Population growth and colonization of Steller sea lions in the Glacier Bay region of southeastern Alaska: 1970s–2009

We estimated trends in numbers of Steller sea lions in the Glacier Bay region of the eastern population from the 1970s to 2009. We documented the colonization of several new haul‐outs and the transition of one haul‐out (Graves Rocks) to a rookery, assessed seasonal patterns in distribution, and compared counts from different observation platforms. Sea lions increased in the region by 8.2%/yr (95%CI = 6.4%–10.0%), with the most growth at South Marble Island in Glacier Bay (16.6%/yr, 1991–2009) and rapid growth in Cross Sound. Seasonal patterns in the distribution of sea lions were likely influenced by new breeding opportunities and the seasonal availability of prey. Factors that likely contributed to the exceptional growth include availability of new habitat following deglaciation, immigration, redistribution, decreases in mortality, and ecosystem‐level changes. The rapid increase in sea lion numbers in this region is of particular interest in light of dramatic declines in the western population and evidence that Steller sea lions from both the eastern and western populations colonized the Graves Rocks rookery. The colonization and rookery development in this dynamic area may signal the reversal of the reproductive isolation of the two populations.     

DECLINES IN HARBOR SEAL (PHOCA VITULINA) NUMBERS IN GLACIER BAY NATIONAL PARK, ALASKA, 1992–2002

Glacier Bay National Park had one of the largest breeding aggregations of harbor seals in Alaska, and it is functionally the only marine reserve for harbor seals in Alaska; yet, numbers of seals in the Bay are declining rapidly. Understanding why seals in Glacier Bay are declining may clarify their minimal habitat needs. We estimated population trends using models that controlled for environmental and observer‐related factors. In 1992, 6,200 seals were counted on icebergs in a tidewater glacial fjord and at terrestrial sites; by 2002 only 2,550 seals were counted at these same haul‐outs. Numbers of non‐pups in the glacial fjord declined by 6.6%/yr (?39%/8 yr) in June and by 9.6%/yr (?63%/11 yr) in August and at all other haul‐outs by 14.5%/yr (?75%/10 yr) during August. In the glacial fjord the number of pups remained steady from 1994 to 1999 and made up an increasing proportion of seals counted (5.4%/yr), and the proportion of pups peaked at 34%–36%. The rapid declines do not appear to be due to changes in seal behavior or redistribution. The declines reinforce genetic evidence that harbor seals in Glacier Bay are demographically isolated from other populations and indicate that current management stocks need to be redefined. Changes in Glacier Bay's ecosystem and population demographic data from the glacial fjord suggest that interspecific competition and predation are likely factors in the declines.     

KILLER WHALES AND MARINE MAMMAL TRENDS IN THE NORTH PACIFIC—A RE-EXAMINATION OF EVIDENCE FOR SEQUENTIAL MEGAFAUNA COLLAPSE AND THE PREY-SWITCHING HYPOTHESIS

Springer et al . (2003) contend that sequential declines occurred in North Pacific populations of harbor and fur seals, Steller sea lions, and sea otters. They hypothesize that these were due to increased predation by killer whales, when industrial whaling's removal of large whales as a supposed primary food source precipitated a prey switch. Using a regional approach, we reexamined whale catch data, killer whale predation observations, and the current biomass and trends of potential prey, and found little support for the prey-switching hypothesis. Large whale biomass in the Bering Sea did not decline as much as suggested by Springer et al ., and much of the reduction occurred 50–100 yr ago, well before the declines of pinnipeds and sea otters began; thus, the need to switch prey starting in the 1970s is doubtful. With the sole exception that the sea otter decline followed the decline of pinnipeds, the reported declines were not in fact sequential. Given this, it is unlikely that a sequential megafaunal collapse from whales to sea otters occurred. The spatial and temporal patterns of pinniped and sea otter population trends are more complex than Springer et al . suggest, and are often inconsistent with their hypothesis. Populations remained stable or increased in many areas, despite extensive historical whaling and high killer whale abundance. Furthermore, observed killer whale predation has largely involved pinnipeds and small cetaceans; there is little evidence that large whales were ever a major prey item in high latitudes. Small cetaceans (ignored by Springer et al .) were likely abundant throughout the period. Overall, we suggest that the Springer et al . hypothesis represents a misleading and simplistic view of events and trophic relationships within this complex marine ecosystem.     

