys in Alaska show that the decline of the western stock of Steller sea lions continues.  For non-pups and pups alike, the decline has been about 4%-5% per year.  Incomplete aerial survey results from Southeast Alaska suggests a slight decline in numbers of non-pups, but pup counts at the three rookeries in Southeast showed a marginal increase since 1997.  Taken together, and considering non-pup and pup count results from the past several years, all indications are that the Steller sea lion population in Southeast Alaska remains stable.