Organochlorine exposure and health effects in stranded Steller sea lions (Eumetopias jubatus) and Pacific harbor seals (Phoca vitulina richardii) from Oregon and southern Washington coasts

Organochlorine (OC) pesticides and polychlorinated biphenyls (PCBs) have been detected in a variety of marine mammal species at levels associated with adverse health effects. Little is known about OC levels and impacts on health in pinnipeds with different life histories. We determined the health and levels of 18 OC pesticides and 16 PCB congeners in blubber samples from 20 Steller sea lions and 39 Pacific harbor seals stranded from Oregon and Southern Washington. The most commonly detected OC at the highest concentration was p,p′- dichlorodiphenyldichloroethylene (DDE). PCBs were detected in all samples as well. Hypothesis testing indicated that diseased Steller sea lions (males and females combined) had higher contaminant concentrations than healthy Steller sea lions, and diseased Pacific harbor seals had higher concentrations of total OCs than healthy animals. Differences were also noted between diseased and healthy animals when looking at individual sexes of each species. Diseased Steller sea lions had higher mean contaminant levels than diseased harbor seals and healthy Steller sea lions had higher mean contaminant concentrations than healthy Pacific harbor seals. These results show that species differences exist in both contaminant loads and sensitivity to contaminants, which may be due to differences in life histories and physiology.     

New records of South American sea lion <Emphasis Type="Italic">Otaria flavescens</Emphasis> predation on southern rockhopper penguins <Emphasis Type="Italic">Eudyptes chrysocome</Emphasis> at Staten Island,Argentina

Predation by fur seals and sea lions has been identified, among others, as a potential cause of the declines in rockhopper penguin populations. Here, we report a multiple predation event of southern rockhopper penguins Eudyptes chrysocome by South American sub-adult male sea lions Otaria flavescens at Staten Island, Argentina. The sea lions attacked and preyed on penguins mostly using a wait and rush tactic at sea, but in some cases, penguins were also pursued on land. Although observations suggested that only a few sea lions are involved in predation, further research is necessary to elucidate the importance of this predation in the rate of population decline.     

Interactions between killer whales (<Emphasis Type="Italic">Orcinus orca</Emphasis>) and Steller sea lions (<Emphasis Type="Italic">Eumetopias jubatus</Emphasis>) in the vicinity of Brat Chirpoev Island,Kuril Islands

The behaviors of breeding Steller sea lions in response to encounters with killer whales near the shore were observed on Brat Chirpoev Island, Kuril Islands between May and July 2002–2007. Approaches by killer whales and sea lion behavior was observed visually and recorded. Killer whales approached the rookery 104 times during the entire period of observations (289 days). In most cases (n = 95), beached sea lions did not show any apparent reactions to the presence of killer whales, and there were no observed interactions. Sea lions showed agitation during nine of the approaches; five of these events were considered to be predation attempts. The killer whales attacked the sea lions three times, however all the attacks were unsuccessful. We recorded two different types of responses towards the killer whales: (1) beaching on the shore (three times) and (2) mass exodus from the rookery with subsequent formation of a tight, actively swimming and vocalizing group (six times). The latter is the first recorded observation of this behavior for Steller sea lions. The observation suggests a low degree of interactions between these two species near the studied rookery. Despite the numerous observations of killer whales near the rookery, there were no observations of direct predation on sea lions. It is likely the killer whale predation has little or no direct impact on the Steller sea lion population on Brat Chirpoev Islands during the breeding period.     

KILLER WHALES, ORCINUS ORCA, IN THE SOUTHEASTERN BERING SEA: RECENT SIGHTINGS andPREDATION ON OTHER MARINE MAMMALS

An unusual number of killer whales appeared in inshore waters of the southeastern Bering Sea in summer 1989 and 1990. Multiple sightings occurred in Bristol and Kuskokwim bays where killer whales had been seen only rarely in previous years. Three animals became stranded on mud flats in Kuskokwim Bay but were able to free themselves on a high tide. Killer whales were observed interacting with salmon, harbor seals, Steller sea lions, walruses, and beluga whales. Detailed observations were made of killer whales attacking belugas in the Naknek River. Local conditions and behavioral adaptations may reduce the susceptibility of belugas to killer whale predation. Continued killer whale activity in this area would be unlikely to affect fish resources, but might have some influence on beluga whales.     

MAJOR DECLINE IN NUMBER OF HARBOR SEALS, PHOCA VITULINA RICHARDSI, ON TUGIDAK ISLAND, GULF OF ALASKA

Tugidak Island, located in the Gulf of Alaska, was once the site of one of largest local concentrations of harbor seals ( Phoca vitulina richardsi ) in the world. This population, which probably consisted of about 20,500 animals in the mid-1960s declined by about 85% between 1976 and 1988. The population appeared to decline more rapidly during the late 1970s than during the 1980s. Causes for the decline are not apparent. There appear to be both similarities and dissimilarities between this decline and recent declines in abundance of northern fur seals ( Callorhinus ursinus ) and Steller sea lions ( Eumetopias jubatus ) in the Bering Sea and Gulf of Alaska.     

ASSESSING KILLER WHALE PREDATION ON STELLER SEA LIONS FROM FIELD OBSERVATIONS IN KENAI FJORDS, ALASKA

The behavioral and predatory patterns of Gulf of Alaska (GOA) transient killer whales ( Orcinus orca ) were studied between 2000 and 2005 using remote video and vessel-based observations near the Chiswell Island Steller sea lion ( Eumetopias jubatus ) rookery and in the broader Kenai Fjords (KF) region of the northern GOA. GOA transient killer whales were observed on 118 d over the 6-yr period; the median group size was two (range: 1–9). Nine predation events were observed from vessels and an additional sixteen were inferred from remote video studies; all involved Steller sea lions. Estimates from field observations suggest that fifty-nine sea lions were consumed over the summer seasons of 2002–2005; whereas estimates based on published caloric requirements of transient killer whales would suggest a loss of 103 sea lions over the same time period. GOA transients spent a large proportion (43%) of their time resting which may be a strategy for conserving energy. Predation on sea lion pups at the Chiswell Island rookery was greatest during years when a single killer whale was foraging alone and when a 1.5-yr-old calf was evidently being trained to handle prey. Predation on pups was low during years when killer whales were foraging in groups and were observed and presumed to be taking mostly juvenile sea lions. Our study suggests that GOA transients are having a minor effect on the recovery of Steller sea lions in the GOA.     

Harbor seal population decline in the Aleutian Archipelago

Populations of Steller sea lions, northern fur seals, and northern sea otters declined substantially during recent decades in the Bering Sea and Aleutian Islands region, yet the population status of harbor seals has not been assessed adequately. We determined that counts obtained during skiff‐based surveys conducted in 1977–1982 represent the earliest estimate of harbor seal abundance throughout the Aleutian Islands. By comparing counts from 106 islands surveyed in 1977–1982 (8,601 seals) with counts from the same islands during a 1999 aerial survey (2,859 seals), we observed a 67% decline over the ～20‐yr period. Regionally, the largest decline of 86% was in the western Aleutians (n= 7 islands), followed by 66% in the central Aleutians (n= 64 islands), and 45% in the eastern Aleutians (n= 35 islands). Harbor seal counts decreased at the majority of islands in each region, the number of islands with >100 seals decreased ～70%, and the number of islands with no seals counted increased ～80%, indicating that harbor seal abundance throughout the Aleutian Islands was substantially lower in the late 1990s than in the 1970s and 1980s.     

Serologic surveillance of pathogens in a declining harbor seal (Phoca vitulina) population in Glacier Bay National Park, Alaska, USA and a reference site

The harbor seal population in Glacier Bay National Park, Alaska, has declined by over 70% since 1992. The reasons for this decline are not known. We examined serum antibodies and feces for evidence of exposure to multiple pathogens in this population. We also studied harbor seals from a reference site on Kodiak Island. In 2007, we found antibodies against Leptospira spp. in 31% of specimens from harbor seals in Glacier Bay, but no detectable serum antibodies in samples from Kodiak. In 2008, no samples had detectable antibodies against Leptospira spp. No serum antibodies against Toxoplasma gondii, morbilliviruses, or presence of Cryptosporidium in fecal samples were detected. However, Giardia was found in 6% of the fecal samples from Glacier Bay. Our results indicate that the harbor seal population in Glacier Bay National Park could be immunologically na?ve to distemper viruses and therefore vulnerable to these pathogens. Given the relatively low prevalence of antibodies and low titers, pathogens likely are not the reason for the harbor seal decline in Glacier Bay.     

Coxiella burnetii infection of marine mammals in the Pacific Northwest, 1997-2010

Q fever is a zoonotic disease caused by the bacterium Coxiella burnetii. Humans are commonly exposed via inhalation of aerosolized bacteria derived from the waste products of domesticated sheep and goats, and particularly from products generated during parturition. However, many other species can be infected with C. burnetii, and the host range and full zoonotic potential of C. burnetii is unknown. Two cases of C. burnetii infection in marine mammal placenta have been reported, but it is not known if this infection is common in marine mammals. To address this issue, placenta samples were collected from Pacific harbor seals (Phoca vitulina richardsi), harbor porpoises (Phocoena phocoena), and Steller sea lions (Eumetopias jubatus). Coxiella burnetii was detected by polymerase chain reaction (PCR) in the placentas of Pacific harbor seals (17/27), harbor porpoises (2/6), and Steller sea lions (1/2) collected in the Pacific Northwest. A serosurvey of 215 Pacific harbor seals sampled in inland and outer coastal areas of the Pacific Northwest showed that 34.0% (73/215) had antibodies against either Phase 1 or Phase 2 C. burnetii. These results suggest that C. burnetii infection is common among marine mammals in this region.     

Beneath the surface: Profiling blubber depth in pinnipeds with infrared imaging

Infrared thermography (IRT) was assessed as a non-invasive tool to evaluate body condition in juvenile female harbor seals (Phoca vitulina), (n=6) and adult female Steller sea lions (Eumetopias jubatus), (n=2). Surface temperature determined by IRT and blubber depth assessed with portable imaging ultrasound were monitored concurrently at eight body sites over the course of a year in long-term captive individuals under controlled conditions. Site-specific differences in surface temperature were noted between winter and summer in both species. Overall, surface temperature was slightly higher and more variable in harbor seals (9.8±0.6 °C) than Steller sea lions (9.1±0.5 °C). Limited site-specific relationships were found between surface temperature and blubber thickness, however, insulation level alone explained a very small portion of the variance. Therefore, while validated IRT data collection can potentially provide valuable information on the health, condition and metabolic state of an animal, it cannot provide a generalized proxy for blubber depth.     

INVESTIGATING TROPHIC RELATIONSHIPS OF PINNIPEDS IN ALASKA AND WASHINGTON USING STABLE ISOTOPE RATIOS OF NITROGEN AND CARBON

We measured stable-nitrogen (δ¹⁵N) and stable-carbon (δ¹³C) isotope ratios in muscle and hair from 7 northern fur seals (Callorhinus ursinus) from the Pribilof Islands, Alaska, and 27 Steller sea lions (Eumetopias jubatus), and 14 harbor seals (Phoca vitulina) from the Gulf of Alaska and coast of Washington State, in order to contrast dietary information derived from isotopic vs. available conventional dietary studies. Stable-nitrogen-isotope analysis of muscle revealed that harbor seals were enriched over sea lions (mean δ¹⁵N = 18.6‰vs. 17.5‰) which were in turn enriched over northern fur seals (mean δ¹⁵N = 16.6‰). Trophic segregation among these species likely results primarily from differential reliance on herring (Clupea harengus), Atka mackerel (Pleurogrammus monopterygius), and large vs. small walleye pollock (Theregra chalcogramma). According to their δ¹⁵N values, adult male Steller sea lions showed a higher trophic position than adult females (mean δ¹⁵N: 18.0‰vs. 17.2‰), whereas adult female northern fur seals were trophically higher than juvenile male fur seals (mean δ¹⁵N: 16.5‰vs. 15.0‰). Each of these observed differences likely resulted from differential reliance on squid or differences in the size range of pollock consumed. Three northern fur seal pups showed higher δ¹⁵N enrichment over adults (mean 17.7‰vs. 15.8‰) due to their reliance on their mother's milk. Stable-carbon isotope measurements of hair revealed a cline toward more negative values with latitude. Segregation in hair δ¹³C between Steller sea lions and harbor seals off the coast of Washington (mean δ¹³C: ?13.6‰vs.?15.0‰) reflected the greater association of harbor seals with freshwater input from the Columbia River. Our study demonstrates the utility of the stable isotope approach to augment conventional dietary analyses of pinnipeds and other marine mammals.     

Diet of Pacific sleeper shark, a potential Steller sea lion predator, in the north-east Pacific Ocean

Pacific sleeper sharks Somniosus pacificus were captured near Steller sea lion Eumetopias jubatus rookeries during the period when Steller sea lion pups are most vulnerable to Pacific sleeper shark predation (first water entrance and weaning). Analysis of stomach contents revealed that teleosts were the dominant prey in August and cephalopods were the dominant prey in May ( n = 198). Marine mammals were found in 15% of stomachs regardless of season, but no Steller sea lion tissues were detected. Molecular genetic analysis identified grey whale Eschrichtius robustus and harbour seal Phoca vitulina remains in some Pacific sleeper shark stomachs. Most mammals were cetacean and at least 70% of the cetaceans were probably scavenged. Although Pacific sleeper shark and Steller sea lion ranges overlapped, so predation could potentially occur, the diet study suggested that predation on Steller sea lions is unlikely, at least when pups first enter the water or during weaning. Harbour seals were infrequent prey and may have been consumed alive. Pacific sleeper sharks consume fast-swimming prey like Pacific salmon Oncorhynchus sp., most likely live animals rather than scavenged animals. Pacific sleeper sharks appeared to be opportunistic consumers of the available prey and carrion, feeding both on the bottom and in the water column, and their diet shifted to teleosts and cetacean carrion as the fish grew larger.     

Temporal records of δ13C and δ15N in North Pacific pinnipeds: inferences regarding environmental change and diet

Sea lion and seal populations in Alaskan waters underwent various degrees of decline during the latter half of the twentieth century and the cause(s) for the declines remain uncertain. The stable carbon (¹³C/¹²C) and nitrogen (¹⁵N/¹⁴N) isotope ratios in bone collagen from wild Steller sea lions (Eumetopias jubatus), northern fur seals (Callorhinus ursinus) and harbor seals (Phoca vitulina) from the Bering Sea and Gulf of Alaska were measured for the period 1951-1997 to test the hypothesis that a change in trophic level may have occurred during this interval and contributed to the population declines. A significant change in '¹⁵N in pinniped tissues over time would imply a marked change in trophic level. No significant change in bone collagen '¹⁵N was found for any of the three species during the past 47 years in either the Bering Sea or the Gulf of Alaska. However, the ¹⁵N in the Steller sea lion collagen was significantly higher than both northern fur seals and harbor seals. A significant decline in '¹³C (almost 2 ‰ over the 47 years) was evident in Steller sea lions, while a declining trend, though not significant, was evident in harbor seals and northern fur seals. Changes in foraging location, in combination with a trophic shift, may offer one possible explanation. Nevertheless, a decrease in '¹³C over time with no accompanying change in '¹⁵N suggests an environmental change affecting the base of the foodweb rather than a trophic level change due to prey switching. A decline in the seasonal primary production in the region, possibly resulting from decreased phytoplankton growth rates, would exhibit itself as a decline in '¹³C. Declining production could be an indication of a reduced carrying capacity in the North Pacific Ocean. Sufficient quantities of optimal prey species may have fallen below threshold sustaining densities for these pinnipeds, particularly for yearlings and subadults who have not yet developed adequate foraging skills.     

Impact of changing diet regimes on Steller sea lion body condition

A leading theory for the cause of the decline of Steller sea lions is nutritional stress, which led to chronic high juvenile mortality and possibly episodic adult mortality. Nutritional stress may have resulted from either poor quality or low abundance of prey. The objective of this study was to determine whether we could predict shifts in body condition (i.e., body mass or body fat content) over different seasons associated with a change in diet (i.e., toward lower quality prey). Captive Steller sea lions (n= 3) were fed three different diet regimes, where Diet 1 approximated the diet in the Kodiak area in the 1970s prior to the documented decline in that area, Diet 2 approximated the species composition in the Kodiak area after the decline had begun, and Diet 3 approximated the diet in southeast Alaska where the Steller sea lion population has been increasing for over 25 yr. All the animals used in this study were still growing and gained mass regardless of diet. Body fat (%) varied between 13% and 28%, but was not consistently high or low for any diet regime or season. Mean intake (in kg) of Diet 2 was significantly greater for all sea lions during all seasons. All animals did, however, tend to gain less body mass on Diets 2 and 3, as well as during the breeding and postbreeding seasons. They also tended to gain more mass during the winter and on Diet 1, though these differences were not statistically significant. Thus, changing seasonal physiology of Steller sea lions appears to have more impact on body condition than quality of prey, provided sufficient quantity of prey is available. Steller sea lions are opportunistic predators and are evidently able to thrive on a variety of prey. Our results indicate that Steller sea lions are capable of compensating for prey of low quality.     

FORAGING STRATEGIES OF SYMPATRIC KILLER WHALE (ORCINUS ORCA) POPULATIONS IN PRINCE WILLIAM SOUND, ALASKA

Killer whales ( Orcinus orca ) feed on a wide variety of fish, cephalopods, and marine mammals throughout their cosmopolitan range; however, the dietary breadth that characterizes the species is not reflected in all populations. Here, we present the findings of a 14-yr study of the diet and feeding habits of killer whales in Prince William Sound, Alaska. Two non-associating forms of killer whale, termed resident and transient (Bigg et al. 1987), were identified. All prey seen taken by transients were marine mammals, including harbor seals ( Phoca vitulina ), Dall's porpoises ( Phocoenoides dalli ), Steller sea lions ( Eumetopias jubatus ), and harbor porpoises ( Phocoena phocoena ). Resident killer whales appeared to prey principally on salmon ( Oncorhynchus spp.), preferring coho salmon ( O. kisutch ) over other, more abundant salmon species. Pacific herring ( Clupea pallasi ) and Pacific halibut ( Hippocampus stenolepis ) were also taken. Resident killer whales frequently were seen to interact in non-predatory ways with Steller sea lions and Dall's porpoises, while transients were not. Differences in the social organization and behavior of the resident and transient killer whales in Prince William Sound are discussed in the light of the dietary differences documented here.     

PINNIPED POPULATION DYNAMICS IN CENTRAL CALIFORNIA: CORRELATIONS WITH SEA SURFACE TEMPERATURE AND UPWELLING INDICES

We investigated effects of marine climate variability on pinniped populations and assessed the initial stages of recovery following implementation of the U. S. Marine Mammal Protection Act (MMPA) based on long-term (1973-1997) population surveys at the South Farallon Islands and Point Reyes Peninsula, central California. California sea lions increased over the study period, with peak numbers observed during and after major El Niño events. The rate of increase for California sea lions appears to have decreased in recent years. Steller sea lions decreased at the South Farallon Islands and remain depleted at Point Reyes Peninsula. Harbor seal populations increased in a logistic and non-linear fashion at Point Reyes Peninsula and the South Farallon Islands, respectively. Harbor seals were more abundant at the South Farallon Islands during years of relatively high sea-surface temperature, which may be related to their inability to find sufficient prey in coastal waters under these conditions. Northern elephant seal abundance increased in a logistic fashion over the study period at both the South Farallon Islands and Point Reyes Peninsula; however, productivity at the South Farallon Islands decreased in recent years. Maximum haulout numbers for elephant seals at the South Farallon Islands increased in the 1970s, maintained an asymptote throughout the 1980s and early 1990s, but recently declined; additional studies are needed to investigate which age classes are associated with this decline. Protection afforded by the MMPA has facilitated partial to full recovery of all populations except for Steller sea lion. Oceanographic relationships do not appear to confound interpretations of population recovery and may help to explain changes in the Steller sea lion population.     

LONG-TERM TRENDS IN HARBOR SEAL NUMBERS AT TUGIDAK ISLAND AND NANVAK BAY, ALASKA

We conducted land‐based counts of harbor seals (Phoca vitulina richardii) and collected related environmental data at Tugidak Island (Gulf of Alaska, 1994–2000) and Nanvak Bay (Bristol Bay, 1990–2000) to estimate population trends and identify factors influencing counts. At Tugidak Island, the seal population declined substantially during molting from 1976 through the 1980s, stabilized in the early 1990s, and increased at a moderate rate (3.4%/yr, CI: 1.0%–5.8%) from 1994 to 2000. Pups and all seals ashore during pupping increased at higher annual rates of 5.4% (CI: 2.2%–8.8%) and 8.3% (CI: 4.5%–12.3%) from 1994 to 2000 at Tugidak Island. At Nanvak Bay seals declined in abundance between 1975 and 1990 but increased during the 1990s at 9.2%/yr (CI: 7.2%–11.3%) during pupping and 2.1%/yr (CI: 0.6%–3.6%) during molting. Date and time‐of‐day were significant covariates in all analyses. Factors that led to declines at Tugidak Island and Nanvak Bay have seemingly abated sufficiently such that these populations are currently increasing, though still greatly reduced from the 1970s. Index sites are useful adjuncts to aerial surveys, providing survey‐related information not always available from aerial counts, which is useful in survey design and data analysis.